Two-dimensional conductors with interactions and disorder from particle-vortex duality

NASA Astrophysics Data System (ADS)

Goldman, H.; Mulligan, M.; Raghu, S.; Torroba, G.; Zimet, M.

2017-12-01

We study Dirac fermions in two spatial dimensions (2D) coupled to strongly fluctuating U (1 ) gauge fields in the presence of quenched disorder. Such systems are dual to theories of free Dirac fermions, which are vortices of the original theory. In analogy to superconductivity, when these fermionic vortices localize, the original system becomes a perfect conductor, and when the vortices possess a finite conductivity, the original fermions do as well. We provide several realizations of this principle and thereby introduce examples of strongly interacting 2D metals that evade Anderson localization.

Instantons and Massless Fermions in Two Dimensions

DOE R&D Accomplishments Database

Callan, C. G. Jr.; Dashen, R.; Gross, D. J.

1977-05-01

The role of instantons in the breakdown of chiral U(N) symmetry is studied in a two dimensional model. Chiral U(1) is always destroyed by the axial vector anomaly. For N = 2 chiral SU(N) is also spontaneously broken yielding massive fermions and three (decoupled) Goldstone bosons. For N greater than or equal to 3 the fermions remain massless. Realistic four dimensional theories are believed to behave in a similar way but the critical N above which the fermions cease to be massive is not known in four dimensions.

How gauge covariance of the fermion and boson propagators in QED constrain the effective fermion-boson vertex

DOE PAGES

Jia, Shaoyang; Pennington, M. R.

2016-12-12

In this paper, we derive the gauge covariance requirement imposed on the QED fermion-photon three-point function within the framework of a spectral representation for fermion propagators. When satisfied, such requirement ensures solutions to the fermion propagator Schwinger-Dyson equation (SDE) in any covariant gauge with arbitrary numbers of spacetime dimensions to be consistent with the Landau-Khalatnikov-Fradkin transformation (LKFT). The general result has been verified by the special cases of three and four dimensions. Additionally, we present the condition that ensures the vacuum polarization is independent of the gauge parameter. Finally, as an illustration, we show how the gauge technique dimensionally regularizedmore » in four dimensions does not satisfy the covariance requirement.« less

Diffusion in higher dimensional SYK model with complex fermions

NASA Astrophysics Data System (ADS)

Cai, Wenhe; Ge, Xian-Hui; Yang, Guo-Hong

2018-01-01

We construct a new higher dimensional SYK model with complex fermions on bipartite lattices. As an extension of the original zero-dimensional SYK model, we focus on the one-dimension case, and similar Hamiltonian can be obtained in higher dimensions. This model has a conserved U(1) fermion number Q and a conjugate chemical potential μ. We evaluate the thermal and charge diffusion constants via large q expansion at low temperature limit. The results show that the diffusivity depends on the ratio of free Majorana fermions to Majorana fermions with SYK interactions. The transport properties and the butterfly velocity are accordingly calculated at low temperature. The specific heat and the thermal conductivity are proportional to the temperature. The electrical resistivity also has a linear temperature dependence term.

Experimental Observation of Three-Component New Fermions in Topological Semimetal MoP

NASA Astrophysics Data System (ADS)

Lv, B. Q.; Feng, Z.-L.; Xu, Q.-N.; Ma, J.-Z.; Kong, L.-Y.; Richard, Pierre; Huang, Y.-B.; Strocov, V. N.; Fang, C.; Weng, H.-M.; Shi, Y.-G.; Qian, Tian; Ding, Hong; Paul Scherrer Institute, Swiss Light Source, CH-5232 Villigen PSI, Switzerland Team; Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics Team; University of Chinese Academy of Sciences, Beijing 100190, China Team; Collaborative Innovation Center of Quantum Matter, Beijing, China Collaboration; Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Team

Condensed matter systems can host quasiparticle excitations that are analogues to elementary particles such as Majorana, Weyl, and Dirac fermions. Recent advances in band theory have expanded the classification of fermions in crystals, and revealed crystal symmetry-protected electron excitations that have no high-energy counterparts. Here, using angle-resolved photoemission spectroscopy, we demonstrate the existence of a triply degenerate point in the electronic structure of MoP crystal, where the quasiparticle excitations are beyond the Majorana-Weyl-Dirac classification. Furthermore, we observe pairs of Weyl points in the bulk electronic structure coexisting with the new fermions, thus introducing a platform for studying the interplay between different types of fermions. We thank Binbin Fu, Nan Xu, and Xin Gao for the assistance in the ARPES experiments.

Hamiltonian Cycle Enumeration via Fermion-Zeon Convolution

NASA Astrophysics Data System (ADS)

Staples, G. Stacey

2017-12-01

Beginning with a simple graph having finite vertex set V, operators are induced on fermion and zeon algebras by the action of the graph's adjacency matrix and combinatorial Laplacian on the vector space spanned by the graph's vertices. When the graph is simple (undirected with no loops or multiple edges), the matrices are symmetric and the induced operators are self-adjoint. The goal of the current paper is to recover a number of known graph-theoretic results from quantum observables constructed as linear operators on fermion and zeon Fock spaces. By considering an "indeterminate" fermion/zeon Fock space, a fermion-zeon convolution operator is defined whose trace recovers the number of Hamiltonian cycles in the graph. This convolution operator is a quantum observable whose expectation reveals the number of Hamiltonian cycles in the graph.

Chiral Majorana fermion modes in a quantum anomalous Hall insulator-superconductor structure.

PubMed

He, Qing Lin; Pan, Lei; Stern, Alexander L; Burks, Edward C; Che, Xiaoyu; Yin, Gen; Wang, Jing; Lian, Biao; Zhou, Quan; Choi, Eun Sang; Murata, Koichi; Kou, Xufeng; Chen, Zhijie; Nie, Tianxiao; Shao, Qiming; Fan, Yabin; Zhang, Shou-Cheng; Liu, Kai; Xia, Jing; Wang, Kang L

2017-07-21

Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing. Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Hartree-Fock treatment of Fermi polarons using the Lee-Low-Pine transformation

NASA Astrophysics Data System (ADS)

Kain, Ben; Ling, Hong Y.

2017-09-01

We consider the Fermi polaron problem at zero temperature, where a single impurity interacts with noninteracting host fermions. We approach the problem starting with a Fröhlich-like Hamiltonian where the impurity is described with canonical position and momentum operators. We apply the Lee-Low-Pine (LLP) transformation to change the fermionic Fröhlich Hamiltonian into the fermionic LLP Hamiltonian, which describes a many-body system containing host fermions only. We adapt the self-consistent Hartree-Fock (HF) approach, first proposed by Edwards, to the fermionic LLP Hamiltonian in which a pair of host fermions with momenta k and k' interact with a potential proportional to k .k' . We apply the HF theory, which has the advantage of not restricting the number of particle-hole pairs, to repulsive Fermi polarons in one dimension. When the impurity and host fermion masses are equal our variational ansatz, where HF orbitals are expanded in terms of free-particle states, produces results in excellent agreement with McGuire's exact analytical results based on the Bethe ansatz. This work raises the prospect of using the HF ansatz and its time-dependent generalization as building blocks for developing all-coupling theories for both equilibrium and nonequilibrium Fermi polarons in higher dimensions.

Type-III and IV interacting Weyl points

NASA Astrophysics Data System (ADS)

Nissinen, J.; Volovik, G. E.

2017-04-01

3+1-dimensional Weyl fermions in interacting systems are described by effective quasi-relativistic Green's functions parametrized by a 16-element matrix e α μ in an expansion around the Weyl point. The matrix e α μ can be naturally identified as an effective tetrad field for the fermions. The correspondence between the tetrad field and an effective quasi-relativistic metric gμν governing the Weyl fermions allows for the possibility to simulate different classes of metric fields emerging in general relativity in interacting Weyl semimetals. According to this correspondence, there can be four types of Weyl fermions, depending on the signs of the components g 00 and g 00 of the effective metric. In addition to the conventional type-I fermions with a tilted Weyl cone and type-II fermions with an overtilted Weyl cone for g 00 > 0 and, respectively, g 00 > 0 or g 00 < 0, we find additional "type-III" and "type-IV" Weyl fermions with instabilities (complex frequencies) for g 00 < 0 and g 00 > 0 or g 00 < 0, respectively. While the type-I and type-II Weyl points allow us to simulate the black hole event horizon at an interface where g 00 changes sign, the type-III Weyl point leads to effective spacetimes with closed timelike curves.

Simple Z2 lattice gauge theories at finite fermion density

NASA Astrophysics Data System (ADS)

Prosko, Christian; Lee, Shu-Ping; Maciejko, Joseph

2017-11-01

Lattice gauge theories are a powerful language to theoretically describe a variety of strongly correlated systems, including frustrated magnets, high-Tc superconductors, and topological phases. However, in many cases gauge fields couple to gapless matter degrees of freedom, and such theories become notoriously difficult to analyze quantitatively. In this paper we study several examples of Z2 lattice gauge theories with gapless fermions at finite density, in one and two spatial dimensions, that are either exactly soluble or whose solution reduces to that of a known problem. We consider complex fermions (spinless and spinful) as well as Majorana fermions and study both theories where Gauss' law is strictly imposed and those where all background charge sectors are kept in the physical Hilbert space. We use a combination of duality mappings and the Z2 slave-spin representation to map our gauge theories to models of gauge-invariant fermions that are either free, or with on-site interactions of the Hubbard or Falicov-Kimball type that are amenable to further analysis. In 1D, the phase diagrams of these theories include free-fermion metals, insulators, and superconductors, Luttinger liquids, and correlated insulators. In 2D, we find a variety of gapped and gapless phases, the latter including uniform and spatially modulated flux phases featuring emergent Dirac fermions, some violating Luttinger's theorem.

Application of fermionic marginal constraints to hybrid quantum algorithms

NASA Astrophysics Data System (ADS)

Rubin, Nicholas C.; Babbush, Ryan; McClean, Jarrod

2018-05-01

Many quantum algorithms, including recently proposed hybrid classical/quantum algorithms, make use of restricted tomography of the quantum state that measures the reduced density matrices, or marginals, of the full state. The most straightforward approach to this algorithmic step estimates each component of the marginal independently without making use of the algebraic and geometric structure of the marginals. Within the field of quantum chemistry, this structure is termed the fermionic n-representability conditions, and is supported by a vast amount of literature on both theoretical and practical results related to their approximations. In this work, we introduce these conditions in the language of quantum computation, and utilize them to develop several techniques to accelerate and improve practical applications for quantum chemistry on quantum computers. As a general result, we demonstrate how these marginals concentrate to diagonal quantities when measured on random quantum states. We also show that one can use fermionic n-representability conditions to reduce the total number of measurements required by more than an order of magnitude for medium sized systems in chemistry. As a practical demonstration, we simulate an efficient restoration of the physicality of energy curves for the dilation of a four qubit diatomic hydrogen system in the presence of three distinct one qubit error channels, providing evidence these techniques are useful for pre-fault tolerant quantum chemistry experiments.

Heavy fermion behavior explained by bosons

NASA Technical Reports Server (NTRS)

Kallio, A.; Poykko, S.; Apaja, V.

1995-01-01

Conventional heavy fermion (HF) theories require existence of massive fermions. We show that heavy fermion phenomena can also be simply explained by existence of bosons with moderate mass but temperature dependent concentration below the formation temperature T(sub B), which in turn is close to room temperature. The bosons B(++) are proposed to be in chemical equilibrium with a system of holes h(+): B(++) = h(+) + h(+). This equilibrium is governed by a boson breaking function f(T), which determines the decreasing boson density and the increasing fermion density with increasing temperature. Since HF-compounds are hybridized from minimum two elements, we assume in addition existence of another fermion component h(sub s)(+) with temperature independent density. This spectator component is thought to be the main agent in binding the bosons in analogy with electronic or muonic molecules. Using a linear boson breaking function we can explain temperature dependence of the giant linear specific heat coefficient gamma(T) coming essentially from bosons. The maxima in resistivity, Hall coefficient, and susceptibility are explained by boson localization effects due to the Wigner crystallization. The antiferromagnetic transitions in turn are explained by similar localization of the pairing fermion system when their density n(sub h)(T(sub FL)) becomes lower than n(sub WC), the critical density of Wigner crystallization. The model applies irrespective whether a compound is superconducting or not. The same model explains the occurrence of low temperature antiferromagnetism also in high-T(sub c) superconductors. The double transition in UPt3 is proposed to be due to the transition of the pairing fermion liquid from spin polarized to unpolarized state.

Infrared problem in non-Abelian gauge theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yao, Y.

1976-03-22

I extend the Bloch--Nordsieck idea to show that in the lowest nontrivial order of radiative correction the fermion--fermion and gauge-meson--fermion scattering rates are finite, provided that they are averaged over the initial and summed over the final internal spin states. Questions of the physical gauge coupling and infrared slavery are discussed. (AIP)

Superalgebra and fermion-boson symmetry

PubMed Central

Miyazawa, Hironari

2010-01-01

Fermions and bosons are quite different kinds of particles, but it is possible to unify them in a supermultiplet, by introducing a new mathematical scheme called superalgebra. In this article we discuss the development of the concept of symmetry, starting from the rotational symmetry and finally arriving at this fermion-boson (FB) symmetry. PMID:20228617

Relaxation of Fermionic Excitations in a Strongly Attractive Fermi Gas in an Optical Lattice

DTIC Science & Technology

2011-09-27

decreases both with temperature and deviation of the fermion density from half filling. We show that quasiparticle and phase degrees of freedom are...the interaction strength to the bandwidth of the system. Thus, at strong coupling, the fermionic quasiparticles and the motion of the bosonic molecules

Role of four-fermion interaction and impurity in the states of two-dimensional semi-Dirac materials.

PubMed

Wang, Jing

2018-03-28

We study the effects of four-fermion interaction and impurity on the low-energy states of 2D semi-Dirac materials by virtue of the unbiased renormalization group approach. The coupled flow equations that govern the energy-dependent evolutions of all correlated interaction parameters are derived after taking into account one-loop corrections from the interplay between four-fermion interaction and impurity. Whether and how four-fermion interaction and impurity influence the low-energy properties of 2D semi-Dirac materials are discreetly explored and addressed attentively. After carrying out the standard renormalization group analysis, we find that both trivial insulating and nontrivial semimetal states are qualitatively stable against all four kinds of four-fermion interactions. However, while switching on both four-fermion interaction and impurity, certain insulator-semimetal phase transitions and the distance of Dirac nodal points can be respectively induced and modified due to their strong interplay and intimate competition. Moreover, several non-Fermi liquid behaviors that deviate from the conventional Fermi liquids are exhibited at the lowest-energy limit.

Bose-Fermi degeneracies in large N adjoint QCD

DOE PAGES

Basar, Gokce; Cherman, Aleksey; McGady, David

2015-07-06

Here, we analyze the large N limit of adjoint QCD, an SU( N) gauge theory with N f flavors of massless adjoint Majorana fermions, compactified on S 3 × S 1. We focus on the weakly-coupled confining small- S 3 regime. If the fermions are given periodic boundary conditions on S 1, we show that there are large cancellations between bosonic and fermionic contributions to the twisted partition function. These cancellations follow a pattern previously seen in the context of misaligned supersymmetry, and lead to the absence of Hagedorn instabilities for any S 1 size L, even though the bosonicmore » and fermionic densities of states both have Hagedorn growth. Adjoint QCD stays in the confining phase for any L ~ N 0, explaining how it is able to enjoy large N volume independence for any L. The large N boson-fermion cancellations take place in a setting where adjoint QCD is manifestly non-supersymmetric at any finite N, and are consistent with the recent conjecture that adjoint QCD has emergent fermionic symmetries in the large N limit.« less

Parallelized traveling cluster approximation to study numerically spin-fermion models on large lattices

NASA Astrophysics Data System (ADS)

Mukherjee, Anamitra; Patel, Niravkumar D.; Bishop, Chris; Dagotto, Elbio

2015-06-01

Lattice spin-fermion models are important to study correlated systems where quantum dynamics allows for a separation between slow and fast degrees of freedom. The fast degrees of freedom are treated quantum mechanically while the slow variables, generically referred to as the "spins," are treated classically. At present, exact diagonalization coupled with classical Monte Carlo (ED + MC) is extensively used to solve numerically a general class of lattice spin-fermion problems. In this common setup, the classical variables (spins) are treated via the standard MC method while the fermion problem is solved by exact diagonalization. The "traveling cluster approximation" (TCA) is a real space variant of the ED + MC method that allows to solve spin-fermion problems on lattice sizes with up to 103 sites. In this publication, we present a novel reorganization of the TCA algorithm in a manner that can be efficiently parallelized. This allows us to solve generic spin-fermion models easily on 104 lattice sites and with some effort on 105 lattice sites, representing the record lattice sizes studied for this family of models.

Fermionic entanglement that survives a black hole

NASA Astrophysics Data System (ADS)

Martín-Martínez, Eduardo; León, Juan

2009-10-01

We introduce an arbitrary number of accessible modes when analyzing bipartite entanglement degradation due to Unruh effect between two partners Alice and Rob. Under the single mode approximation (SMA) a fermion field only had a few accessible levels due to Pauli exclusion principle conversely to bosonic fields which had an infinite number of excitable levels. This was argued to justify entanglement survival in the fermionic case in the SMA infinite acceleration limit. Here we relax SMA. Hence, an infinite number of modes are excited as the observer Rob accelerates, even for a fermion field. We will prove that, despite this analogy with the bosonic case, entanglement loss is limited. We will show that this comes from fermionic statistics through the characteristic structure it imposes on the infinite dimensional density matrix for Rob. Surprisingly, the surviving entanglement is independent of the specific maximally entangled state chosen, the kind of fermionic field analyzed, and the number of accessible modes considered. We shall discuss whether this surviving entanglement goes beyond the purely statistical correlations, giving insight concerning the black hole information paradox.

Role of four-fermion interaction and impurity in the states of two-dimensional semi-Dirac materials

NASA Astrophysics Data System (ADS)

Wang, Jing

2018-03-01

We study the effects of four-fermion interaction and impurity on the low-energy states of 2D semi-Dirac materials by virtue of the unbiased renormalization group approach. The coupled flow equations that govern the energy-dependent evolutions of all correlated interaction parameters are derived after taking into account one-loop corrections from the interplay between four-fermion interaction and impurity. Whether and how four-fermion interaction and impurity influence the low-energy properties of 2D semi-Dirac materials are discreetly explored and addressed attentively. After carrying out the standard renormalization group analysis, we find that both trivial insulating and nontrivial semimetal states are qualitatively stable against all four kinds of four-fermion interactions. However, while switching on both four-fermion interaction and impurity, certain insulator-semimetal phase transitions and the distance of Dirac nodal points can be respectively induced and modified due to their strong interplay and intimate competition. Moreover, several non-Fermi liquid behaviors that deviate from the conventional Fermi liquids are exhibited at the lowest-energy limit.

Direct evidence for a magnetic f-electron–mediated pairing mechanism of heavy-fermion superconductivity in CeCoIn5

PubMed Central

Van Dyke, John S.; Massee, Freek; Allan, Milan P.; Davis, J. C. Séamus; Petrovic, Cedomir; Morr, Dirk K.

2014-01-01

To identify the microscopic mechanism of heavy-fermion Cooper pairing is an unresolved challenge in quantum matter studies; it may also relate closely to finding the pairing mechanism of high-temperature superconductivity. Magnetically mediated Cooper pairing has long been the conjectured basis of heavy-fermion superconductivity but no direct verification of this hypothesis was achievable. Here, we use a novel approach based on precision measurements of the heavy-fermion band structure using quasiparticle interference imaging to reveal quantitatively the momentum space (k-space) structure of the f-electron magnetic interactions of CeCoIn5. Then, by solving the superconducting gap equations on the two heavy-fermion bands Ekα,β with these magnetic interactions as mediators of the Cooper pairing, we derive a series of quantitative predictions about the superconductive state. The agreement found between these diverse predictions and the measured characteristics of superconducting CeCoIn5 then provides direct evidence that the heavy-fermion Cooper pairing is indeed mediated by f-electron magnetism. PMID:25062692

Thermal transitions, pseudogap behavior, and BCS-BEC crossover in Fermi-Fermi mixtures

NASA Astrophysics Data System (ADS)

Karmakar, Madhuparna

2018-03-01

We study the mass imbalanced Fermi-Fermi mixture within the framework of a two-dimensional lattice fermion model. Based on the thermodynamic and species-dependent quasiparticle behavior, we map out the finite-temperature phase diagram of this system and show that unlike the balanced Fermi superfluid, there are now two different pseudogap regimes as PG-I and PG-II. While within the PG-I regime both the fermionic species are pseudogapped, PG-II corresponds to the regime where pseudogap feature survives only in the light species. We believe that the single-particle spectral features that we discuss in this paper are observable through the species-resolved radio-frequency spectroscopy and momentum-resolved photoemission spectroscopy measurements on systems such as 6Li-40K mixture. We further investigate the interplay between the population and mass imbalances and report that at a fixed population imbalance, the BCS-BEC crossover in a Fermi-Fermi mixture would require a critical interaction (Uc) for the realization of the uniform superfluid state. The effect of imbalance in mass on the exotic Fulde-Ferrell-Larkin-Ovchinnikov superfluid phase has been probed in detail in terms of the thermodynamic and quasiparticle behavior of this phase. It has been observed that in spite of the s -wave symmetry of the pairing field, a nodal superfluid gap is realized in the Larkin-Ovchinnikov regime. Our results on the various thermal scales and regimes are expected to serve as benchmarks for the experimental observations on 6Li-40K mixture.

Nanoscale Measurements of Magnetism & Spin Coherence in Semiconductors

DTIC Science & Technology

2015-12-17

superconductor created by magnetic defects. These energy-resolved studies are distinct from typical spin-selective measurements performed previously using...MacDonald, B. A. Bernevig, A. Yazdani. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor , Science, (10 2014): 602... superconductor , Physical Review B, (07 2013): 0. doi: 10.1103/PhysRevB.88.020407 TOTAL: 4 Number of Papers published in peer-reviewed journals

Zitterbewegung in time-reversal Weyl semimetals

NASA Astrophysics Data System (ADS)

Huang, Tongyun; Ma, Tianxing; Wang, Li-Gang

2018-06-01

We perform a systematic study of the Zitterbewegung effect of fermions, which are described by a Gaussian wave with broken spatial-inversion symmetry in a three-dimensional low-energy Weyl semimetal. Our results show that the motion of fermions near the Weyl points is characterized by rectilinear motion and Zitterbewegung oscillation. The ZB oscillation is affected by the width of the Gaussian wave packet, the position of the Weyl node, and the chirality and anisotropy of the fermions. By introducing a one-dimensional cosine potential, the new generated massless fermions have lower Fermi velocities, which results in a robust relativistic oscillation. Modulating the height and periodicity of periodic potential demonstrates that the ZB effect of fermions in the different Brillouin zones exhibits quasi-periodic behavior. These results may provide an appropriate system for probing the Zitterbewegung effect experimentally.

Trial wave functions for a composite Fermi liquid on a torus

NASA Astrophysics Data System (ADS)

Fremling, M.; Moran, N.; Slingerland, J. K.; Simon, S. H.

2018-01-01

We study the two-dimensional electron gas in a magnetic field at filling fraction ν =1/2 . At this filling the system is in a gapless state which can be interpreted as a Fermi liquid of composite fermions. We construct trial wave functions for the system on a torus, based on this idea, and numerically compare these to exact wave functions for small systems found by exact diagonalization. We find that the trial wave functions give an excellent description of the ground state of the system, as well as its charged excitations, in all momentum sectors. We analyze the dispersion of the composite fermions and the Berry phase associated with dragging a single fermion around the Fermi surface and comment on the implications of our results for the current debate on whether composite fermions are Dirac fermions.

Wigner functions for fermions in strong magnetic fields

NASA Astrophysics Data System (ADS)

Sheng, Xin-li; Rischke, Dirk H.; Vasak, David; Wang, Qun

2018-02-01

We compute the covariant Wigner function for spin-(1/2) fermions in an arbitrarily strong magnetic field by exactly solving the Dirac equation at non-zero fermion-number and chiral-charge densities. The Landau energy levels as well as a set of orthonormal eigenfunctions are found as solutions of the Dirac equation. With these orthonormal eigenfunctions we construct the fermion field operators and the corresponding Wigner-function operator. The Wigner function is obtained by taking the ensemble average of the Wigner-function operator in global thermodynamical equilibrium, i.e., at constant temperature T and non-zero fermion-number and chiral-charge chemical potentials μ and μ_5, respectively. Extracting the vector and axial-vector components of the Wigner function, we reproduce the currents of the chiral magnetic and separation effect in an arbitrarily strong magnetic field.

Benchmark results in the 2D lattice Thirring model with a chemical potential

NASA Astrophysics Data System (ADS)

Ayyar, Venkitesh; Chandrasekharan, Shailesh; Rantaharju, Jarno

2018-03-01

We study the two-dimensional lattice Thirring model in the presence of a fermion chemical potential. Our model is asymptotically free and contains massive fermions that mimic a baryon and light bosons that mimic pions. Hence, it is a useful toy model for QCD, especially since it, too, suffers from a sign problem in the auxiliary field formulation in the presence of a fermion chemical potential. In this work, we formulate the model in both the world line and fermion-bag representations and show that the sign problem can be completely eliminated with open boundary conditions when the fermions are massless. Hence, we are able accurately compute a variety of interesting quantities in the model, and these results could provide benchmarks for other methods that are being developed to solve the sign problem in QCD.

Projective flatness in the quantisation of bosons and fermions

NASA Astrophysics Data System (ADS)

Wu, Siye

2015-07-01

We compare the quantisation of linear systems of bosons and fermions. We recall the appearance of projectively flat connection and results on parallel transport in the quantisation of bosons. We then discuss pre-quantisation and quantisation of fermions using the calculus of fermionic variables. We define a natural connection on the bundle of Hilbert spaces and show that it is projectively flat. This identifies, up to a phase, equivalent spinor representations constructed by various polarisations. We introduce the concept of metaplectic correction for fermions and show that the bundle of corrected Hilbert spaces is naturally flat. We then show that the parallel transport in the bundle of Hilbert spaces along a geodesic is a rescaled projection provided that the geodesic lies within the complement of a cut locus. Finally, we study the bundle of Hilbert spaces when there is a symmetry.

Renormalization of Coulomb interactions in a system of two-dimensional tilted Dirac fermions

NASA Astrophysics Data System (ADS)

Lee, Yu-Wen; Lee, Yu-Li

2018-01-01

We investigate the effects of long-ranged Coulomb interactions in a tilted Dirac semimetal in two dimensions by using the perturbative renormalization-group (RG) method. Depending on the magnitude of the tilting parameter, the undoped system can have either Fermi points (type I) or Fermi lines (type II). Previous studies usually performed the renormalization-group transformations by integrating out the modes with large momenta. This is problematic when the Fermi surface is open, like type-II Dirac fermions. In this work we study the effects of Coulomb interactions, following the spirit of Shankar [Rev. Mod. Phys. 66, 129 (1994), 10.1103/RevModPhys.66.129], by introducing a cutoff in the energy scale around the Fermi surface and integrating out the high-energy modes. For type-I Dirac fermions, our result is consistent with that of the previous work. On the other hand, we find that for type-II Dirac fermions, the magnitude of the tilting parameter increases monotonically with lowering energies. This implies the stability of type-II Dirac fermions in the presence of Coulomb interactions, in contrast with previous results. Furthermore, for type-II Dirac fermions, the velocities in different directions acquire different renormalization even if they have the same bare values. By taking into account the renormalization of the tilting parameter and the velocities due to the Coulomb interactions, we show that while the presence of a charged impurity leads only to charge redistribution around the impurity for type-I Dirac fermions, for type-II Dirac fermions, the impurity charge is completely screened, albeit with a very long screening length. The latter indicates that the temperature dependence of physical observables are essentially determined by the RG equations we derived. We illustrate this by calculating the temperature dependence of the compressibility and specific heat of the interacting tilted Dirac fermions.

Grassmann phase space methods for fermions. II. Field theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dalton, B.J., E-mail: bdalton@swin.edu.au; Jeffers, J.; Barnett, S.M.

In both quantum optics and cold atom physics, the behaviour of bosonic photons and atoms is often treated using phase space methods, where mode annihilation and creation operators are represented by c-number phase space variables, with the density operator equivalent to a distribution function of these variables. The anti-commutation rules for fermion annihilation, creation operators suggests the possibility of using anti-commuting Grassmann variables to represent these operators. However, in spite of the seminal work by Cahill and Glauber and a few applications, the use of Grassmann phase space methods in quantum-atom optics to treat fermionic systems is rather rare, thoughmore » fermion coherent states using Grassmann variables are widely used in particle physics. This paper presents a phase space theory for fermion systems based on distribution functionals, which replace the density operator and involve Grassmann fields representing anti-commuting fermion field annihilation, creation operators. It is an extension of a previous phase space theory paper for fermions (Paper I) based on separate modes, in which the density operator is replaced by a distribution function depending on Grassmann phase space variables which represent the mode annihilation and creation operators. This further development of the theory is important for the situation when large numbers of fermions are involved, resulting in too many modes to treat separately. Here Grassmann fields, distribution functionals, functional Fokker–Planck equations and Ito stochastic field equations are involved. Typical applications to a trapped Fermi gas of interacting spin 1/2 fermionic atoms and to multi-component Fermi gases with non-zero range interactions are presented, showing that the Ito stochastic field equations are local in these cases. For the spin 1/2 case we also show how simple solutions can be obtained both for the untrapped case and for an optical lattice trapping potential.« less

Fermion localization on a split brane

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chumbes, A. E. R.; Vasquez, A. E. O.; Hott, M. B.

2011-05-15

In this work we analyze the localization of fermions on a brane embedded in five-dimensional, warped and nonwarped, space-time. In both cases we use the same nonlinear theoretical model with a nonpolynomial potential featuring a self-interacting scalar field whose minimum energy solution is a soliton (a kink) which can be continuously deformed into a two-kink. Thus a single brane splits into two branes. The behavior of spin 1/2 fermions wave functions on the split brane depends on the coupling of fermions to the scalar field and on the geometry of the space-time.

DOE Office of Scientific and Technical Information (OSTI.GOV)

He, Qing Lin; Pan, Lei; Stern, Alexander L.

Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantummore » computing.« less

Two-dimensional conductors with interactions and disorder from particle-vortex duality

DOE PAGES

Goldman, H.; Mulligan, M.; Raghu, S.; ...

2017-12-27

Here, we study Dirac fermions in two spatial dimensions (2D) coupled to strongly fluctuating U(1) gauge fields in the presence of quenched disorder. Such systems are dual to theories of free Dirac fermions, which are vortices of the original theory. In analogy to superconductivity, when these fermionic vortices localize, the original system becomes a perfect conductor, and when the vortices possess a finite conductivity, the original fermions do as well. We provide several realizations of this principle and thereby introduce examples of strongly interacting 2D metals that evade Anderson localization.

A search for excited fermions in electron-proton collisions at HERA

NASA Astrophysics Data System (ADS)

Derrick, M.; Krakauer, D.; Magill, S.; Musgrave, B.; Repond, J.; Schlereth, J.; Stanek, R.; Talaga, R. L.; Thron, J.; Arzarello, F.; Ayad, R.; Bari, G.; Basile, M.; Bellagamba, L.; Boscherini, D.; Bruni, A.; Bruni, G.; Bruni, P.; Romeo, G. Cara; Castellini, G.; Chiarini, M.; Cifarelli, L.; Cindolo, F.; Ciralli, F.; Contin, A.; D'Auria, S.; Frasconi, F.; Gialas, I.; Giusti, P.; Iacobucci, G.; Laurenti, G.; Levi, G.; Margotti, A.; Massam, T.; Nania, R.; Nemoz, C.; Palmonari, F.; Polini, A.; Sartorelli, G.; Timellini, R.; Garcia, Y. Zamora; Zichichi, A.; Bargende, A.; Crittenden, J.; Desch, K.; Diekmann, B.; Doeker, T.; Eckart, M.; Feld, L.; Frey, A.; Geerts, M.; Geitz, G.; Grothe, M.; Hartmann, H.; Haun, D.; Heinloth, K.; Hilger, E.; Jakob, H.-P.; Katz, U. F.; Mari, S. M.; Mass, A.; Mengel, S.; Mollen, J.; Paul, E.; Rembser, Ch.; Schattevoy, R.; Schneider, J.-L.; Schramm, D.; Stamm, J.; Wedemeyer, R.; Campbell-Robson, S.; Cassidy, A.; Dyce, N.; Foster, B.; George, S.; Gilmore, R.; Heath, G. P.; Heath, H. F.; Llewellyn, T. J.; Morgado, C. J. S.; Norman, D. J. P.; O'Mara, J. A.; Tapper, R. J.; Wilson, S. S.; Yoshida, R.; Rau, R. R.; Arneodo, M.; Iannotti, L.; Schioppa, M.; Susinno, G.; Bernstein, A.; Caldwell, A.; Parsons, J. A.; Ritz, S.; Sciulli, F.; Straub, P. B.; Wai, L.; Yang, S.; Borzemski, P.; Chwastowski, J.; Eskreys, A.; Piotrzkowski, K.; Zachara, M.; Zawiejski, L.; Adamczyk, L.; Bednarek, B.; Eskreys, K.; Jeleń, K.; Kisielewska, D.; Kowalski, T.; Rulikowska-Zarebska, E.; Suszycki, L.; Zajac, J.; Kedzierski, T.; Kotański, A.; Przybycień, M.; Bauerdick, L. A. T.; Behrens, U.; Bienlein, J. K.; Böttcher, S.; Coldewey, C.; Drews, G.; Flasiński, M.; Gilkinson, D. J.; Göttlicher, P.; Gutjahr, B.; Haas, T.; Hain, W.; Hasell, D.; Heßling, H.; Hultschig, H.; Iga, Y.; Joos, P.; Kasemann, M.; Klanner, R.; Koch, W.; Köpke, L.; Kötz, U.; Kowalski, H.; Kroger, W.; Krüger, J.; Labs, J.; Ladage, A.; Löhr, B.; Löwe, M.; Lüke, D.; Mańczak, O.; Ng, J. S. T.; Nickel, S.; Notz, D.; Ohrenberg, K.; Roco, M.; Rohde, M.; Roldán, J.; Schneekloth, U.; Schulz, W.; Selonke, F.; Stiliaris, E.; Voß, T.; Westphal, D.; Wolf, G.; Youngman, C.; Grabosch, H. J.; Leich, A.; Meyer, A.; Rethfeldt, C.; Schlenstedt, S.; Barbagli, G.; Pelfer, P.; Anzivino, G.; Maccarrone, G.; de Pasquale, S.; Qian, S.; Votano, L.; Bamberger, A.; Freidhof, A.; Poser, T.; Söldner-Rembold, S.; Schroeder, J.; Theisen, G.; Trefzger, T.; Brook, N. H.; Bussey, P. J.; Doyle, A. T.; Fleck, I.; Jamieson, V. A.; Saxon, D. H.; Utley, M. L.; Wilson, A. S.; Dannemann, A.; Holm, U.; Horstmann, D.; Kammerlocher, H.; Krebs, B.; Neumann, T.; Sinkus, R.; Wick, K.; Badura, E.; Burow, B. D.; Fürtjes, A.; Hagge, L.; Lohrmann, E.; Mainusch, J.; Milewski, J.; Nakahata, M.; Pavel, N.; Poelz, G.; Schott, W.; Terron, J.; Zetsche, F.; Bacon, T. C.; Beuselinck, R.; Butterworth, I.; Gallo, E.; Harris, V. L.; Hung, B. H.; Long, K. R.; Miller, D. B.; Morawitz, P. P. O.; Prinias, A.; Sedgbeer, J. K.; Whitfield, A. F.; Mallik, U.; McCliment, E.; Wang, M. Z.; Wang, S. M.; Wu, J. T.; Zhang, Y.; Cloth, P.; Filges, D.; An, S. H.; Hong, S. M.; Nam, S. W.; Park, S. K.; Suh, M. H.; Yon, S. H.; Imlay, R.; Kartik, S.; Kim, H.-J.; McNeil, R. R.; Metcalf, W.; Nadendla, V. K.; Barreiro, F.; Cases, G.; Graciani, R.; Hernández, J. M.; Hervás, L.; Labarga, L.; Del Peso, J.; Puga, J.; de Trocóniz, J. F.; Ikraiam, F.; Mayer, J. K.; Smith, G. R.; Corriveau, F.; Hanna, D. S.; Hartmann, J.; Hung, L. W.; Lim, J. N.; Matthews, C. G.; Patel, P. M.; Sinclair, L. E.; Stairs, D. G.; St. Laurent, M.; Ullmann, R.; Zacek, G.; Bashkirov, V.; Dolgoshein, B. A.; Stifutkin, A.; Bashindzhagyan, G. L.; Ermolov, P. F.; Gladilin, L. K.; Golubkov, Y. A.; Kobrin, V. D.; Kuzmin, V. A.; Proskuryakov, A. S.; Savin, A. A.; Shcheglova, L. M.; Solomin, A. N.; Zotov, N. P.; Bentvelsen, S.; Botje, M.; Chlebana, F.; Dake, A.; Engelen, J.; de Jong, P.; de Kamps, M.; Kooijman, P.; Kruse, A.; O'Dell, V.; Tenner, A.; Tiecke, H.; Verkerke, W.; Vreeswijk, M.; Wiggers, L.; de Wolf, E.; van Woudenberg, R.; Acosta, D.; Bylsma, B.; Durkin, L. S.; Honscheid, K.; Li, C.; Ling, T. Y.; McLean, K. W.; Murray, W. N.; Park, I. H.; Romanowski, T. A.; Seidlein, R.; Bailey, D. S.; Blair, G. A.; Byrne, A.; Cashmore, R. J.; Cooper-Sarkar, A. M.; Daniels, D.; Devenish, R. C. E.; Harnew, N.; Lancaster, M.; Luffman, P. E.; Lindemann, L.; McFall, J.; Nath, C.; Quadt, A.; Uijterwaal, H.; Walczak, R.; Wilson, F. F.; Yip, T.; Abbiendi, G.; Bertolin, A.; Brugnera, R.; Carlin, R.; Dal Corso, F.; de Giorgi, M.; Dosselli, U.; Limentani, S.; Morandin, M.; Posocco, M.; Stanco, L.; Stroili, R.; Voci, C.; Bulmahn, J.; Butterworth, J. M.; Feild, R. G.; Oh, B. Y.; Whitmore, J. J.; D'Agostini, G.; Iori, M.; Marini, G.; Mattioli, M.; Nigro, A.; Tassi, E.; Hart, J. C.; McCubbin, N. A.; Prytz, K.; Shah, T. P.; Short, T. L.; Barberis, E.; Cartiglia, N.; Dubbs, T.; Heusch, C.; van Hook, M.; Hubbard, B.; Lockman, W.; Rahn, J. T.; Sadrozinski, H. F.-W.; Seiden, A.; Biltzinger, J.; Seifert, R. J.; Walenta, A. H.; Zech, G.; Abramowicz, H.; Briskin, G.; Dagan, S.; Levy, A.; Hasegawa, T.; Hazumi, M.; Ishii, T.; Kuze, M.; Mine, S.; Nagasawa, Y.; Nagira, T.; Nakao, M.; Suzuki, I.; Tokushuku, K.; Yamada, S.; Yamazaki, Y.; Chiba, M.; Hamatsu, R.; Hirose, T.; Homma, K.; Kitamura, S.; Nagayama, S.; Nakamitsu, Y.; Cirio, R.; Costa, M.; Ferrero, M. I.; Lamberti, L.; Maselli, S.; Peroni, C.; Sacchi, R.; Solano, A.; Staiano, A.; Dardo, M.; Bailey, D. C.; Bandyopadhyay, D.; Benard, F.; Brkic, M.; Crombie, M. B.; Gingrich, D. M.; Hartner, G. F.; Joo, K. K.; Levman, G. M.; Martin, J. F.; Orr, R. S.; Sampson, C. R.; Teuscher, R. J.; Catterall, C. D.; Jones, T. W.; Kaziewicz, P. B.; Lane, J. B.; Saunders, R. L.; Shulman, J.; Blankenship, K.; Kochocki, J.; Lu, B.; Mo, L. W.; Bogusz, W.; Charchula, K.; Ciborowski, J.; Gajewski, J.; Grzelak, G.; Kasprazak, M.; Krzyżanowski, M.; Muchorowski, K.; Nowak, R. J.; Pawlak, J. M.; Tymieniecka, T.; Wróblewski, A. K.; Zakrzewski, J. A.; Żarnecki, A. F.; Adamus, M.; Eisenberg, Y.; Glasman, C.; Karshon, U.; Revel, D.; Shapira, A.; Ali, I.; Behrens, B.; Dasu, S.; Fordham, C.; Foudas, C.; Goussiou, A.; Loveless, R. J.; Reeder, D. D.; Silverstein, S.; Smith, W. H.; Tsurugai, T.; Bhadra, S.; Frisken, W. R.; Furutani, K. M.

1995-12-01

A search for excited states of the standard model fermions was performed using the ZEUS detector at the HERA electron-proton collider, operating at a centre of mass energy of 296 GeV. In a sample corresponding to an integrated luminosity of 0.55 pb-1, no evidence was found for any resonant state decaying into final states composed of a fermion and a gauge boson. Limits on the coupling strength times branching ratio of excited fermions are presented for masses between 50 GeV and 250 GeV, extending previous search regions significantly.

The 't Hooft vertex for staggered fermions and flavor-singlet mesons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Donald, Gordon C.; Davies, Christine T.H.; Follana, Eduardo

2011-01-01

We derive the ’t Hooft vertex for staggered fermions and examine its symmetries for nonzero lattice spacing. We also derive a set of structural properties for the eigenvectors of the staggered Dirac operator, which should emerge in the continuum limit, if staggered fermions yield four species. This property also is needed for flavor-taste-singlet correlators to behave correctly. We then test numerically whether the needed structure arises: it does. This structure and symmetry of (unrooted) staggered fermions also imply that Creutz’s (latest) objections to the rooted determinant are without foundation.

Cosmological BCS mechanism and the big bang singularity

NASA Astrophysics Data System (ADS)

Alexander, Stephon; Biswas, Tirthabir

2009-07-01

We provide a novel mechanism that resolves the big bang singularity present in Friedman-Lemaitre-Robertson-Walker space-times without the need for ghost fields. Building on the fact that a four-fermion interaction arises in general relativity when fermions are covariantly coupled, we show that at early times the decrease in scale factor enhances the correlation between pairs of fermions. This enhancement leads to a BCS-like condensation of the fermions and opens a gap dynamically driving the Hubble parameter H to zero and results in a nonsingular bounce, at least in some special cases.

Bootstrapping 3D fermions

DOE PAGES

Iliesiu, Luca; Kos, Filip; Poland, David; ...

2016-03-17

We study the conformal bootstrap for a 4-point function of fermions in 3D. We first introduce an embedding formalism for 3D spinors and compute the conformal blocks appearing in fermion 4-point functions. Using these results, we find general bounds on the dimensions of operators appearing in the ψ × ψ OPE, and also on the central charge C T. We observe features in our bounds that coincide with scaling dimensions in the GrossNeveu models at large N. Finally, we also speculate that other features could coincide with a fermionic CFT containing no relevant scalar operators.

Two-dimensional conductors with interactions and disorder from particle-vortex duality

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goldman, H.; Mulligan, M.; Raghu, S.

Here, we study Dirac fermions in two spatial dimensions (2D) coupled to strongly fluctuating U(1) gauge fields in the presence of quenched disorder. Such systems are dual to theories of free Dirac fermions, which are vortices of the original theory. In analogy to superconductivity, when these fermionic vortices localize, the original system becomes a perfect conductor, and when the vortices possess a finite conductivity, the original fermions do as well. We provide several realizations of this principle and thereby introduce examples of strongly interacting 2D metals that evade Anderson localization.

Fluctuations of a q-deformed fermion gas

NASA Astrophysics Data System (ADS)

Zeng, Qijun; Ge, Jing; Luo, Yongsong

2018-05-01

The theory of q-deformed fermions is one of the theories of q-deformed oscillators. Within the framework of this theory and the traditional fluctuation theory, we investigate fluctuations of q-deformed fermion gas and obtain the expressions of fluctuations of the internal energy U, the particle number N and the correlation of fluctuations of the two physical quantities above. Further numerical calculation reveals that fluctuations of such a system have some interesting and particular features. We consider that this work may provide much insight into the theory of q fermions, and may also be helpful for the theory of q-deformed oscillators.

General structure of fermion two-point function and its spectral representation in a hot magnetized medium

NASA Astrophysics Data System (ADS)

Das, Aritra; Bandyopadhyay, Aritra; Roy, Pradip K.; Mustafa, Munshi G.

2018-02-01

We have systematically constructed the general structure of the fermion self-energy and the effective quark propagator in the presence of a nontrivial background such as a hot magnetized medium. This is applicable to both QED and QCD. The hard thermal loop approximation has been used for the heat bath. We have also examined transformation properties of the effective fermion propagator under some of the discrete symmetries of the system. Using the effective fermion propagator we have analyzed the fermion dispersion spectra in a hot magnetized medium along with the spinor for each fermion mode obtained by solving the modified Dirac equation. The fermion spectra is found to reflect the discrete symmetries of the two-point functions. We note that for a chirally symmetric theory the degenerate left- and right-handed chiral modes in vacuum or in a heat bath get separated and become asymmetric in the presence of a magnetic field without disturbing the chiral invariance. The obtained general structure of the two-point functions is verified by computing the three-point function, which agrees with the existing results in one-loop order. Finally, we have computed explicitly the spectral representation of the two-point functions which would be very important to study the spectral properties of the hot magnetized medium corresponding to QED and QCD with background magnetic field.

I.I. Rabi in Atomic, Molecular & Optical Physics Prize Talk: Strongly Interacting Fermi Gases of Atoms and Molecules

NASA Astrophysics Data System (ADS)

Zwierlein, Martin

2017-04-01

Strongly interacting fermions govern physics at all length scales, from nuclear matter to modern electronic materials and neutron stars. The interplay of the Pauli principle with strong interactions can give rise to exotic properties that we do not understand even at a qualitative level. In recent years, ultracold Fermi gases of atoms have emerged as a new type of strongly interacting fermionic matter that can be created and studied in the laboratory with exquisite control. Feshbach resonances allow for unitarity limited interactions, leading to scale invariance, universal thermodynamics and a superfluid phase transition already at 17 Trapped in optical lattices, fermionic atoms realize the Fermi-Hubbard model, believed to capture the essence of cuprate high-temperature superconductors. Here, a microscope allows for single-atom, single-site resolved detection of density and spin correlations, revealing the Pauli hole as well as anti-ferromagnetic and doublon-hole correlations. Novel states of matter are predicted for fermions interacting via long-range dipolar interactions. As an intriguing candidate we created stable fermionic molecules of NaK at ultralow temperatures featuring large dipole moments and second-long spin coherence times. In some of the above examples the experiment outperformed the most advanced computer simulations of many-fermion systems, giving hope for a new level of understanding of strongly interacting fermions.

PROCEEDINGS OF RIKEN BNL RESEARCH CENTER WORKSHOP ENTITLED - DOMAIN WALL FERMIONS AT TEN YEARS (VOLUME 84)

DOE Office of Scientific and Technical Information (OSTI.GOV)

BLUM,T.; SONI,A.

The workshop was held to mark the 10th anniversary of the first numerical simulations of QCD using domain wall fermions initiated at BNL. It is very gratifying that in the intervening decade widespread use of domain wall and overlap fermions is being made. It therefore seemed appropriate at this stage for some ''communal introspection'' of the progress that has been made, hurdles that need to be overcome, and physics that can and should be done with chiral fermions. The meeting was very well attended, drawing about 60 registered participants primarily from Europe, Japan and the US. It was quite remarkablemore » that pioneers David Kaplan, Herbert Neuberger, Rajamani Narayanan, Yigal Shamir, Sinya Aoki, and Pavlos Vranas all attended the workshop. Comparisons between domain wall and overlap formulations, with their respective advantages and limitations, were discussed at length, and a broad physics program including pion and kaon physics, the epsilon regime, nucleon structure, and topology, among others, emerged. New machines and improved algorithms have played a key role in realizing realistic dynamical fermion lattice simulations (small quark mass, large volume, and so on), so much in fact that measurements are now as costly. Consequently, ways to make the measurements more efficient were also discussed. We were very pleased to see the keen and ever growing interest in chiral fermions in our community and the significant strides our colleagues have made in bringing chiral fermions to the fore of lattice QCD calculations. Their contributions made the workshop a success, and we thank them deeply for sharing their time and ideas. Finally, we must especially acknowledge Norman Christ and Bob Mawhinney for their early and continued collaboration without which the success of domain wall fermions would not have been possible.« less

Multiple Types of Topological Fermions in Transition Metal Silicides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tang, Peizhe; Zhou, Quan; Zhang, Shou -Cheng

Exotic massless fermionic excitations with nonzero Berry flux, other than the Dirac and Weyl fermions, could exist in condensed matter systems under the protection of crystalline symmetries, such as spin-1 excitations with threefold degeneracy and spin-3/2 Rarita-Schwinger-Weyl fermions. Herein, by using the ab initio density functional theory, we show that these unconventional quasiparticles coexist with type-I and type-II Weyl fermions in a family of transition metal silicides, including CoSi, RhSi, RhGe, and CoGe, when spin-orbit coupling is considered. Their nontrivial topology results in a series of extensive Fermi arcs connecting projections of these bulk excitations on the side surface, whichmore » is confirmed by (001) surface electronic spectra of CoSi. Additionally, these stable arc states exist within a wide energy window around the Fermi level, which makes them readily accessible in angle-resolved photoemission spectroscopy measurements.« less

Chiral fermions in asymptotically safe quantum gravity

NASA Astrophysics Data System (ADS)

Meibohm, J.; Pawlowski, J. M.

2016-05-01

We study the consistency of dynamical fermionic matter with the asymptotic safety scenario of quantum gravity using the functional renormalisation group. Since this scenario suggests strongly coupled quantum gravity in the UV, one expects gravity-induced fermion self-interactions at energies of the Planck scale. These could lead to chiral symmetry breaking at very high energies and thus to large fermion masses in the IR. The present analysis which is based on the previous works (Christiansen et al., Phys Rev D 92:121501, 2015; Meibohm et al., Phys Rev D 93:084035, 2016), concludes that gravity-induced chiral symmetry breaking at the Planck scale is avoided for a general class of NJL-type models. We find strong evidence that this feature is independent of the number of fermion fields. This finding suggests that the phase diagram for these models is topologically stable under the influence of gravitational interactions.

Conceptual Foundations of Soliton Versus Particle Dualities Toward a Topological Model for Matter

NASA Astrophysics Data System (ADS)

Kouneiher, Joseph

2016-06-01

The idea that fermions could be solitons was actually confirmed in theoretical models in 1975 in the case when the space-time is two-dimensional and with the sine-Gordon model. More precisely S. Coleman showed that two different classical models end up describing the same fermions particle, when the quantum theory is constructed. But in one model the fermion is a quantum excitation of the field and in the other model the particle is a soliton. Hence both points of view can be reconciliated.The principal aim in this paper is to exhibit a solutions of topological type for the fermions in the wave zone, where the equations of motion are non-linear field equations, i.e. using a model generalizing sine- Gordon model to four dimensions, and describe the solutions for linear and circular polarized waves. In other words, the paper treat fermions as topological excitations of a bosonic field.

Two- and four-dimensional representations of the PT - and CPT -symmetric fermionic algebras

NASA Astrophysics Data System (ADS)

Beygi, Alireza; Klevansky, S. P.; Bender, Carl M.

2018-03-01

Fermionic systems differ from their bosonic counterparts, the main difference with regard to symmetry considerations being that T2=-1 for fermionic systems. In PT -symmetric quantum mechanics an operator has both PT and CPT adjoints. Fermionic operators η , which are quadratically nilpotent (η2=0 ), and algebras with PT and CPT adjoints can be constructed. These algebras obey different anticommutation relations: η ηPT+ηPTη =-1 , where ηPT is the PT adjoint of η , and η ηCPT+ηCPTη =1 , where ηCPT is the CPT adjoint of η . This paper presents matrix representations for the operator η and its PT and CPT adjoints in two and four dimensions. A PT -symmetric second-quantized Hamiltonian modeled on quantum electrodynamics that describes a system of interacting fermions and bosons is constructed within this framework and is solved exactly.

Chiral fermions in asymptotically safe quantum gravity.

PubMed

Meibohm, J; Pawlowski, J M

2016-01-01

We study the consistency of dynamical fermionic matter with the asymptotic safety scenario of quantum gravity using the functional renormalisation group. Since this scenario suggests strongly coupled quantum gravity in the UV, one expects gravity-induced fermion self-interactions at energies of the Planck scale. These could lead to chiral symmetry breaking at very high energies and thus to large fermion masses in the IR. The present analysis which is based on the previous works (Christiansen et al., Phys Rev D 92:121501, 2015; Meibohm et al., Phys Rev D 93:084035, 2016), concludes that gravity-induced chiral symmetry breaking at the Planck scale is avoided for a general class of NJL-type models. We find strong evidence that this feature is independent of the number of fermion fields. This finding suggests that the phase diagram for these models is topologically stable under the influence of gravitational interactions.

Free-fermion descriptions of parafermion chains and string-net models

NASA Astrophysics Data System (ADS)

Meichanetzidis, Konstantinos; Turner, Christopher J.; Farjami, Ashk; Papić, Zlatko; Pachos, Jiannis K.

2018-03-01

Topological phases of matter remain a focus of interest due to their unique properties: fractionalization, ground-state degeneracy, and exotic excitations. While some of these properties can occur in systems of free fermions, their emergence is generally associated with interactions between particles. Here, we quantify the role of interactions in general classes of topological states of matter in one and two spatial dimensions, including parafermion chains and string-net models. Surprisingly, we find that certain topological states can be exactly described by free fermions, while others saturate the maximum possible distance from their optimal free-fermion description [C. J. Turner et al., Nat. Commun. 8, 14926 (2017), 10.1038/ncomms14926]. Our work opens the door to understanding the complexity of topological models by establishing new types of fermionization procedures to describe their low-energy physics, thus making them amenable to experimental realizations.

Phase diagram and re-entrant fermionic entanglement in a hybrid Ising-Hubbard ladder

NASA Astrophysics Data System (ADS)

Sousa, H. S.; Pereira, M. S. S.; de Oliveira, I. N.; Strečka, J.; Lyra, M. L.

2018-05-01

The degree of fermionic entanglement is examined in an exactly solvable Ising-Hubbard ladder, which involves interacting electrons on the ladder's rungs described by Hubbard dimers at half-filling on each rung, accounting for intrarung hopping and Coulomb terms. The coupling between neighboring Hubbard dimers is assumed to have an Ising-like nature. The ground-state phase diagram consists of four distinct regions corresponding to the saturated paramagnetic, the classical antiferromagnetic, the quantum antiferromagnetic, and the mixed classical-quantum phase. We have exactly computed the fermionic concurrence, which measures the degree of quantum entanglement between the pair of electrons on the ladder rungs. The effects of the hopping amplitude, the Coulomb term, temperature, and magnetic fields on the fermionic entanglement are explored in detail. It is shown that the fermionic concurrence displays a re-entrant behavior when quantum entanglement is being generated at moderate temperatures above the classical saturated paramagnetic ground state.

Multiple Types of Topological Fermions in Transition Metal Silicides

DOE PAGES

Tang, Peizhe; Zhou, Quan; Zhang, Shou -Cheng

2017-11-17

Exotic massless fermionic excitations with nonzero Berry flux, other than the Dirac and Weyl fermions, could exist in condensed matter systems under the protection of crystalline symmetries, such as spin-1 excitations with threefold degeneracy and spin-3/2 Rarita-Schwinger-Weyl fermions. Herein, by using the ab initio density functional theory, we show that these unconventional quasiparticles coexist with type-I and type-II Weyl fermions in a family of transition metal silicides, including CoSi, RhSi, RhGe, and CoGe, when spin-orbit coupling is considered. Their nontrivial topology results in a series of extensive Fermi arcs connecting projections of these bulk excitations on the side surface, whichmore » is confirmed by (001) surface electronic spectra of CoSi. Additionally, these stable arc states exist within a wide energy window around the Fermi level, which makes them readily accessible in angle-resolved photoemission spectroscopy measurements.« less

Similarity-transformed dyson mapping and SDG-interacting boson hamiltonian

NASA Astrophysics Data System (ADS)

Navrátil, P.; Dobeš, J.

1991-10-01

The sdg-interacting boson hamiltonian is constructed from the fermion shell-model input. The seniority boson mapping as given by the similarity-transformed Dyson boson mapping is used. The s, d, and g collective boson amplitudes are determined consistently from the mapped hamiltonian. Influence of the starting shell-model parameters is discussed. Calculations for the Sm isotopic chain and for the 148Sm, 150Nd, and 196Pt nuclei are presented. Calculated energy levels as well as E2 and E4 properties agree rather well with experimental ones. To obtain such agreement, the input shell-model parameters cannot be fixed at a constant set for several nuclei but have to be somewhat varied, especially in the deformed region. Possible reasons for this variation are discussed. Effects of the explicit g-boson consideration are shown.

Matrix pentagons

NASA Astrophysics Data System (ADS)

Belitsky, A. V.

2017-10-01

The Operator Product Expansion for null polygonal Wilson loop in planar maximally supersymmetric Yang-Mills theory runs systematically in terms of multi-particle pentagon transitions which encode the physics of excitations propagating on the color flux tube ending on the sides of the four-dimensional contour. Their dynamics was unraveled in the past several years and culminated in a complete description of pentagons as an exact function of the 't Hooft coupling. In this paper we provide a solution for the last building block in this program, the SU(4) matrix structure arising from internal symmetry indices of scalars and fermions. This is achieved by a recursive solution of the Mirror and Watson equations obeyed by the so-called singlet pentagons and fixing the form of the twisted component in their tensor decomposition. The non-singlet, or charged, pentagons are deduced from these by a limiting procedure.

Sign problem and Monte Carlo calculations beyond Lefschetz thimbles

DOE PAGES

Alexandru, Andrei; Basar, Gokce; Bedaque, Paulo F.; ...

2016-05-10

We point out that Monte Carlo simulations of theories with severe sign problems can be profitably performed over manifolds in complex space different from the one with fixed imaginary part of the action (“Lefschetz thimble”). We describe a family of such manifolds that interpolate between the tangent space at one critical point (where the sign problem is milder compared to the real plane but in some cases still severe) and the union of relevant thimbles (where the sign problem is mild but a multimodal distribution function complicates the Monte Carlo sampling). As a result, we exemplify this approach using amore » simple 0+1 dimensional fermion model previously used on sign problem studies and show that it can solve the model for some parameter values where a solution using Lefschetz thimbles was elusive.« less

Holographic mutual information of two disjoint spheres

NASA Astrophysics Data System (ADS)

Chen, Bin; Fan, Zhong-Ying; Li, Wen-Ming; Zhang, Cheng-Yong

2018-04-01

We study quantum corrections to holographic mutual information for two disjoint spheres at a large separation by using the operator product expansion of the twist field. In the large separation limit, the holographic mutual information is vanishing at the semiclassical order, but receive quantum corrections from the fluctuations. We show that the leading contributions from the quantum fluctuations take universal forms as suggested from the boundary CFT. We find the universal behavior for the scalar, the vector, the tensor and the fermionic fields by treating these fields as free fields propagating in the fixed background and by using the 1 /n prescription. In particular, for the fields with gauge symmetries, including the massless vector boson and massless graviton, we find that the gauge parts in the propagators play an indispensable role in reading the leading order corrections to the bulk mutual information.

The Fermionic Projector, entanglement and the collapse of the wave function

NASA Astrophysics Data System (ADS)

Finster, Felix

2011-07-01

After a brief introduction to the fermionic projector approach, we review how entanglement and second quantized bosonic and fermionic fields can be described in this framework. The constructions are discussed with regard to decoherence phenomena and the measurement problem. We propose a mechanism leading to the collapse of the wave function in the quantum mechanical measurement process.

Drude Conductivity of Dirac Fermions in Graphene

DTIC Science & Technology

2010-01-01

interband transitions, as required by the sum rule. Our surprising observation indicates that many-body effects and Dirac fermion-impurity interactions...reduction of free electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum...dimensions. Researchers have demonstrated in graphene exotic Dirac fermion phenomena ranging from anomalous quantum Hall effects 1,2 to Klein tunneling 3 in

New excitations in the Thirring model

NASA Astrophysics Data System (ADS)

Cortés, J. L.; Gamboa, J.; Schmidt, I.; Zanelli, J.

1998-12-01

The quantization of the massless Thirring model in the light-cone using functional methods is considered. The need to compactify the coordinate x- in the light-cone spacetime implies that the quantum effective action for left-handed fermions contains excitations similar to abelian instantons produced by composite of left-handed fermions. Right-handed fermions don't have a similar effective action. Thus, quantum mechanically, chiral symmetry must be broken as a result of the topological excitations. The conserved charge associated to the topological states is quantized. Different cases with only fermionic excitations or bosonic excitations or both can occur depending on the boundary conditions and the value of the coupling.

Polar Kerr effect studies of time reversal symmetry breaking states in heavy fermion superconductors

DOE PAGES

Schemm, E. R.; Levenson-Falk, E. M.; Kapitulnik, A.

2016-11-30

The connection between chiral superconductivity and topological order has emerged as an active direction in research as more instances of both have been identified in condensed matter systems. Moreover, with the notable exception of 3He-B, all of the known or suspected chiral – that is to say time-reversal symmetry-breaking (TRSB) – superfluids arise in heavy fermion superconductors, although the vast majority of heavy fermion superconductors preserve time-reversal symmetry. We review recent experimental efforts to identify TRSB states in heavy fermion systems via measurement of polar Kerr effect, which is a direct consequence of TRSB.

Theoretical princi les of constructing the equations of motion for a spin color-charged particle in gauge and fermion fields

NASA Astrophysics Data System (ADS)

Markov, Yu. A.; Shishmarev, A. A.

2010-11-01

Based on the most general principles of materiality, gauge, and re-parameterized invariance, the problem of constructing an action describing the dynamics of a classical color-charged particle moving in external non-Abelian gauge and fermion fields is considered. The case of a linear Lagrangian dependence on the external fermion fields is discussed. Within the framework of the description of the color degree of freedom of the particle with half-integer spin by the Grassmann color charges, a new concept of the Grassmann color source of the particle being a fermion analog of the conventional color current is introduced.

A beam splitter for Dirac-Weyl fermions through the Goos-Hänchen-like shift

NASA Astrophysics Data System (ADS)

Zheng, Ren-fei; Zhou, Lu; Zhang, Weiping

2017-12-01

We propose a method of realizing an effective beam splitter for Dirac-Weyl fermions through the Goos-Hänchen-like shift. It is implemented via the birefringence of a wave packet of pseudospin-3/2 Dirac-Weyl fermions impinging upon a potential barrier. It is shown that experimentally observable spatial separation between the transmitted fermions with helicity-1/2 and 3/2 can be generated by the Goos-Hänchen-like shift. The dependence of Goos-Hänchen-like shift and the corresponding transmission probability on the incident angle, the height and width of the potential barrier are carefully studied.

The Rational Hybrid Monte Carlo algorithm

NASA Astrophysics Data System (ADS)

Clark, Michael

2006-12-01

The past few years have seen considerable progress in algorithmic development for the generation of gauge fields including the effects of dynamical fermions. The Rational Hybrid Monte Carlo (RHMC) algorithm, where Hybrid Monte Carlo is performed using a rational approximation in place the usual inverse quark matrix kernel is one of these developments. This algorithm has been found to be extremely beneficial in many areas of lattice QCD (chiral fermions, finite temperature, Wilson fermions etc.). We review the algorithm and some of these benefits, and we compare against other recent algorithm developements. We conclude with an update of the Berlin wall plot comparing costs of all popular fermion formulations.

Ladder physics in the spin fermion model

NASA Astrophysics Data System (ADS)

Tsvelik, A. M.

2017-05-01

A link is established between the spin fermion (SF) model of the cuprates and the approach based on the analogy between the physics of doped Mott insulators in two dimensions and the physics of fermionic ladders. This enables one to use nonperturbative results derived for fermionic ladders to move beyond the large-N approximation in the SF model. It is shown that the paramagnon exchange postulated in the SF model has exactly the right form to facilitate the emergence of the fully gapped d -Mott state in the region of the Brillouin zone at the hot spots of the Fermi surface. Hence, the SF model provides an adequate description of the pseudogap.

Stability of the two-dimensional Fermi polaron

NASA Astrophysics Data System (ADS)

Griesemer, Marcel; Linden, Ulrich

2018-02-01

A system composed of an ideal gas of N fermions interacting with an impurity particle in two space dimensions is considered. The interaction between impurity and fermions is given in terms of two-body point interactions whose strength is determined by the two-body binding energy, which is a free parameter of the model. If the mass of the impurity is 1.225 times larger than the mass of a fermion, it is shown that the energy is bounded below uniformly in the number N of fermions. This result improves previous, N-dependent lower bounds, and it complements a recent, similar bound for the Fermi polaron in three space dimensions.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kodama, Yuta; Kokubu, Kento; Sawado, Nobuyuki

We construct brane solutions in 6-dimensional Einstein-Skyrme systems. A class of baby-Skyrmion solutions realizes warped compactification of the extra dimensions and gravity localization on the brane for the negative bulk cosmological constant. Coupling of the fermions with brane Skyrmions leads to brane localized fermions. In terms of the level crossing picture, emergence of the massive localized modes are observed. The nonlinear nature of Skyrmions brings richer information for the fermions' level structure. It comprises doubly degenerate lowest plus single excited modes. Three generations of fundamental fermions are associated with this distinctive structure. The mass hierarchy of quarks or leptons appearedmore » in terms of slightly deformed baby Skyrmions with topological charge three.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chinotti, M.; Pal, A.; Ren, W. J.

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. These quasi-two-dimensional bismuth layers based materials were recently designed and provide an arena for studying the interplay between anisotropic Dirac fermions, magnetism, and structural changes, allowing the formation of Weyl fermions in condensed matter. We perform an optical investigation of YbMnBi 2 , a representative type-II Weyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal EuMnBi 2 . This comparative study allows us to disentangle the optical fingerprints of type-II Weyl fermions, but also challengesmore » the present theoretical understanding of their electrodynamic response.« less

Fermion localization and resonances on a de Sitter thick brane

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu Yuxiao; Yang Jie; Zhao Zhenhua

2009-09-15

In C. A. S. Almeida, R. Casana, M. M. Ferreira, Jr., and A. R. Gomes, Phys. Rev. D 79, 125022 (2009), the simplest Yukawa coupling {eta}{psi}{phi}{chi}{psi} was considered for a two-scalar-generated Bloch brane model. Fermionic resonances for both chiralities were obtained, and their appearance is related to branes with internal structure. Inspired on this result, we investigate the localization and resonance spectrum of fermions on a one-scalar-generated de Sitter thick brane with a class of scalar-fermion couplings {eta}{psi}{phi}{sup k}{psi} with positive odd integer k. A set of massive fermionic resonances for both chiralities is obtained when provided large coupling constantmore » {eta}. We find that the masses and lifetimes of left and right chiral resonances are almost the same, which demonstrates that it is possible to compose massive Dirac fermions from the left and right chiral resonances. The resonance with lower mass has longer lifetime. For a same set of parameters, the number of resonances increases with k and the lifetime of the lower level resonance for larger k is much longer than the one for smaller k.« less

Superfluidity of identical fermions in an optical lattice: Atoms and polar molecules

NASA Astrophysics Data System (ADS)

Fedorov, A. K.; Yudson, V. I.; Shlyapnikov, G. V.

2018-02-01

In this work we discuss the emergence of p-wave superfluids of identical fermions in 2D lattices. The optical lattice potential manifests itself in an interplay between an increase in the density of states on the Fermi surface and the modification of the fermion-fermion interaction (scattering) amplitude. The density of states is enhanced due to an increase of the effective mass of atoms. In deep lattices, for short-range interacting atoms the scattering amplitude is strongly reduced compared to free space due to a small overlap of wavefunctions of fermions sitting in the neighboring lattice sites, which suppresses the p-wave superfluidity. However, we show that for a moderate lattice depth there is still a possibility to create atomic p-wave superfluids with sizable transition temperatures. The situation is drastically different for fermionic polar molecules. Being dressed with a microwave field, they acquire a dipole-dipole attractive tail in the interaction potential. Then, due to a long-range character of the dipole-dipole interaction, the effect of the suppression of the scattering amplitude in 2D lattices is absent. This leads to the emergence of a stable topological px + ipy superfluid of identical microwave-dressed polar molecules.

Parallelized traveling cluster approximation to study numerically spin-fermion models on large lattices

DOE PAGES

Mukherjee, Anamitra; Patel, Niravkumar D.; Bishop, Chris; ...

2015-06-08

Lattice spin-fermion models are quite important to study correlated systems where quantum dynamics allows for a separation between slow and fast degrees of freedom. The fast degrees of freedom are treated quantum mechanically while the slow variables, generically referred to as the “spins,” are treated classically. At present, exact diagonalization coupled with classical Monte Carlo (ED + MC) is extensively used to solve numerically a general class of lattice spin-fermion problems. In this common setup, the classical variables (spins) are treated via the standard MC method while the fermion problem is solved by exact diagonalization. The “traveling cluster approximation” (TCA)more » is a real space variant of the ED + MC method that allows to solve spin-fermion problems on lattice sizes with up to 10 3 sites. In this paper, we present a novel reorganization of the TCA algorithm in a manner that can be efficiently parallelized. Finally, this allows us to solve generic spin-fermion models easily on 10 4 lattice sites and with some effort on 10 5 lattice sites, representing the record lattice sizes studied for this family of models.« less

Nontrivial Berry phase in magnetic BaMnSb2 semimetal

PubMed Central

Huang, Silu; Shelton, W. A.; Plummer, E. W.; Jin, Rongying

2017-01-01

The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov–de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material. PMID:28539436

Parallelized traveling cluster approximation to study numerically spin-fermion models on large lattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mukherjee, Anamitra; Patel, Niravkumar D.; Bishop, Chris

Lattice spin-fermion models are quite important to study correlated systems where quantum dynamics allows for a separation between slow and fast degrees of freedom. The fast degrees of freedom are treated quantum mechanically while the slow variables, generically referred to as the “spins,” are treated classically. At present, exact diagonalization coupled with classical Monte Carlo (ED + MC) is extensively used to solve numerically a general class of lattice spin-fermion problems. In this common setup, the classical variables (spins) are treated via the standard MC method while the fermion problem is solved by exact diagonalization. The “traveling cluster approximation” (TCA)more » is a real space variant of the ED + MC method that allows to solve spin-fermion problems on lattice sizes with up to 10 3 sites. In this paper, we present a novel reorganization of the TCA algorithm in a manner that can be efficiently parallelized. Finally, this allows us to solve generic spin-fermion models easily on 10 4 lattice sites and with some effort on 10 5 lattice sites, representing the record lattice sizes studied for this family of models.« less

Color superfluidity of neutral ultracold fermions in the presence of color-flip and color-orbit fields

NASA Astrophysics Data System (ADS)

Kurkcuoglu, Doga Murat; Sá de Melo, C. A. R.

2018-02-01

We describe how color superfluidity is modified in the presence of color-flip and color-orbit fields in the context of ultracold atoms and discuss connections between this problem and that of color superconductivity in quantum chromodynamics. We study the case of s -wave contact interactions between different colors and we identify several superfluid phases, with five being nodal and one being fully gapped. When our system is described in a mixed-color basis, the superfluid order parameter tensor is characterized by six independent components with explicit momentum dependence induced by color-orbit coupling. The nodal superfluid phases are topological in nature and the low-temperature phase diagram of the color-flip field versus the interaction parameter exhibits a pentacritical point, where all five nodal color superfluid phases converge. These results are in sharp contrast to the case of zero color-flip and color-orbit fields, where the system has perfect U(3) symmetry and possesses a superfluid phase that is characterized by fully gapped quasiparticle excitations with a single complex order parameter with no momentum dependence and by inert unpaired fermions representing a nonsuperfluid component. In the latter case, just a crossover between a Bardeen-Cooper-Schrieffer and a Bose-Einstein-condensation superfluid occurs. Furthermore, we analyze the order parameter tensor in a total pseudospin basis, investigate its momentum dependence in the singlet, triplet, and quintet sectors, and compare the results with the simpler case of spin-1/2 fermions in the presence of spin-flip and spin-orbit fields, where only singlet and triplet channels arise. Finally, we analyze in detail spectroscopic properties of color superfluids in the presence of color-flip and color-orbit fields, such as the quasiparticle excitation spectrum, momentum distribution, and density of states to help characterize all the encountered topological quantum phases, which can be realized in fermionic isotopes of lithium, potassium, and ytterbium atoms with three internal states trapped.

NNLO corrections to top-pair production at hadron colliders: the all-fermionic scattering channels

NASA Astrophysics Data System (ADS)

Czakon, Michal; Mitov, Alexander

2012-12-01

This is a second paper in our ongoing calculation of the next-to-next-to-leading order (NNLO) QCD correction to the total inclusive top-pair production cross-section at hadron colliders. In this paper we calculate the reaction qoverline{q}to toverline{t}+qoverline{q} which was not considered in our previous work on qoverline{q}to toverline{t}+X [1] due to its phenomenologically negligible size. We also calculate all remaining fermion-pair-initiated partonic channels q{q^' }} , q{{overline{q}}^' }} and qq that contribute to top-pair production starting from NNLO. The contributions of these reactions to the total cross-section for top-pair production at the Tevatron and LHC are small, at the permil level. The most interesting feature of these reactions is their characteristic logarithmic rise in the high energy limit. We compute the constant term in the leading power behavior in this limit, and achieve precision that is an order of magnitude better than the precision of a recent theoretical prediction for this constant. All four partonic reactions computed in this paper are included in our numerical program Top++. The calculation of the NNLO corrections to the two remaining partonic reactions, qgto toverline{t}+X and ggto toverline{t}+X , is ongoing.

On the simulation of indistinguishable fermions in the many-body Wigner formalism

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sellier, J.M., E-mail: jeanmichel.sellier@gmail.com; Dimov, I.

2015-01-01

The simulation of quantum systems consisting of interacting, indistinguishable fermions is an incredible mathematical problem which poses formidable numerical challenges. Many sophisticated methods addressing this problem are available which are based on the many-body Schrödinger formalism. Recently a Monte Carlo technique for the resolution of the many-body Wigner equation has been introduced and successfully applied to the simulation of distinguishable, spinless particles. This numerical approach presents several advantages over other methods. Indeed, it is based on an intuitive formalism in which quantum systems are described in terms of a quasi-distribution function, and highly scalable due to its Monte Carlo nature.more » In this work, we extend the many-body Wigner Monte Carlo method to the simulation of indistinguishable fermions. To this end, we first show how fermions are incorporated into the Wigner formalism. Then we demonstrate that the Pauli exclusion principle is intrinsic to the formalism. As a matter of fact, a numerical simulation of two strongly interacting fermions (electrons) is performed which clearly shows the appearance of a Fermi (or exchange–correlation) hole in the phase-space, a clear signature of the presence of the Pauli principle. To conclude, we simulate 4, 8 and 16 non-interacting fermions, isolated in a closed box, and show that, as the number of fermions increases, we gradually recover the Fermi–Dirac statistics, a clear proof of the reliability of our proposed method for the treatment of indistinguishable particles.« less

Universal Behavior of Quantum Spin Liquid and Optical Conductivity in the Insulator Herbertsmithite

NASA Astrophysics Data System (ADS)

Shaginyan, V. R.; Msezane, A. Z.; Stephanovich, V. A.; Popov, K. G.; Japaridze, G. S.

2018-04-01

We analyze optical conductivity with the goal to demonstrate experimental manifestation of a new state of matter, the so-called fermion condensate. Fermion condensates are realized in quantum spin liquids, exhibiting typical behavior of heavy-fermion metals. Measurements of the low-frequency optical conductivity collected on the geometrically frustrated insulator herbertsmithite provide important experimental evidence of the nature of its quantum spin liquid composed of spinons. To analyze recent measurements of the herbertsmithite optical conductivity at different temperatures, we employ a model of strongly correlated quantum spin liquid located near the fermion condensation phase transition. Our theoretical analysis of the optical conductivity allows us to expose the physical mechanism of its temperature dependence. We also predict a dependence of the optical conductivity on a magnetic field. We consider an experimental manifestation (optical conductivity) of a new state of matter (so-called fermion condensate) realized in quantum spin liquids, for, in many ways, they exhibit typical behavior of heavy-fermion metals. Measurements of the low-frequency optical conductivity collected on the geometrically frustrated insulator herbertsmithite produce important experimental evidence of the nature of its quantum spin liquid composed of spinons. To analyze recent measurements of the herbertsmithite optical conductivity at different temperatures, we employ a model of a strongly correlated quantum spin liquid located near the fermion condensation phase transition. Our theoretical analysis of the optical conductivity allows us to reveal the physical mechanism of its temperature dependence. We also predict a dependence of the optical conductivity on a magnetic field.

Competing forces in five-dimensional fermion condensation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoon, Jongmin; Peskin, Michael E.

We study fermion condensation in the Randall-Sundrum background as a setting for composite Higgs models. We formalize the computation of the Coleman-Weinberg potential and present a simple, general formula. Using this tool, we study the competition of fermion multiplets with different boundary conditions, to find conditions for creating a little hierarchy with the Higgs field expectation value much smaller than the intrinsic Randall-Sundrum mass scale.

Berry Phase in Lattice QCD.

PubMed

Yamamoto, Arata

2016-07-29

We propose the lattice QCD calculation of the Berry phase, which is defined by the ground state of a single fermion. We perform the ground-state projection of a single-fermion propagator, construct the Berry link variable on a momentum-space lattice, and calculate the Berry phase. As the first application, the first Chern number of the (2+1)-dimensional Wilson fermion is calculated by the Monte Carlo simulation.

Lattice fermions

NASA Technical Reports Server (NTRS)

Wilczek, Frank

1987-01-01

A simple heuristic proof of the Nielsen-Ninomaya theorem is given. A method is proposed whereby the multiplication of fermion species on a lattice is reduced to the minimal doubling, in any dimension, with retention of appropriate chiral symmetries. Also, it is suggested that use of spatially thinned fermion fields is likely to be a useful and appropriate approximation in QCD - in any case, it is a self-checking one.

Competing forces in five-dimensional fermion condensation

DOE PAGES

Yoon, Jongmin; Peskin, Michael E.

2017-12-27

We study fermion condensation in the Randall-Sundrum background as a setting for composite Higgs models. We formalize the computation of the Coleman-Weinberg potential and present a simple, general formula. Using this tool, we study the competition of fermion multiplets with different boundary conditions, to find conditions for creating a little hierarchy with the Higgs field expectation value much smaller than the intrinsic Randall-Sundrum mass scale.

Competing forces in five-dimensional fermion condensation

NASA Astrophysics Data System (ADS)

Yoon, Jongmin; Peskin, Michael E.

2017-12-01

We study fermion condensation in the Randall-Sundrum background as a setting for composite Higgs models. We formalize the computation of the Coleman-Weinberg potential and present a simple, general formula. Using this tool, we study the competition of fermion multiplets with different boundary conditions, to find conditions for creating a little hierarchy with the Higgs field expectation value much smaller than the intrinsic Randall-Sundrum mass scale.

48 CFR 16.403 - Fixed-price incentive contracts.

Code of Federal Regulations, 2010 CFR

2010-10-01

... relationship of total final negotiated cost to total target cost. The final price is subject to a price ceiling, negotiated at the outset. The two forms of fixed-price incentive contracts, firm target and successive targets, are further described in 16.403-1 and 16.403-2 below. (b) Application. A fixed-price incentive...

48 CFR 16.403 - Fixed-price incentive contracts.

Code of Federal Regulations, 2011 CFR

2011-10-01

... relationship of total final negotiated cost to total target cost. The final price is subject to a price ceiling, negotiated at the outset. The two forms of fixed-price incentive contracts, firm target and successive targets, are further described in 16.403-1 and 16.403-2 below. (b) Application. A fixed-price incentive...

Ladder physics in the spin fermion model

DOE PAGES

Tsvelik, A. M.

2017-05-01

A link is established between the spin fermion (SF) model of the cuprates and the approach based on the analogy between the physics of doped Mott insulators in two dimensions and the physics of fermionic ladders. This enables one to use nonperturbative results derived for fermionic ladders to move beyond the large-N approximation in the SF model. Here, it is shown that the paramagnon exchange postulated in the SF model has exactly the right form to facilitate the emergence of the fully gapped d-Mott state in the region of the Brillouin zone at the hot spots of the Fermi surface.more » Hence, the SF model provides an adequate description of the pseudogap.« less

Weak antilocalization of composite fermions in graphene

NASA Astrophysics Data System (ADS)

Laitinen, Antti; Kumar, Manohar; Hakonen, Pertti J.

2018-02-01

We demonstrate experimentally that composite fermions in monolayer graphene display weak antilocalization. Our experiments deal with fractional quantum Hall (FQH) states in high-mobility, suspended graphene Corbino disks in the vicinity of ν =1 /2 . We find a strong temperature dependence of conductivity σ away from half filling, which is consistent with the expected electron-electron interaction-induced gaps in the FQH state. At half filling, however, the temperature dependence of conductivity σ (T ) becomes quite weak, as anticipated for a Fermi sea of composite fermions, and we find a logarithmic dependence of σ on T . The sign of this quantum correction coincides with the weak antilocalization of graphene composite fermions, indigenous to chiral Dirac particles.

Numerical studies of a model fermion-boson system

NASA Astrophysics Data System (ADS)

Cheng, T.; Gospodarczyk, E. R.; Su, Q.; Grobe, R.

2010-02-01

We study the spectral and dynamical properties of a simplified model system of interacting fermions and bosons. The spatial discretization and an effective truncation of the Hilbert space permit us to compute the distribution of the bare fermions and bosons in the energy eigenstates of the coupled system. These states represent the physical particles and are used to examine the validity of the analytical predictions by perturbation theory and by the Greenberg-Schweber approximation that assumes all fermions are at rest. As an example of our numerical framework, we examine how a bare electron can trigger the creation of a cloud of virtual bosons around. We relate this cloud to the properties of the associated energy eigenstates.

Yang-Mills matrix mechanics and quantum phases

NASA Astrophysics Data System (ADS)

Pandey, Mahul; Vaidya, Sachindeo

The SU(2) Yang-Mills matrix model coupled to fundamental fermions is studied in the adiabatic limit, and quantum critical behavior is seen at special corners of the gauge field configuration space. The quantum scalar potential for the gauge field induced by the fermions diverges at the corners, and is intimately related to points of enhanced degeneracy of the fermionic Hamiltonian. This in turn leads to superselection sectors in the Hilbert space of the gauge field, the ground states in different sectors being orthogonal to each other. The SU(2) Yang-Mills matrix model coupled to two Weyl fermions has three quantum phases. When coupled to a massless Dirac fermion, the number of quantum phases is four. One of these phases is the color-spin locked phase. This paper is an extended version of the lectures given by the second author (SV) at the International Workshop on Quantum Physics: Foundations and Applications, Bangalore, in February 2016, and is based on [1].

Effect of boson on-site repulsion on the superfluidity in the boson-fermion-Hubbard model

NASA Astrophysics Data System (ADS)

Sajna, A. S.; Micnas, R.

2018-03-01

We analyze the finite-temperature phase diagram of the boson-fermion-Hubbard model with Feshbach converting interaction, using the coherent-state path-integral method. We show that depending on the position of the bosonic band, this type of interaction, even if weak, can drive the system into the resonant superfluid phase in the strong bosonic interaction limit. It turns out that this phase can exist for an arbitrary number of fermions (i.e., fermionic concentration between 0 and 2), but with the bosonic particle number very close to an integer value. We point out that the standard time-of-flight method in optical lattice experiments can be an adequate technique to confirm the existence of this resonant phase. Moreover, in the nonresonant regime, the enhancement of the critical temperature of the superfluid phase due to Feshbach interaction is also observed. We account for this interesting phenomena for a hole- or particlelike pairing mechanism depending on the system density and mutual location of the fermionic and bosonic bands.

Running coupling from gluon and ghost propagators in the Landau gauge: Yang-Mills theories with adjoint fermions

NASA Astrophysics Data System (ADS)

Bergner, Georg; Piemonte, Stefano

2018-04-01

Non-Abelian gauge theories with fermions transforming in the adjoint representation of the gauge group (AdjQCD) are a fundamental ingredient of many models that describe the physics beyond the Standard Model. Two relevant examples are N =1 supersymmetric Yang-Mills (SYM) theory and minimal walking technicolor, which are gauge theories coupled to one adjoint Majorana and two adjoint Dirac fermions, respectively. While confinement is a property of N =1 SYM, minimal walking technicolor is expected to be infrared conformal. We study the propagators of ghost and gluon fields in the Landau gauge to compute the running coupling in the MiniMom scheme. We analyze several different ensembles of lattice Monte Carlo simulations for the SU(2) adjoint QCD with Nf=1 /2 ,1 ,3 /2 , and 2 Dirac fermions. We show how the running of the coupling changes as the number of interacting fermions is increased towards the conformal window.

Electronic structure of heavy fermion system CePt 2In 7 from angle-resolved photoemission spectroscopy

DOE PAGES

Shen, Bing; Yu, Li; Liu, Kai; ...

2017-06-01

We have carried out high-resolution angle-resolved photoemission measurements on the Cebased heavy fermion compound CePt 2In 7 that exhibits stronger two-dimensional character than the prototypical heavy fermion system CeCoIn 5. Multiple Fermi surface sheets and a complex band structure are clearly resolved. We have also performed detailed band structure calculations on CePt 2In 7. The good agreement found between our measurements and the calculations suggests that the band renormalization effect is rather weak in CePt 2In 7. A comparison of the common features of the electronic structure of CePt 2In 7 and CeCoIn5 indicates that CeCoIn 5 shows a muchmore » stronger band renormalization effect than CePt 2In 7. These results provide new information for understanding the heavy fermion behaviors and unconventional superconductivity in Ce-based heavy fermion systems.« less

Hybrid Monte Carlo approach to the entanglement entropy of interacting fermions

NASA Astrophysics Data System (ADS)

Drut, Joaquín E.; Porter, William J.

2015-09-01

The Monte Carlo calculation of Rényi entanglement entropies Sn of interacting fermions suffers from a well-known signal-to-noise problem, even for a large number of situations in which the infamous sign problem is absent. A few methods have been proposed to overcome this issue, such as ensemble switching and the use of auxiliary partition-function ratios. Here, we present an approach that builds on the recently proposed free-fermion decomposition method; it incorporates entanglement in the probability measure in a natural way; it takes advantage of the hybrid Monte Carlo algorithm (an essential tool in lattice quantum chromodynamics and other gauge theories with dynamical fermions); and it does not suffer from noise problems. This method displays no sign problem for the same cases as other approaches and is therefore useful for a wide variety of systems. As a proof of principle, we calculate S2 for the one-dimensional, half-filled Hubbard model and compare with results from exact diagonalization and the free-fermion decomposition method.

Multipartite entanglement in fermionic systems via a geometric measure

NASA Astrophysics Data System (ADS)

Lari, Behzad; Durganandini, P.; Joag, Pramod S.

2010-12-01

We study multipartite entanglement in a system consisting of indistinguishable fermions. Specifically, we have proposed a geometric entanglement measure for N spin-(1)/(2) fermions distributed over 2L modes (single-particle states). The measure is defined on the 2L qubit space isomorphic to the Fock space for 2L single-particle states. This entanglement measure is defined for a given partition of 2L modes containing m⩾2 subsets. Thus this measure applies to m⩽2L partite fermionic systems where L is any finite number, giving the number of sites. The Hilbert spaces associated with these subsets may have different dimensions. Further, we have defined the local quantum operations with respect to a given partition of modes. This definition is generic and unifies different ways of dividing a fermionic system into subsystems. We have shown, using a representative case, that the geometric measure is invariant under local unitary operators corresponding to a given partition. We explicitly demonstrate the use of the measure to calculate multipartite entanglement in some correlated electron systems.

Entanglement and the fermion sign problem in auxiliary field quantum Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Broecker, Peter; Trebst, Simon

2016-08-01

Quantum Monte Carlo simulations of fermions are hampered by the notorious sign problem whose most striking manifestation is an exponential growth of sampling errors with the number of particles. With the sign problem known to be an NP-hard problem and any generic solution thus highly elusive, the Monte Carlo sampling of interacting many-fermion systems is commonly thought to be restricted to a small class of model systems for which a sign-free basis has been identified. Here we demonstrate that entanglement measures, in particular the so-called Rényi entropies, can intrinsically exhibit a certain robustness against the sign problem in auxiliary-field quantum Monte Carlo approaches and possibly allow for the identification of global ground-state properties via their scaling behavior even in the presence of a strong sign problem. We corroborate these findings via numerical simulations of fermionic quantum phase transitions of spinless fermions on the honeycomb lattice at and below half filling.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ortiz, Gerardo, E-mail: ortizg@indiana.edu; Cobanera, Emilio

We investigate Majorana modes of number-conserving fermionic superfluids from both basic physics principles, and concrete models perspectives. After reviewing a criterion for establishing topological superfluidity in interacting systems, based on many-body fermionic parity switches, we reveal the emergence of zero-energy modes anticommuting with fermionic parity. Those many-body Majorana modes are constructed as coherent superpositions of states with different number of fermions. While realization of Majorana modes beyond mean field is plausible, we show that the challenge to quantum-control them is compounded by particle-conservation, and more realistic protocols will have to balance engineering needs with astringent constraints coming from superselection rules.more » Majorana modes in number-conserving systems are the result of a peculiar interplay between quantum statistics, fermionic parity, and an unusual form of spontaneous symmetry breaking. We test these ideas on the Richardson–Gaudin–Kitaev chain, a number-conserving model solvable by way of the algebraic Bethe ansatz, and equivalent in mean field to a long-range Kitaev chain.« less

The linear -- non-linear frontier for the Goldstone Higgs

DOE PAGES

Gavela, M. B.; Kanshin, K.; Machado, P. A. N.; ...

2016-12-01

The minimalmore » $SO(5)/SO(4)$ sigma model is used as a template for the ultraviolet completion of scenarios in which the Higgs particle is a low-energy remnant of some high-energy dynamics, enjoying a (pseudo) Nambu-Goldstone boson ancestry. Varying the $$\\sigma$$ mass allows to sweep from the perturbative regime to the customary non-linear implementations. The low-energy benchmark effective non-linear Lagrangian for bosons and fermions is obtained, determining as well the operator coefficients including linear corrections. At first order in the latter, three effective bosonic operators emerge which are independent of the explicit soft breaking assumed. The Higgs couplings to vector bosons and fermions turn out to be quite universal: the linear corrections are proportional to the explicit symmetry breaking parameters. Furthermore, we define an effective Yukawa operator which allows a simple parametrization and comparison of different heavy fermion ultraviolet completions. In addition, one particular fermionic completion is explored in detail, obtaining the corresponding leading low-energy fermionic operators.« less

Spectral properties of four-time fermionic Green's functions

DOE PAGES

Shvaika, A. M.

2016-09-01

The spectral relations for the four-time fermionic Green's functions are derived in the most general case. The terms which correspond to the zero-frequency anomalies, known before only for the bosonic Green's functions, are separated and their connection with the second cumulants of the Boltzmann distribution function is elucidated. Furthermore, the high-frequency expansions of the four-time fermionic Green's functions are provided for different directions in the frequency space.

Spectral properties of four-time fermionic Green's functions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shvaika, A. M.

The spectral relations for the four-time fermionic Green's functions are derived in the most general case. The terms which correspond to the zero-frequency anomalies, known before only for the bosonic Green's functions, are separated and their connection with the second cumulants of the Boltzmann distribution function is elucidated. Furthermore, the high-frequency expansions of the four-time fermionic Green's functions are provided for different directions in the frequency space.

Local moment relaxation in heavy-fermion compounds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simanek, E.; Sasahara, K.

1987-02-01

The Korringa relaxation rate for a local moment of an impurity in a heavy fermion compound is calculated using the model of Yoshimori and Kasai. Consistent with the recent ESR data for local moments in UBe/sub 13/, the relaxation rate is found to be unaffected by the heavy fermion renormalizations. This result can be traced to the single-site approximation and the weak k dependence of the conduction electron self-energy.

How Kondo-holes create intense nanoscale heavy-fermion hybridization disorder

PubMed Central

Hamidian, Mohammad H.; Schmidt, Andrew R.; Firmo, Inês A.; Allan, Milan P.; Bradley, Phelim; Garrett, Jim D.; Williams, Travis J.; Luke, Graeme M.; Dubi, Yonatan; Balatsky, Alexander V.; Davis, J. C.

2011-01-01

Replacing a magnetic atom by a spinless atom in a heavy-fermion compound generates a quantum state often referred to as a “Kondo-hole”. No experimental imaging has been achieved of the atomic-scale electronic structure of a Kondo-hole, or of their destructive impact [Lawrence JM, et al. (1996) Phys Rev B 53:12559–12562] [Bauer ED, et al. (2011) Proc Natl Acad Sci. 108:6857–6861] on the hybridization process between conduction and localized electrons which generates the heavy-fermion state. Here we report visualization of the electronic structure at Kondo-holes created by substituting spinless thorium atoms for magnetic uranium atoms in the heavy-fermion system URu2Si2. At each thorium atom, an electronic bound state is observed. Moreover, surrounding each thorium atom we find the unusual modulations of hybridization strength recently predicted to occur at Kondo-holes [Figgins J, Morr DK (2011) Phys Rev Lett 107:066401]. Then, by introducing the “hybridization gapmap” technique to heavy-fermion studies, we discover intense nanoscale heterogeneity of hybridization due to a combination of the randomness of Kondo-hole sites and the long-range nature of the hybridization oscillations. These observations provide direct insight into both the microscopic processes of heavy-fermion forming hybridization and the macroscopic effects of Kondo-hole doping. PMID:22006302

Massless Dirac fermions trapping in a quasi-one-dimensional n p n junction of a continuous graphene monolayer

NASA Astrophysics Data System (ADS)

Bai, Ke-Ke; Qiao, Jia-Bin; Jiang, Hua; Liu, Haiwen; He, Lin

2017-05-01

Massless Dirac fermions in graphene provide unprecedented opportunities to realize the Klein paradox, which is one of the most exotic and striking properties of relativistic particles. In the seminal theoretical work [M. I. Katsnelson et al., Nat. Phys. 2, 620 (2006), 10.1038/nphys384], it was predicted that the massless Dirac fermions can pass through one-dimensional (1D) potential barriers unimpededly at normal incidence. Such a result seems to preclude confinement of the massless Dirac fermions in graphene by using 1D potential barriers. Here, we demonstrate both experimentally and theoretically that massless Dirac fermions can be trapped in a quasi-1D n p n junction of a continuous graphene monolayer. Because of highly anisotropic transmission of the massless Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in graphene), charge carriers incident at large oblique angles will be reflected from one edge of the junction with high probability and continue to bounce from the opposite edge. Consequently, these electrons are trapped for a finite time to form quasibound states in the quasi-1D n p n junction. The quasibound states seen as pronounced resonances are probed and the quantum interference patterns arising from these states are directly visualized in our scanning tunneling microscope measurements.

Determination of Fermi contour and spin polarization of ν = 3 2 composite fermions via ballistic commensurability measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kamburov, D.; Mueed, M. A.; Jo, I.

2014-12-01

We report ballistic transport commensurability minima in the magnetoresistance of nu = 3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to nu = 3/2, which we explain quantitatively by comparing three CF density models and concluding that the nu = 3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe themore » shape and size of the CF Fermi contour. At a fixed electron density of similar or equal to 1.8x10(11) cm(-2), as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B-parallel to) leads to a significant distortion of the CF Fermi contour as B-parallel to couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the nu = 1/2 CF case at comparable B-parallel to. Moreover, the applied B-parallel to further spin-polarizes the nu = 3/2 CFs as deduced from the positions of the commensurability minima.« less

The Fermionic Signature Operator and Hadamard States in the Presence of a Plane Electromagnetic Wave

NASA Astrophysics Data System (ADS)

Finster, Felix; Reintjes, Moritz

2017-05-01

We give a non-perturbative construction of a distinguished state for the quantized Dirac field in Minkowski space in the presence of a time-dependent external field of the form of a plane electromagnetic wave. By explicit computation of the fermionic signature operator, it is shown that the Dirac operator has the strong mass oscillation property. We prove that the resulting fermionic projector state is a Hadamard state.

Staggered fermions, zero modes, and flavor-singlet mesons

DOE PAGES

Donald, Gordon C; Davies, Christine T.H.; Follana, Eduardo; ...

2011-09-12

We examine the taste structure of eigenvectors of the staggered-fermion Dirac operator. We derive a set of conditions on the eigenvectors of modes with small eigenvalues (near-zero modes), such that staggered fermions reproduce the 't Hooft vertex in the continuum limit. We also show that, assuming these conditions, the correlators of flavor-singlet mesons are free of contributions singular in 1/m, where m is the quark mass. This conclusion holds also when a single flavor of sea quark is represented by the fourth root of the staggered-fermion determinant. We then test numerically, using the HISQ action, whether these conditions hold onmore » realistic lattice gauge fields. We find that the needed structure does indeed emerge.« less

Counting local integrals of motion in disordered spinless-fermion and Hubbard chains

NASA Astrophysics Data System (ADS)

Mierzejewski, Marcin; Kozarzewski, Maciej; Prelovšek, Peter

2018-02-01

We develop a procedure which systematically generates all conserved operators in the disordered models of interacting fermions. Among these operators, we identify and count the independent and local integrals of motion (LIOM), which represent the hallmark of the many-body localization (MBL). The method is tested first on the prototype disordered chain of interacting spinless fermions. As expected for full MBL, we find for large enough disorder NM=2M-1 independent and quasilocal LIOM with support on M consecutive sites. On the other hand, the study of the disordered Hubbard chain reveals that 3M-1

Split Orthogonal Group: A Guiding Principle for Sign-Problem-Free Fermionic Simulations

NASA Astrophysics Data System (ADS)

Wang, Lei; Liu, Ye-Hua; Iazzi, Mauro; Troyer, Matthias; Harcos, Gergely

2015-12-01

We present a guiding principle for designing fermionic Hamiltonians and quantum Monte Carlo (QMC) methods that are free from the infamous sign problem by exploiting the Lie groups and Lie algebras that appear naturally in the Monte Carlo weight of fermionic QMC simulations. Specifically, rigorous mathematical constraints on the determinants involving matrices that lie in the split orthogonal group provide a guideline for sign-free simulations of fermionic models on bipartite lattices. This guiding principle not only unifies the recent solutions of the sign problem based on the continuous-time quantum Monte Carlo methods and the Majorana representation, but also suggests new efficient algorithms to simulate physical systems that were previously prohibitive because of the sign problem.

Particlelike solutions of the Einstein-Dirac equations

NASA Astrophysics Data System (ADS)

Finster, Felix; Smoller, Joel; Yau, Shing-Tung

1999-05-01

The coupled Einstein-Dirac equations for a static, spherically symmetric system of two fermions in a singlet spinor state are derived. Using numerical methods, we construct an infinite number of solitonlike solutions of these equations. The stability of the solutions is analyzed. For weak coupling (i.e., small rest mass of the fermions), all the solutions are linearly stable (with respect to spherically symmetric perturbations), whereas for stronger coupling, both stable and unstable solutions exist. For the physical interpretation, we discuss how the energy of the fermions and the (ADM) mass behave as functions of the rest mass of the fermions. Although gravitation is not renormalizable, our solutions of the Einstein-Dirac equations are regular and well behaved even for strong coupling.

Magnetotransport in Layered Dirac Fermion System Coupled with Magnetic Moments

NASA Astrophysics Data System (ADS)

Iwasaki, Yoshiki; Morinari, Takao

2018-03-01

We theoretically investigate the magnetotransport of Dirac fermions coupled with localized moments to understand the physical properties of the Dirac material EuMnBi2. Using an interlayer hopping form, which simplifies the complicated interaction between the layers of Dirac fermions and the layers of magnetic moments in EuMnBi2, the theory reproduces most of the features observed in this system. The hysteresis observed in EuMnBi2 can be caused by the valley splitting that is induced by the spin-orbit coupling and the external magnetic field with the molecular field created by localized moments. Our theory suggests that the magnetotransport in EuMnBi2 is due to the interplay among Dirac fermions, localized moments, and spin-orbit coupling.

Electrodynamic response of the type-II Weyl semimetal YbMnBi 2

DOE PAGES

Chinotti, M.; Pal, A.; Ren, W. J.; ...

2016-12-01

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. These quasi-two-dimensional bismuth layers based materials were recently designed and provide an arena for studying the interplay between anisotropic Dirac fermions, magnetism, and structural changes, allowing the formation of Weyl fermions in condensed matter. We perform an optical investigation of YbMnBi 2 , a representative type-II Weyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal EuMnBi 2 . This comparative study allows us to disentangle the optical fingerprints of type-II Weyl fermions, but also challengesmore » the present theoretical understanding of their electrodynamic response.« less

Millicharge or decay: a critical take on Minimal Dark Matter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nobile, Eugenio Del; Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova and INFN, Sezione di Padova,Via Marzolo 8, 35131 Padova; Nardecchia, Marco

2016-04-26

Minimal Dark Matter (MDM) is a theoretical framework highly appreciated for its minimality and yet its predictivity. Of the two only viable candidates singled out in the original analysis, the scalar eptaplet has been found to decay too quickly to be around today, while the fermionic quintuplet is now being probed by indirect Dark Matter (DM) searches. It is therefore timely to critically review the MDM paradigm, possibly pointing out generalizations of this framework. We propose and explore two distinct directions. One is to abandon the assumption of DM electric neutrality in favor of absolutely stable, millicharged DM candidates whichmore » are part of SU(2){sub L} multiplets with integer isospin. Another possibility is to lower the cutoff of the model, which was originally fixed at the Planck scale, to allow for DM decays. We find new viable MDM candidates and study their phenomenology in detail.« less

CLS 2+1 flavor simulations at physical light-and strange-quark masses

NASA Astrophysics Data System (ADS)

Mohler, Daniel; Schaefer, Stefan; Simeth, Jakob

2018-03-01

We report recent efforts by CLS to generate an ensemble with physical lightand strange-quark masses in a lattice volume of 192 × 963 at β = 3:55 corresponding to a lattice spacing of 0:064 fm. This ensemble is being generated as part of the CLS 2+1 flavor effort with improved Wilson fermions. Our simulations currently cover 5 lattice spacings ranging from 0:039 fm to 0:086 fm at various pion masses along chiral trajectories with either the sum of the quark masses kept fixed, or with the strange-quark mass at the physical value. The current status of simulations is briefly reviewed, including a short discussion of measured autocorrelation times and of the main features of the simulations. We then proceed to discuss the thermalization strategy employed for the generation of the physical quark-mass ensemble and present first results for some simple observables. Challenges encountered in the simulation are highlighted.

Entanglement Holographic Mapping of Many-Body Localized System by Spectrum Bifurcation Renormalization Group

NASA Astrophysics Data System (ADS)

You, Yi-Zhuang; Qi, Xiao-Liang; Xu, Cenke

We introduce the spectrum bifurcation renormalization group (SBRG) as a generalization of the real-space renormalization group for the many-body localized (MBL) system without truncating the Hilbert space. Starting from a disordered many-body Hamiltonian in the full MBL phase, the SBRG flows to the MBL fixed-point Hamiltonian, and generates the local conserved quantities and the matrix product state representations for all eigenstates. The method is applicable to both spin and fermion models with arbitrary interaction strength on any lattice in all dimensions, as long as the models are in the MBL phase. In particular, we focus on the 1 d interacting Majorana chain with strong disorder, and map out its phase diagram using the entanglement entropy. The SBRG flow also generates an entanglement holographic mapping, which duals the MBL state to a fragmented holographic space decorated with small blackholes.

Orthogonality catastrophe and fractional exclusion statistics

NASA Astrophysics Data System (ADS)

Ares, Filiberto; Gupta, Kumar S.; de Queiroz, Amilcar R.

2018-02-01

We show that the N -particle Sutherland model with inverse-square and harmonic interactions exhibits orthogonality catastrophe. For a fixed value of the harmonic coupling, the overlap of the N -body ground state wave functions with two different values of the inverse-square interaction term goes to zero in the thermodynamic limit. When the two values of the inverse-square coupling differ by an infinitesimal amount, the wave function overlap shows an exponential suppression. This is qualitatively different from the usual power law suppression observed in the Anderson's orthogonality catastrophe. We also obtain an analytic expression for the wave function overlaps for an arbitrary set of couplings, whose properties are analyzed numerically. The quasiparticles constituting the ground state wave functions of the Sutherland model are known to obey fractional exclusion statistics. Our analysis indicates that the orthogonality catastrophe may be valid in systems with more general kinds of statistics than just the fermionic type.

Electromagnetic corrections to the hadronic vacuum polarization of the photon within QEDL and QEDM

NASA Astrophysics Data System (ADS)

Bussone, Andrea; Della Morte, Michele; Janowski, Tadeusz

2018-03-01

We compute the leading QED corrections to the hadronic vacuum polarization (HVP) of the photon, relevant for the determination of leptonic anomalous magnetic moments, al. We work in the electroquenched approximation and use dynamical QCD configurations generated by the CLS initiative with two degenerate flavors of nonperturbatively O(a)-improved Wilson fermions. We consider QEDL and QEDM to deal with the finite-volume zero modes. We compare results for the Wilson loops with exact analytical determinations. In addition we make sure that the volumes and photon masses used in QEDM are such that the correct dispersion relation is reproduced by the energy levels extracted from the charged pions two-point functions. Finally we compare results for pion masses and the HVP between QEDL and QEDM. For the vacuum polarization, corrections with respect to the pure QCD case, at fixed pion masses, turn out to be at the percent level.

Millicharge or decay: a critical take on Minimal Dark Matter

DOE PAGES

Nobile, Eugenio Del; Nardecchia, Marco; Panci, Paolo

2016-04-26

Minimal Dark Matter (MDM) is a theoretical framework highly appreciated for its minimality and yet its predictivity. Of the two only viable candidates singled out in the original analysis, the scalar eptaplet has been found to decay too quickly to be around today, while the fermionic quintuplet is now being probed by indirect Dark Matter (DM) searches. It is therefore timely to critically review the MDM paradigm, possibly pointing out generalizations of this framework. We propose and explore two distinct directions. One is to abandon the assumption of DM electric neutrality in favor of absolutely stable, millicharged DM candidates whichmore » are part of SU(2)L multiplets with integer isospin. Another possibility is to lower the cutoff of the model, which was originally fixed at the Planck scale, to allow for DM decays. We find new viable MDM candidates and study their phenomenology in detail.« less

Orthogonality catastrophe and fractional exclusion statistics.

PubMed

Ares, Filiberto; Gupta, Kumar S; de Queiroz, Amilcar R

2018-02-01

We show that the N-particle Sutherland model with inverse-square and harmonic interactions exhibits orthogonality catastrophe. For a fixed value of the harmonic coupling, the overlap of the N-body ground state wave functions with two different values of the inverse-square interaction term goes to zero in the thermodynamic limit. When the two values of the inverse-square coupling differ by an infinitesimal amount, the wave function overlap shows an exponential suppression. This is qualitatively different from the usual power law suppression observed in the Anderson's orthogonality catastrophe. We also obtain an analytic expression for the wave function overlaps for an arbitrary set of couplings, whose properties are analyzed numerically. The quasiparticles constituting the ground state wave functions of the Sutherland model are known to obey fractional exclusion statistics. Our analysis indicates that the orthogonality catastrophe may be valid in systems with more general kinds of statistics than just the fermionic type.

Tan's contact and the phase distribution of repulsive Fermi gases: Insights from quantum chromodynamics noise analyses

NASA Astrophysics Data System (ADS)

Porter, William J.; Drut, Joaquín E.

2017-05-01

Path-integral analyses originally pioneered in the study of the complex-phase problem afflicting lattice calculations of finite-density quantum chromodynamics are generalized to nonrelativistic Fermi gases with repulsive interactions. Using arguments similar to those previously applied to relativistic theories, we show that the analogous problem in nonrelativistic systems manifests itself naturally in Tan's contact as a nontrivial cancellation between terms with varied dependence on extensive thermodynamic quantities. We analyze that case under the assumption of a Gaussian phase distribution, which is supported by our Monte Carlo calculations and perturbative considerations. We further generalize these results to observables other than the contact, as well as to polarized systems and systems with fixed particle number. Our results are quite general in that they apply to repulsive multicomponent fermions, they are independent of dimensionality or trapping potential, and they hold in the ground state as well as at finite temperature.

Conductivity predictions for the 5/2 fractional quantum Hall state using the composite fermion superconductor model

NASA Astrophysics Data System (ADS)

Foster, Kerwin Crayton

The fractional quantum Hall effect (FQHE) occurs when a two-dimensional electron gas is placed in a strong magnetic field at low temperatures. When this effect occurs the Hall resistance, RH, defined to be the Hall voltage divided by the current, is quantized, with RH = (1/nu)h/ e2 where nu = p/q is the Landau level filling fraction; and p and q are relatively prime integers. For almost all observed FQHE states, q is odd with one notable exception: the nu = 5/2 FQHE state. Understanding the nature of this incompressible even-denominator state is one of the central questions in the theory of the FQHE and is the subject of this Dissertation. We use a powerful theoretical tool for studying the FQHE: composite fermion theory. Composite fermions can be viewed as electrons bound to an even number of magnetic flux quanta. Jain has shown that the FQHE for electrons can be viewed as an integer quantum Hall effect (p = 1) for composite fermions. More recently, Halperin, Lee and Read developed a successful theory of the compressible nu = 1/2 state using composite fermions. There is now compelling theoretical evidence that the 5/2 state is a so-called Moore-Read state---a state which can be viewed as a spin-polarized p-wave superconductor of composite fermions. We have developed a semi-phenomenological description of this state by modifying the Halperin-Lee-Read theory, adding a p-wave pairing interaction between composite fermions by hand. The electromagnetic response functions for the resulting superconducting state of composite fermions are then calculated. We show that these response functions exhibit the expected BCS 'coherence factor' effects, such as the Hebel-Slichter peak. Using the composite fermion response functions, we then calculate the corresponding electronic response functions using Chern-Simons theory. We find that in the electronic response, the most striking coherence factor effects (e.g., the Hebel-Slichter peak) are strongly suppressed. However, the low-temperature o = 2Delta threshold behavior does show clear coherence factor effects. Finally, we use our model to predict the wave-vector and frequency dependence of the longitudinal conductivity, sigmaxx( q, o), which can be measured in surface-acoustic-wave propagation experiments.

Fermion Superfluidity

NASA Technical Reports Server (NTRS)

Strecker, Kevin; Truscott, Andrew; Partridge, Guthrie; Chen, Ying-Cheng

2003-01-01

Dual evaporation gives 50 million fermions at T = 0.1 T(sub F). Demonstrated suppression of interactions by coherent superposition - applicable to atomic clocks. Looking for evidence of Cooper pairing and superfluidity.

Calculation of K →π π decay amplitudes with improved Wilson fermion action in lattice QCD

NASA Astrophysics Data System (ADS)

Ishizuka, N.; Ishikawa, K.-I.; Ukawa, A.; Yoshié, T.

2015-10-01

We present our result for the K →π π decay amplitudes for both the Δ I =1 /2 and 3 /2 processes with the improved Wilson fermion action. Expanding on the earlier works by Bernard et al. and by Donini et al., we show that mixings with four-fermion operators with wrong chirality are absent even for the Wilson fermion action for the parity odd process in both channels due to CPS symmetry. Therefore, after subtraction of an effect from the lower dimensional operator, a calculation of the decay amplitudes is possible without complications from operators with wrong chirality, as for the case with chirally symmetric lattice actions. As a first step to verify the possibility of calculations with the Wilson fermion action, we consider the decay amplitudes at an unphysical quark mass mK˜2 mπ . Our calculations are carried out with Nf=2 +1 gauge configurations generated with the Iwasaki gauge action and nonperturbatively O (a )-improved Wilson fermion action at a =0.091 fm , mπ=280 MeV , and mK=580 MeV on a 323×64 (L a =2.9 fm ) lattice. For the quark loops in the penguin and disconnected contributions in the I =0 channel, the combined hopping parameter expansion and truncated solver method work very well for variance reduction. We obtain, for the first time with a Wilson-type fermion action, that Re A0=60 (36 )×1 0-8 GeV and Im A0=-67 (56 )×1 0-12 GeV for a matching scale q*=1 /a . The dependence on the matching scale q* for these values is weak.

Fermionic influence on inflationary fluctuations

NASA Astrophysics Data System (ADS)

Boyanovsky, Daniel

2016-04-01

Motivated by apparent persistent large scale anomalies in the cosmic microwave background we study the influence of fermionic degrees of freedom on the dynamics of inflaton fluctuations as a possible source of violations of (nearly) scale invariance on cosmological scales. We obtain the nonequilibrium effective action of an inflaton-like scalar field with Yukawa interactions (YD ,M) to light fermionic degrees of freedom both for Dirac and Majorana fields in de Sitter space-time. The effective action leads to Langevin equations of motion for the fluctuations of the inflaton-like field, with self-energy corrections and a stochastic Gaussian noise. We solve the Langevin equation in the super-Hubble limit implementing a dynamical renormalization group resummation. For a nearly massless inflaton its power spectrum of super-Hubble fluctuations is enhanced, P (k ;η )=(H/2 π )2eγt[-k η ] with γt[-k η ]=1/6 π2 [∑i =1 NDYi,D 2+2 ∑j =1 NMYj,M 2]{ln2[-k η ]-2 ln [-k η ]ln [-k η0]} for ND Dirac and NM Majorana fermions, and η0 is the renormalization scale at which the inflaton mass vanishes. The full power spectrum is shown to be renormalization group invariant. These corrections to the super-Hubble power spectrum entail a violation of scale invariance as a consequence of the coupling to the fermionic fields. The effective action is argued to be exact in the limit of a large number of fermionic fields. A cancellation between the enhancement from fermionic degrees of freedom and suppression from light scalar degrees of freedom conformally coupled to gravity suggests the possibility of a finely tuned supersymmetry among these fields.

Classification of compactified su( N c ) gauge theories with fermions in all representations

NASA Astrophysics Data System (ADS)

Anber, Mohamed M.; Vincent-Genod, Loïc

2017-12-01

We classify su( N c ) gauge theories on R^3× S^1 with massless fermions in higher representations obeying periodic boundary conditions along S^1 . In particular, we single out the class of theories that is asymptotically free and weakly coupled in the infrared, and therefore, is amenable to semi-classical treatment. Our study is conducted by carefully identifying the vacua inside the affine Weyl chamber using Verma bases and Frobenius formula techniques. Theories with fermions in pure representations are generally strongly coupled. The only exceptions are the four-index symmetric representation of su(2) and adjoint representation of su( N c ). However, we find a plethora of admissible theories with fermions in mixed representations. A sub-class of these theories have degenerate perturbative vacua separated by domain walls. In particular, su( N c ) theories with fermions in the mixed representations adjoint⊕fundamental and adjoint⊕two-index symmetric admit degenerate vacua that spontaneously break the parity P , charge conjugation C , and time reversal T symmetries. These are the first examples of strictly weakly coupled gauge theories on R^3× S^1 with spontaneously broken C , P , and T symmetries. We also compute the fermion zero modes in the background of monopole-instantons. The monopoles and their composites (topological molecules) proliferate in the vacuum leading to the confinement of electric charges. Interestingly enough, some theories have also accidental degenerate vacua, which are not related by any symmetry. These vacua admit different numbers of fermionic zero modes, and hence, different kinds of topological molecules. The lack of symmetry, however, indicates that such degeneracy might be lifted by higher order corrections. Finally, we study the general phase structure of adjoint⊕fundamental theories in the small circle and decompactification limits.

Foundations of heavy-fermion superconductivity: lattice Kondo effect and Mott physics.

PubMed

Steglich, Frank; Wirth, Steffen

2016-08-01

This article overviews the development of heavy-fermion superconductivity, notably in such rare-earth-based intermetallic compounds which behave as Kondo-lattice systems. Heavy-fermion superconductivity is of unconventional nature in the sense that it is not mediated by electron-phonon coupling. Rather, in most cases the attractive interaction between charge carriers is apparently magnetic in origin. Fluctuations associated with an antiferromagnetic (AF) quantum critical point (QCP) play a major role. The first heavy-fermion superconductor CeCu2Si2 turned out to be the prototype of a larger group of materials for which the underlying, often pressure-induced, AF QCP is likely to be of a three-dimensional (3D) spin-density-wave (SDW) variety. For UBe13, the second heavy-fermion superconductor, a magnetic-field-induced 3D SDW QCP inside the superconducting phase can be conjectured. Such a 'conventional', itinerant QCP can be well understood within Landau's paradigm of order-parameter fluctuations. In contrast, the low-temperature normal-state properties of a few heavy-fermion superconductors are at odds with the Landau framework. They are characterized by an 'unconventional', local QCP which may be considered a zero-temperature 4 f-orbital selective Mott transition. Here, as concluded for YbRh2Si2, the breakdown of the Kondo effect concurring with the AF instability gives rise to an abrupt change of the Fermi surface. Very recently, superconductivity was discovered for this compound at ultra-low temperatures. Therefore, YbRh2Si2 along with CeRhIn5 under pressure provide a natural link between the large group of about fifty low-temperature heavy-fermion superconductors and other families of unconventional superconductors with substantially higher T c, e.g. the doped Mott insulators of the perovskite-type cuprates and the organic charge-transfer salts.

Auxiliary-fermion approach to critical fluctuations in the two-dimensional quantum antiferromagnetic Heisenberg model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brinckmann, Jan; Woelfle, Peter

2004-11-01

The nearest-neighbor quantum antiferromagnetic (AF) Heisenberg model for spin-1/2 on a two-dimensional square lattice is studied in the auxiliary-fermion representation. Expressing spin operators by canonical fermionic particles requires a constraint on the fermion charge Q{sub i}=1 on each lattice site i, which is imposed approximately through the thermal average. The resulting interacting fermion system is first treated in mean-field theory (MFT), which yields an AF ordered ground state and spin waves in quantitative agreement with conventional spin-wave theory. At finite temperature a self-consistent approximation beyond mean field is required in order to fulfill the Mermin-Wagner theorem. We first discuss amore » fully self-consistent approximation, where fermions are renormalized due to fluctuations of their spin density, in close analogy to FLEX. While static properties like the correlation length, {xi}(T){proportional_to}exp(aJ/T), come out correctly, the dynamical response lacks the magnon-like peaks which would reflect the appearance of short-range order at low T. This drawback, which is caused by overdamping, is overcome in a 'minimal self-consistent approximation' (MSCA), which we derive from the equations of motion. The MSCA features dynamical scaling at small energy and temperature and is qualitatively correct both in the regime of order-parameter relaxation at long wavelengths {lambda}>{xi} and in the short-range-order regime at {lambda}<{xi}. We also discuss the impact of vertex corrections and the problem of pseudo-gap formation in the single-particle density of states due to long-range fluctuations. Finally we show that the (short-range) magnetic order in MFT and MSCA helps to fulfill the constraint on the local fermion occupancy.« less

Unitarity violation in noninteger dimensional Gross-Neveu-Yukawa model

NASA Astrophysics Data System (ADS)

Ji, Yao; Kelly, Michael

2018-05-01

We construct an explicit example of unitarity violation in fermionic quantum field theories in noninteger dimensions. We study the two-point correlation function of four-fermion operators. We compute the one-loop anomalous dimensions of these operators in the Gross-Neveu-Yukawa model. We find that at one-loop order, the four-fermion operators split into three classes with one class having negative norms. This implies that the theory violates unitarity, following the definition in Ref. [1].

Unconventional States of Matter with Cold Atoms and Dipolar Molecules

DTIC Science & Technology

2014-08-20

ferromagnetic state. For alkaline-earth fermions, the large SU(2N) symmetry greatly enhances quantum spin fluctuations, which give rises to novel...both bosons and fermions, novel quantum magnetism with large spin SU(2N) al- kaline fermions, novel topological states with synthetic gauge fields...presented in Sect. 1.1. The study of novel quantum magnetism with large spin alkaline earth atoms is presented in Sect. 1.2. In Sect. 1.3, we present our

Fermion-to-qubit mappings with varying resource requirements for quantum simulation

NASA Astrophysics Data System (ADS)

Steudtner, Mark; Wehner, Stephanie

2018-06-01

The mapping of fermionic states onto qubit states, as well as the mapping of fermionic Hamiltonian into quantum gates enables us to simulate electronic systems with a quantum computer. Benefiting the understanding of many-body systems in chemistry and physics, quantum simulation is one of the great promises of the coming age of quantum computers. Interestingly, the minimal requirement of qubits for simulating Fermions seems to be agnostic of the actual number of particles as well as other symmetries. This leads to qubit requirements that are well above the minimal requirements as suggested by combinatorial considerations. In this work, we develop methods that allow us to trade-off qubit requirements against the complexity of the resulting quantum circuit. We first show that any classical code used to map the state of a fermionic Fock space to qubits gives rise to a mapping of fermionic models to quantum gates. As an illustrative example, we present a mapping based on a nonlinear classical error correcting code, which leads to significant qubit savings albeit at the expense of additional quantum gates. We proceed to use this framework to present a number of simpler mappings that lead to qubit savings with a more modest increase in gate difficulty. We discuss the role of symmetries such as particle conservation, and savings that could be obtained if an experimental platform could easily realize multi-controlled gates.

New directions in the pursuit of Majorana fermions in solid state systems.

PubMed

Alicea, Jason

2012-07-01

The 1937 theoretical discovery of Majorana fermions-whose defining property is that they are their own anti-particles-has since impacted diverse problems ranging from neutrino physics and dark matter searches to the fractional quantum Hall effect and superconductivity. Despite this long history the unambiguous observation of Majorana fermions nevertheless remains an outstanding goal. This review paper highlights recent advances in the condensed matter search for Majorana that have led many in the field to believe that this quest may soon bear fruit. We begin by introducing in some detail exotic 'topological' one- and two-dimensional superconductors that support Majorana fermions at their boundaries and at vortices. We then turn to one of the key insights that arose during the past few years; namely, that it is possible to 'engineer' such exotic superconductors in the laboratory by forming appropriate heterostructures with ordinary s-wave superconductors. Numerous proposals of this type are discussed, based on diverse materials such as topological insulators, conventional semiconductors, ferromagnetic metals and many others. The all-important question of how one experimentally detects Majorana fermions in these setups is then addressed. We focus on three classes of measurements that provide smoking-gun Majorana signatures: tunneling, Josephson effects and interferometry. Finally, we discuss the most remarkable properties of condensed matter Majorana fermions-the non-Abelian exchange statistics that they generate and their associated potential for quantum computation.

Dirac materials

NASA Astrophysics Data System (ADS)

Wehling, T. O.; Black-Schaffer, A. M.; Balatsky, A. V.

2014-01-01

A wide range of materials, like d-wave superconductors, graphene, and topological insulators, share a fundamental similarity: their low-energy fermionic excitations behave as massless Dirac particles rather than fermions obeying the usual Schrodinger Hamiltonian. This emergent behavior of Dirac fermions in condensed matter systems defines the unifying framework for a class of materials we call "Dirac materials''. In order to establish this class of materials, we illustrate how Dirac fermions emerge in multiple entirely different condensed matter systems and we discuss how Dirac fermions have been identified experimentally using electron spectroscopy techniques (angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy). As a consequence of their common low-energy excitations, this diverse set of materials shares a significant number of universal properties in the low-energy (infrared) limit. We review these common properties including nodal points in the excitation spectrum, density of states, specific heat, transport, thermodynamic properties, impurity resonances, and magnetic field responses, as well as discuss many-body interaction effects. We further review how the emergence of Dirac excitations is controlled by specific symmetries of the material, such as time-reversal, gauge, and spin-orbit symmetries, and how by breaking these symmetries a finite Dirac mass is generated. We give examples of how the interaction of Dirac fermions with their distinct real material background leads to rich novel physics with common fingerprints such as the suppression of back scattering and impurity-induced resonant states.

Thermoelectric Transport Signatures of Dirac Composite Fermions in the Half-Filled Landau Level

NASA Astrophysics Data System (ADS)

Potter, Andrew C.; Serbyn, Maksym; Vishwanath, Ashvin

2016-07-01

The half-filled Landau level is expected to be approximately particle-hole symmetric, which requires an extension of the Halperin-Lee-Read (HLR) theory of the compressible state observed at this filling. Recent work indicates that, when particle-hole symmetry is preserved, the composite fermions experience a quantized π -Berry phase upon winding around the composite Fermi surface, analogous to Dirac fermions at the surface of a 3D topological insulator. In contrast, the effective low-energy theory of the composite fermion liquid originally proposed by HLR lacks particle-hole symmetry and has vanishing Berry phase. In this paper, we explain how thermoelectric transport measurements can be used to test the Dirac nature of the composite fermions by quantitatively extracting this Berry phase. First, we point out that longitudinal thermopower (Seebeck effect) is nonvanishing because of the unusual nature of particle-hole symmetry in this context and is not sensitive to the Berry phase. In contrast, we find that off-diagonal thermopower (Nernst effect) is directly related to the topological structure of the composite Fermi surface, vanishing for zero Berry phase and taking its maximal value for π Berry phase. In contrast, in purely electrical transport signatures, the Berry phase contributions appear as small corrections to a large background signal, making the Nernst effect a promising diagnostic of the Dirac nature of composite fermions.

Taste symmetry breaking with hypercubic-smeared staggered fermions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bae, Taegil; Adams, David H.; Kim, Hyung-Jin

2008-05-01

We study the impact of hypercubic (HYP) smearing on the size of taste-breaking for staggered fermions, comparing to unimproved and to asqtad-improved staggered fermions. As in previous studies, we find a substantial reduction in taste-breaking compared to unimproved staggered fermions (by a factor of 4-7 on lattices with spacing a{approx_equal}0.1 fm). In addition, we observe that discretization effects of next-to-leading order in the chiral expansion (O(a{sup 2}p{sup 2})) are markedly reduced by HYP smearing. Compared to asqtad valence fermions, we find that taste-breaking in the pion spectrum is reduced by a factor of 2.5-3, down to a level comparable tomore » the expected size of generic O(a{sup 2}) effects. Our results suggest that, once one reaches a lattice spacing of a{approx_equal}0.09 fm, taste-breaking will be small enough after HYP smearing that one can use a modified power counting in which O(a{sup 2})<

Bravyi-Kitaev Superfast simulation of electronic structure on a quantum computer.

PubMed

Setia, Kanav; Whitfield, James D

2018-04-28

Present quantum computers often work with distinguishable qubits as their computational units. In order to simulate indistinguishable fermionic particles, it is first required to map the fermionic state to the state of the qubits. The Bravyi-Kitaev Superfast (BKSF) algorithm can be used to accomplish this mapping. The BKSF mapping has connections to quantum error correction and opens the door to new ways of understanding fermionic simulation in a topological context. Here, we present the first detailed exposition of the BKSF algorithm for molecular simulation. We provide the BKSF transformed qubit operators and report on our implementation of the BKSF fermion-to-qubits transform in OpenFermion. In this initial study of a hydrogen molecule we have compared BKSF, Jordan-Wigner, and Bravyi-Kitaev transforms under the Trotter approximation. The gate count to implement BKSF is lower than Jordan-Wigner but higher than Bravyi-Kitaev. We considered different orderings of the exponentiated terms and found lower Trotter errors than the previously reported for Jordan-Wigner and Bravyi-Kitaev algorithms. These results open the door to the further study of the BKSF algorithm for quantum simulation.

Multiple quantum phase transitions and superconductivity in Ce-based heavy fermions.

PubMed

Weng, Z F; Smidman, M; Jiao, L; Lu, Xin; Yuan, H Q

2016-09-01

Heavy fermions have served as prototype examples of strongly-correlated electron systems. The occurrence of unconventional superconductivity in close proximity to the electronic instabilities associated with various degrees of freedom points to an intricate relationship between superconductivity and other electronic states, which is unique but also shares some common features with high temperature superconductivity. The magnetic order in heavy fermion compounds can be continuously suppressed by tuning external parameters to a quantum critical point, and the role of quantum criticality in determining the properties of heavy fermion systems is an important unresolved issue. Here we review the recent progress of studies on Ce based heavy fermion superconductors, with an emphasis on the superconductivity emerging on the edge of magnetic and charge instabilities as well as the quantum phase transitions which occur by tuning different parameters, such as pressure, magnetic field and doping. We discuss systems where multiple quantum critical points occur and whether they can be classified in a unified manner, in particular in terms of the evolution of the Fermi surface topology.

Destruction of the Kondo effect in the cubic heavy-fermion compound Ce3Pd20Si6

NASA Astrophysics Data System (ADS)

Custers, J.; Lorenzer, K.-A.; Müller, M.; Prokofiev, A.; Sidorenko, A.; Winkler, H.; Strydom, A. M.; Shimura, Y.; Sakakibara, T.; Yu, R.; Si, Q.; Paschen, S.

2012-03-01

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy-fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy-fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure, raising the question of the extent to which heavy-fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy-fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram—a precondition for a unified theoretical description.

Scattering of fermions in the Yukawa theory coupled to unimodular gravity

NASA Astrophysics Data System (ADS)

Gonzalez-Martin, S.; Martin, C. P.

2018-03-01

We compute the lowest order gravitational UV divergent radiative corrections to the S matrix element of the fermion + fermion→ fermion + fermion scattering process in the massive Yukawa theory, coupled either to Unimodular Gravity or to General Relativity. We show that both Unimodular Gravity and General Relativity give rise to the same UV divergent contribution in Dimensional Regularization. This is a nontrivial result, since in the classical action of Unimodular Gravity coupled to the Yukawa theory, the graviton field does not couple neither to the mass operator nor to the Yukawa operator. This is unlike the General Relativity case. The agreement found points in the direction that Unimodular Gravity and General Relativity give rise to the same quantum theory when coupled to matter, as long as the Cosmological Constant vanishes. Along the way we have come across another unexpected cancellation of UV divergences for both Unimodular Gravity and General Relativity, resulting in the UV finiteness of the one-loop and κ y^2 order of the vertex involving two fermions and one graviton only.

Interplay between magnetism and relativistic fermions in Eu doped (Sr/Ba)MnSb2

NASA Astrophysics Data System (ADS)

Liu, Jinyu; Hu, Jin; Zhu, Yanglin; Chuang, Alyssa; Graf, David; Jaime, Marcelo; Balakirev, Fedor; Weickert, Franziska; Zhang, Qiang; Ditusa, John; Wu, Yan; Cao, Huibo; Mao, Zhiqiang

Layered compounds AMnBi2 (A =Ca, Sr, Ba, Eu, and Yb) have been established as Dirac materials with fascinating properties. In our previous work, we have demonstrated that Sr1-y Mn1-z Sb2 (y, z <0.1), isostructural to AMnBi2, not only host relativistic fermions, but also exhibit ferromagnetic properties, with its ferromagnetism being coupled to the relativistic fermions' transport. To gain further insight into the relativistic fermion-magnetism coupling, we have synthesized a series of Eu doped (Sr/Ba)MnSb2 single crystals and found Eu moments order antiferromagnetically. Through neutron scattering experiments, we determined the magnetic structures for Sr1-xEuxMnSb2 with x = 0.2, 0.5, and 0.8. From magnetotransport measurements, we find the Eu antiferromagnetism is also coupled to relativistic fermion transport. More importantly, we observed a novel quantum phase with saturated magnetoresistivity near the quantum limit for the 10% Eu doped BaMnSb2 sample. We will discuss possible mechanisms for this novel phase.

Composite Fermions: Motivation, Successes, and Application to Fractional Quantum Hall Effect in Graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jain, Jainendra

2011-07-15

The fractional quantum Hall effect (FQHE) is one of the most amazing collective states discovered in modern times. A remarkably detailed and accurate understanding of its nonperturbative physics has been achieved in terms of a new class of exotic particles called composite fermions. I will begin with a brief review of the composite fermion theory and its outstanding successes. The rest of the talk will be concerned with fractional quantum Hall effect in graphene, observed recently. I will present results of theoretical studies that demonstrate that composite fermions are formed in graphene as well, but the spin and valley degeneraciesmore » and the linear dispersion of electrons produce interesting new physics relative to that in the usual two-dimensional GaAs systems. Composite fermion theory allows detailed predictions about FQHE in graphene in regimes when either or both of the spin and valley degeneracies are broken. I will discuss the relevance of our theory to recent experiments. This work on FQHE in graphene has been performed in collaboration with Csaba Toke.« less

Fermion-induced quantum critical points in two-dimensional Dirac semimetals

NASA Astrophysics Data System (ADS)

Jian, Shao-Kai; Yao, Hong

2017-11-01

In this paper we investigate the nature of quantum phase transitions between two-dimensional Dirac semimetals and Z3-ordered phases (e.g., Kekule valence-bond solid), where cubic terms of the order parameter are allowed in the quantum Landau-Ginzberg theory and the transitions are putatively first order. From large-N renormalization-group (RG) analysis, we find that fermion-induced quantum critical points (FIQCPs) [Z.-X. Li et al., Nat. Commun. 8, 314 (2017), 10.1038/s41467-017-00167-6] occur when N (the number of flavors of four-component Dirac fermions) is larger than a critical value Nc. Remarkably, from the knowledge of space-time supersymmetry, we obtain an exact lower bound for Nc, i.e., Nc>1 /2 . (Here the "1/2" flavor of four-component Dirac fermions is equivalent to one flavor of four-component Majorana fermions). Moreover, we show that the emergence of two length scales is a typical phenomenon of FIQCPs and obtain two different critical exponents, i.e., ν ≠ν' , by large-N RG calculations. We further give a brief discussion of possible experimental realizations of FIQCPs.

Emergence of gravity, fermion, gauge and Chern-Simons fields during formation of N-dimensional manifolds from joining point-like ones

NASA Astrophysics Data System (ADS)

Sepehri, Alireza; Shoorvazi, Somayyeh

In this paper, we will consider the birth and evolution of fields during formation of N-dimensional manifolds from joining point-like ones. We will show that at the beginning, only there are point-like manifolds which some strings are attached to them. By joining these manifolds, 1-dimensional manifolds are appeared and gravity, fermion, and gauge fields are emerged. By coupling these manifolds, higher dimensional manifolds are produced and higher orders of fermion, gauge fields and gravity are emerged. By decaying N-dimensional manifold, two child manifolds and a Chern-Simons one are born and anomaly is emerged. The Chern-Simons manifold connects two child manifolds and leads to the energy transmission from the bulk to manifolds and their expansion. We show that F-gravity can be emerged during the formation of N-dimensional manifold from point-like manifolds. This type of F-gravity includes both type of fermionic and bosonic gravity. G-fields and also C-fields which are produced by fermionic strings produce extra energy and change the gravity.

Minimally doubled fermions and spontaneous chiral symmetry breaking

NASA Astrophysics Data System (ADS)

Osmanaj (Zeqirllari), Rudina; Hyka (Xhako), Dafina

2018-03-01

Chiral symmetry breaking in massless QCD is a very important feature in the current understanding of low energy physics. Low - lying Dirac modes are suitable to help us understand the spontaneous chiral symmetry breaking, since the formation of a non zero chiral condensate is an effect of their accumulation near zero. The Banks - Casher relation links the spectral density of the Dirac operator to the condensate with an identity that can be read in both directions. In this work we propose a spectral method to achieve a reliable determination of the density of eigenvalues of Dirac operator near zero using the Gauss - Lanczos quadrature. In order to understand better the dynamical chiral symmetry breaking and use the method we propose, we have chosen to work with minimally doubled fermions. These kind of fermions have been proposed as a strictly local discretization of the QCD fermions action, which preserves chiral symmetry at finite cut-off. Being chiral fermions, is easier to work with them and their low - lying Dirac modes and to understand the dynamical spontaneous chiral symmetry breaking.

Experimental evidence of hourglass fermion in the candidate nonsymmorphic topological insulator KHgSb

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ma, Junzhang; Yi, Changjiang; Lv, Baiqing

Topological insulators (TIs) host novel states of quantum matter characterized by nontrivial conducting boundary states connecting valence and conduction bulk bands. All TIs discovered experimentally so far rely on either time-reversal or mirror crystal symmorphic symmetry to protect massless Dirac-like boundary states. Several materials were recently proposed to be TIs with nonsymmorphic symmetry, where a glide mirror protects exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of this new fermion is missing. Using angle-resolved photoemission spectroscopy, we provide experimental evidence of hourglass fermions on the (010) surface of crystalline KHgSb, whereas the (001) surface has no boundary state,more » in agreement with first-principles calculations. Our study will stimulate further research activities of topological properties of nonsymmorphic materials.« less

Scattering length of composite bosons in the three-dimensional BCS-BEC crossover

NASA Astrophysics Data System (ADS)

Salasnich, L.; Bighin, G.

2015-03-01

We study the zero-temperature grand potential of a three-dimensional superfluid made of ultracold fermionic alkali-metal atoms in the BCS-BEC crossover. In particular, we analyze the zero-point energy of both fermionic single-particle excitations and bosonic collective excitations. The bosonic elementary excitations, which are crucial to obtain a reliable equation of state in the Bose-Einstein condensate regime, are obtained with a low-momentum expansion up to the forth order of the quadratic (Gaussian) action of the fluctuating pairing field. By performing a cutoff regularization and renormalization of Gaussian fluctuations, we find that the scattering length aB of composite bosons, bound states of fermionic pairs, is given by aB=(2 /3 ) aF , where aF is the scattering length of fermions.

Experimental evidence of hourglass fermion in the candidate nonsymmorphic topological insulator KHgSb

DOE PAGES

Ma, Junzhang; Yi, Changjiang; Lv, Baiqing; ...

2017-05-05

Topological insulators (TIs) host novel states of quantum matter characterized by nontrivial conducting boundary states connecting valence and conduction bulk bands. All TIs discovered experimentally so far rely on either time-reversal or mirror crystal symmorphic symmetry to protect massless Dirac-like boundary states. Several materials were recently proposed to be TIs with nonsymmorphic symmetry, where a glide mirror protects exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of this new fermion is missing. Using angle-resolved photoemission spectroscopy, we provide experimental evidence of hourglass fermions on the (010) surface of crystalline KHgSb, whereas the (001) surface has no boundary state,more » in agreement with first-principles calculations. Our study will stimulate further research activities of topological properties of nonsymmorphic materials.« less

Solution to the sign problem in a frustrated quantum impurity model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hann, Connor T., E-mail: connor.hann@yale.edu; Huffman, Emilie; Chandrasekharan, Shailesh

2017-01-15

In this work we solve the sign problem of a frustrated quantum impurity model consisting of three quantum spin-half chains interacting through an anti-ferromagnetic Heisenberg interaction at one end. We first map the model into a repulsive Hubbard model of spin-half fermions hopping on three independent one dimensional chains that interact through a triangular hopping at one end. We then convert the fermion model into an inhomogeneous one dimensional model and express the partition function as a weighted sum over fermion worldline configurations. By imposing a pairing of fermion worldlines in half the space we show that all negative weightmore » configurations can be eliminated. This pairing naturally leads to the original frustrated quantum spin model at half filling and thus solves its sign problem.« less

Annihilation of singlet fermionic dark matter into two photons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ettefaghi, M.M.; Moazzemi, R., E-mail: mettefaghi@qom.ac.ir, E-mail: r.moazzemi@qom.ac.ir

2013-02-01

We consider an extension of the standard model in which a singlet fermionic particle, to serve as cold dark matter, and a singlet Higgs are added. We perform a reanalysis on the free parameters. In particular, demanding a correct relic abundance of dark matter, we derive and plot the coupling of the singlet fermion with the singlet Higgs, g{sub s}, versus the dark matter mass. We analytically compute the pair annihilation cross section of singlet fermionic dark matter into two photons. The thermally averaged of this cross section is calculated for wide range of energies and plotted versus dark mattermore » mass using g{sub s} consistent with the relic abundance condition. We also compare our results with the Fermi-Lat observations.« less

Dirac fermions in an antiferromagnetic semimetal

NASA Astrophysics Data System (ADS)

Tang, Peizhe; Zhou, Quan; Xu, Gang; Zhang, Shou-Cheng

2016-12-01

Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry and inversion symmetry . Here we show that Dirac fermions can exist in one type of antiferromagnetic system, where both and are broken but their combination is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections and demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.

Landau Levels of Majorana Fermions in a Spin Liquid.

PubMed

Rachel, Stephan; Fritz, Lars; Vojta, Matthias

2016-04-22

Majorana fermions, originally proposed as elementary particles acting as their own antiparticles, can be realized in condensed-matter systems as emergent quasiparticles, a situation often accompanied by topological order. Here we propose a physical system which realizes Landau levels-highly degenerate single-particle states usually resulting from an orbital magnetic field acting on charged particles-for Majorana fermions. This is achieved in a variant of a quantum spin system due to Kitaev which is distorted by triaxial strain. This strained Kitaev model displays a spin-liquid phase with charge-neutral Majorana-fermion excitations whose spectrum corresponds to that of Landau levels, here arising from a tailored pseudomagnetic field. We show that measuring the dynamic spin susceptibility reveals the Landau-level structure by a remarkable mechanism of probe-induced bound-state formation.

Baryon spectrum of SU(4) composite Higgs theory with two distinct fermion representations

NASA Astrophysics Data System (ADS)

Ayyar, Venkitesh; DeGrand, Thomas; Hackett, Daniel C.; Jay, William I.; Neil, Ethan T.; Shamir, Yigal; Svetitsky, Benjamin

2018-06-01

We use lattice simulations to compute the baryon spectrum of SU(4) lattice gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations simultaneously. This model is closely related to a composite Higgs model in which the chimera baryon made up of fermions from both representations plays the role of a composite top-quark partner. The dependence of the baryon masses on each underlying fermion mass is found to be generally consistent with a quark-model description and large-Nc scaling. We combine our numerical results with experimental bounds on the scale of the new strong sector to estimate a lower bound on the mass of the top-quark partner. We discuss some theoretical uncertainties associated with this estimate.

Gravitationally bound BCS state as dark matter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alexander, Stephon; Cormack, Sam, E-mail: stephon_alexander@brown.edu, E-mail: samuel.c.cormack.gr@dartmouth.edu

2017-04-01

We explore the possibility that fermionic dark matter undergoes a BCS transition to form a superfluid. This requires an attractive interaction between fermions and we describe a possible source of this interaction induced by torsion. We describe the gravitating fermion system with the Bogoliubov-de Gennes formalism in the local density approximation. We solve the Poisson equation along with the equations for the density and gap energy of the fermions to find a self-gravitating, superfluid solution for dark matter halos. In order to produce halos the size of dwarf galaxies, we require a particle mass of ∼ 200 eV. We findmore » a maximum attractive coupling strength before the halo becomes unstable. If dark matter halos do have a superfluid component, this raises the possibility that they contain vortex lines.« less

Birefringent breakup of Dirac fermions on a square optical lattice

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kennett, Malcolm P.; Komeilizadeh, Nazanin; Kaveh, Kamran

2011-05-15

We introduce a lattice model for fermions in a spatially periodic magnetic field that also has spatially periodic hopping amplitudes. We discuss how this model might be realized with cold atoms in an artificial magnetic field on a square optical lattice. When there is an average flux of half a flux quantum per plaquette, the spectrum of low-energy excitations can be described by massless Dirac fermions in which the usually doubly degenerate Dirac cones split into cones with different ''speeds of light.'' These gapless birefringent Dirac fermions arise because of broken chiral symmetry in the kinetic energy term of themore » effective low-energy Hamiltonian. We characterize the effects of various perturbations to the low-energy spectrum, including staggered potentials, interactions, and domain-wall topological defects.« less

Quantum Monte Carlo Simulation of Frustrated Kondo Lattice Models

NASA Astrophysics Data System (ADS)

Sato, Toshihiro; Assaad, Fakher F.; Grover, Tarun

2018-03-01

The absence of the negative sign problem in quantum Monte Carlo simulations of spin and fermion systems has different origins. World-line based algorithms for spins require positivity of matrix elements whereas auxiliary field approaches for fermions depend on symmetries such as particle-hole symmetry. For negative-sign-free spin and fermionic systems, we show that one can formulate a negative-sign-free auxiliary field quantum Monte Carlo algorithm that allows Kondo coupling of fermions with the spins. Using this general approach, we study a half-filled Kondo lattice model on the honeycomb lattice with geometric frustration. In addition to the conventional Kondo insulator and antiferromagnetically ordered phases, we find a partial Kondo screened state where spins are selectively screened so as to alleviate frustration, and the lattice rotation symmetry is broken nematically.

Adding gauge fields to Kaplan's fermions

NASA Astrophysics Data System (ADS)

Blum, T.; Kärkkäinen, Leo

1994-04-01

We experiment with adding dynamical gauge field to Kaplan (defect) fermions. In the case of U (1) gauge theory we use an inhomogenous Higgs mechanism to restrict the 3d gauge dynamics to a planar 2d defect. In our simulations the 3d theory produce the correct 2d gauge dynamics. We measure fermion propagators with dynamical gauge fields. They posses the correct chiral structure. The fermions at the boundary of the support of the gauge field (waveguide) are non-chiral, and have a mass two times heavier than the chiral modes. Moreover, these modes cannot be excited by a source at the defect; implying that they are dynamically decoupled. We have also checked that the anomaly relation is fullfilled for the case of a smooth external gauge field.

Comparison of fixed-bearing and mobile-bearing total knee arthroplasty after high tibial osteotomy.

PubMed

Hernigou, Philippe; Huys, Maxime; Pariat, Jacques; Roubineau, François; Flouzat Lachaniette, Charles Henri; Dubory, Arnaud

2018-02-01

There is no information comparing the results of fixed-bearing total knee replacement and mobile-bearing total knee replacement in the same patients previously treated by high tibial osteotomy. The purpose was therefore to compare fixed-bearing and mobile-bearing total knee replacements in patients treated with previous high tibial osteotomy. We compared the results of 57 patients with osteoarthritis who had received a fixed-bearing prosthesis after high tibial osteotomy with the results of 41 matched patients who had received a rotating platform after high tibial osteotomy. The match was made for length of follow-up period. The mean follow-up was 17 years (range, 15-20 years). The patients were assessed clinically and radiographically. The pre-operative knee scores had no statistically significant differences between the two groups. So was the case with the intra-operative releases, blood loss, thromboembolic complications and infection rates in either group. There was significant improvement in both groups of knees, and no significant difference was observed between the groups (i.e., fixed-bearing and mobile-bearing knees) for the mean Knee Society knee clinical score (95 and 92 points, respectively), or the Knee Society knee functional score (82 and 83 points, respectively) at the latest follow-up. However, the mean post-operative knee motion was higher for the fixed-bearing group (117° versus 110°). In the fixed-bearing group, one knee was revised because of periprosthetic fracture. In the rotating platform mobile-bearing group, one knee was revised because of aseptic loosening of the tibial component. The Kaplan-Meier survivorship for revision at ten years of follow-up was 95.2% for the fixed bearing prosthesis and 91.1% for the rotating platform mobile-bearing prosthesis. Although we did manage to detect significant differences mainly in clinical and radiographic results between the two groups, we found no superiority or inferiority of the mobile-bearing total knee prosthesis over the fixed-bearing total knee prosthesis for patients previously operated by high tibial osteotomy.

Quantum Computation and Simulation Using Neutral Fermionic Atoms

DTIC Science & Technology

2014-06-06

labeled n = 1) Efimov trimer crosses the three-atom scattering threshold. Working in the context of nuclear physics in the early 1970’s, Vitaly Efimov...permit the observation of anti-ferromagnetic ordering in the Hubbard model. (a) Papers published in peer-reviewed journals ( N /A for none) Enter List of...Physics, (06 2011): 0. doi: TOTAL: 7 Number of Papers published in peer-reviewed journals: (b) Papers published in non-peer-reviewed journals ( N /A for

48 CFR 1852.216-74 - Estimated cost and fixed fee.

Code of Federal Regulations, 2010 CFR

2010-10-01

... and Clauses 1852.216-74 Estimated cost and fixed fee. As prescribed in 1816.307-70(b), insert the following clause: Estimated Cost and Fixed Fee (DEC 1991) The estimated cost of this contract is ______ exclusive of the fixed fee of ______. The total estimated cost and fixed fee is ______. (End of clause) [62...

Local Hamiltonian Monte Carlo study of the massive schwinger model, the decoupling of heavy flavours

NASA Astrophysics Data System (ADS)

Ranft, J.

1983-12-01

The massive Schwinger model with two flavours is studied using the local hamiltonian lattice Monte Carlo method. Chiral symmetry breaking is studied using the fermion condensate as order parameter. For a small ratio of the two fermion masses, degeneracy of the two flavours is found. For a large ratio of the masses, the heavy flavour decouples and the light fermion behaves like in the one flavour Schwinger model. On leave from Sektion Physik, Karl-Marx-Universität, Leipzig, GDR.

Fermions tunneling from the Horowitz-Strominger Dilaton black hole

NASA Astrophysics Data System (ADS)

Li, Qiang; Zeng, Xiaoxiong

2009-06-01

Based on the work of Kerner and Mann, fermions tunneling from the Horowitz-Strominger Dilaton black hole on the membrane is studied. Owing to the coupling among electromagnetic field, matter field and gravity field, the Dirac equation of charged particles is introduced, and according to that, the expected emission temperature is obtained. After the self-gravitational interaction is considered, it is found that the tunneling rate of fermions also satisfies the underlying Unitary theory as the case of scalar particles.

Global analysis of fermion mixing with exotics

NASA Technical Reports Server (NTRS)

Nardi, Enrico; Roulet, Esteban; Tommasini, Daniele

1991-01-01

The limits are analyzed on deviation of the lepton and quark weak-couplings from their standard model values in a general class of models where the known fermions are allowed to mix with new heavy particles with exotic SU(2) x U(1) quantum number assignments (left-handed singlets or right-handed doublets). These mixings appear in many extensions of the electroweak theory such as models with mirror fermions, E(sub 6) models, etc. The results update previous analyses and improve considerably the existing bounds.

Gapped fermionic spectrum from a domain wall in seven dimension

NASA Astrophysics Data System (ADS)

Mukhopadhyay, Subir; Rai, Nishal

2018-05-01

We obtain a domain wall solution in maximally gauged seven dimensional supergravity, which interpolates between two AdS spaces and spontaneously breaks a U (1) symmetry. We analyse frequency dependence of conductivity and find power law behaviour at low frequency. We consider certain fermions of supergravity in the background of this domain wall and compute holographic spectral function of the operators in the dual six dimensional theory. We find fermionic operators involving bosons with non-zero expectation value lead to gapped spectrum.

The Feynman-Vernon Influence Functional Approach in QED

NASA Astrophysics Data System (ADS)

Biryukov, Alexander; Shleenkov, Mark

2016-10-01

In the path integral approach we describe evolution of interacting electromagnetic and fermionic fields by the use of density matrix formalism. The equation for density matrix and transitions probability for fermionic field is obtained as average of electromagnetic field influence functional. We obtain a formula for electromagnetic field influence functional calculating for its various initial and final state. We derive electromagnetic field influence functional when its initial and final states are vacuum. We present Lagrangian for relativistic fermionic field under influence of electromagnetic field vacuum.

Coherent vs. incoherent pairing in 2D systems near magnetic instability

NASA Astrophysics Data System (ADS)

Abanov, Ar.; Chubukov, A. V.; Finkel'stein, A. M.

2001-05-01

We study the superconductivity in 2D fermionic systems near antiferromagnetic instability, assuming that the pairing is mediated by spin fluctuations. This pairing involves fully incoherent fermions and diffusive spin excitations. We show that the competition between fermionic incoherence and strong pairing interaction yields the pairing instability temperature Tins which increases and saturates as the magnetic correlation length ξ → ∞. We argue that in this quantum-critical regime the pairing problem is qualitatively different from the BCS one.

On bound-states of the Gross Neveu model with massive fundamental fermions

NASA Astrophysics Data System (ADS)

Frishman, Yitzhak; Sonnenschein, Jacob

2018-01-01

In the search for QFT's that admit boundstates, we reinvestigate the two dimensional Gross-Neveu model, but with massive fermions. By computing the self-energy for the auxiliary boundstate field and the effective potential, we show that there are no bound states around the lowest minimum, but there is a meta-stable bound state around the other minimum, a local one. The latter decays by tunneling. We determine the dependence of its lifetime on the fermion mass and coupling constant.

Magnetic Majorana Fermions

NASA Astrophysics Data System (ADS)

Moessner, Roderich

Condensed matter systems provide emergent mini-universes in which quasiparticles may exist which do not correspond to any experimentally detected elementary particle. Topological quantum materials have been particularly productive in this regard, with the present search focussing on Majorana fermions, known theoretically already for decades. Here, we discuss manifestations of magnetic Majorana fermions in the Kitaev model. We place particular emphasis on their fate when perturbations, such as Heisenberg terms, are added to the ideal model system, and address experimental signatures of their vestiges in phases adjacent to the spin liquid.

Entanglement of Dirac fields in an expanding spacetime

NASA Astrophysics Data System (ADS)

Fuentes, Ivette; Mann, Robert B.; Martín-Martínez, Eduardo; Moradi, Shahpoor

2010-08-01

We study the entanglement generated between Dirac modes in a 2-dimensional conformally flat Robertson-Walker universe. We find radical qualitative differences between the bosonic and fermionic entanglement generated by the expansion. The particular way in which fermionic fields get entangled encodes more information about the underlying spacetime than the bosonic case, thereby allowing us to reconstruct the parameters of the history of the expansion. This highlights the importance of bosonic/fermionic statistics to account for relativistic effects on the entanglement of quantum fields.

Magnetotransport study of Dirac fermions in YbMnBi 2 antiferromagnet

DOE PAGES

Wang, Aifeng; Zaliznyak, I.; Ren, Weijun; ...

2016-10-15

We report quantum transport and Dirac fermions in YbMnBi 2 single crystals. YbMnBi 2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, and small cyclotron mass indicate the presence of Dirac fermions. Lastly, angular-dependent magnetoresistance indicates a possible quasi-two-dimensional Fermi surface, whereas the deviation from the nontrivial Berry phase expected for Dirac states suggests the contribution of parabolic bands at the Fermi level or spin-orbit coupling.

Is the Luttinger Liquid a New State of Matter?

NASA Astrophysics Data System (ADS)

Afonin, V. V.; Petrov, V. Y.

2010-02-01

We are demonstrating that the Luttinger model with short range interaction can be treated as a type of Fermi liquid. In line with the main dogma of Landau’s theory one can define a fermion excitation renormalized by interaction and show that in terms of these fermions any excited state of the system is described by free particles. The fermions are a mixture of renormalized right and left electrons. The electric charge and chirality of the Landau quasi-particle is discussed.

The realization of Majorana fermions in Kitaev Quantum Spin Lattice

NASA Astrophysics Data System (ADS)

Do, Seung-Hwan; Park, Sang-Youn; Yoshitake, Junki; Nasu, Joji; Motome, Yukitoshi; Kwon, Y. S.; Adroja, D. T.; Voneshen, D.; Park, J.-H.; Choi, Kwang-Yong; Ji, Sungdae

The Kitaev honeycomb lattice is envisioned as an ideal host for Majorana fermions that are created out of the spin liquid background. Combining specific heat and neutron scattering experiments with theoretical calculations, here, we establish a hitherto unparalleled spin fractionalization to two species of Majorana fermions in the Kitaev material α-RuCl3. The specific heat data unveil a two-stage release of magnetic entropy by (R/2)ln2 and the T-linear dependence at intermediate temperatures. Our inelastic neutron scattering measurements further corroborate two distinct characters of fractionalized excitations: an Y-like, dispersive, magnetic continuum at higher energies and a dispersionless excitation at low energies around the Brillouin zone center. These dual features are well described by a Ferromagnetic Kitaev model, providing a smoking gun proof of the itinerant and localized Majorana fermions emergent in Kitaev magnets.

Proposal to probe quantum nonlocality of Majorana fermions in tunneling experiments

NASA Astrophysics Data System (ADS)

Sau, Jay D.; Swingle, Brian; Tewari, Sumanta

2015-07-01

Topological Majorana fermion (MF) quasiparticles have been recently suggested to exist in semiconductor quantum wires with proximity induced superconductivity and a Zeeman field. Although the experimentally observed zero bias tunneling peak and a fractional ac-Josephson effect can be taken as necessary signatures of MFs, neither of them constitutes a sufficient "smoking gun" experiment. Since one pair of Majorana fermions share a single conventional fermionic degree of freedom, MFs are in a sense fractionalized excitations. Based on this fractionalization we propose a tunneling experiment that furnishes a nearly unique signature of end state MFs in semiconductor quantum wires. In particular, we show that a "teleportation"-like experiment is not enough to distinguish MFs from pairs of MFs, which are equivalent to conventional zero energy states, but our proposed tunneling experiment, in principle, can make this distinction.

High-order Path Integral Monte Carlo methods for solving strongly correlated fermion problems

NASA Astrophysics Data System (ADS)

Chin, Siu A.

2015-03-01

In solving for the ground state of a strongly correlated many-fermion system, the conventional second-order Path Integral Monte Carlo method is plagued with the sign problem. This is due to the large number of anti-symmetric free fermion propagators that are needed to extract the square of the ground state wave function at large imaginary time. In this work, I show that optimized fourth-order Path Integral Monte Carlo methods, which uses no more than 5 free-fermion propagators, in conjunction with the use of the Hamiltonian energy estimator, can yield accurate ground state energies for quantum dots with up to 20 polarized electrons. The correlations are directly built-in and no explicit wave functions are needed. This work is supported by the Qatar National Research Fund NPRP GRANT #5-674-1-114.

Particle statistics and lossy dynamics of ultracold atoms in optical lattices

NASA Astrophysics Data System (ADS)

Yago Malo, J.; van Nieuwenburg, E. P. L.; Fischer, M. H.; Daley, A. J.

2018-05-01

Experimental control over ultracold quantum gases has made it possible to investigate low-dimensional systems of both bosonic and fermionic atoms. In closed one-dimensional systems there are many similarities in the dynamics of local quantities for spinless fermions and strongly interacting "hard-core" bosons, which on a lattice can be formalized via a Jordan-Wigner transformation. In this study, we analyze the similarities and differences for spinless fermions and hard-core bosons on a lattice in the presence of particle loss. The removal of a single fermion causes differences in local quantities compared with the bosonic case because of the different particle exchange symmetry in the two cases. We identify deterministic and probabilistic signatures of these dynamics in terms of local particle density, which could be measured in ongoing experiments with quantum gas microscopes.

Majorana fermion surface code for universal quantum computation

DOE PAGES

Vijay, Sagar; Hsieh, Timothy H.; Fu, Liang

2015-12-10

In this study, we introduce an exactly solvable model of interacting Majorana fermions realizing Z 2 topological order with a Z 2 fermion parity grading and lattice symmetries permuting the three fundamental anyon types. We propose a concrete physical realization by utilizing quantum phase slips in an array of Josephson-coupled mesoscopic topological superconductors, which can be implemented in a wide range of solid-state systems, including topological insulators, nanowires, or two-dimensional electron gases, proximitized by s-wave superconductors. Our model finds a natural application as a Majorana fermion surface code for universal quantum computation, with a single-step stabilizer measurement requiring no physicalmore » ancilla qubits, increased error tolerance, and simpler logical gates than a surface code with bosonic physical qubits. We thoroughly discuss protocols for stabilizer measurements, encoding and manipulating logical qubits, and gate implementations.« less

Realizing universal Majorana fermionic quantum computation

NASA Astrophysics Data System (ADS)

Wu, Ya-Jie; He, Jing; Kou, Su-Peng

2014-08-01

Majorana fermionic quantum computation (MFQC) was proposed by S. B. Bravyi and A. Yu. Kitaev [Ann. Phys. (NY) 298, 210 (2002), 10.1006/aphy.2002.6254], who indicated that a (nontopological) fault-tolerant quantum computer built from Majorana fermions may be more efficient than that built from distinguishable two-state systems. However, until now scientists have not known how to realize a MFQC in a physical system. In this paper we propose a possible realization of MFQC. We find that the end of a line defect of a p-wave superconductor or superfluid in a honeycomb lattice traps a Majorana zero mode, which becomes the starting point of MFQC. Then we show how to manipulate Majorana fermions to perform universal MFQC, which possesses possibilities for high-level local controllability through individually addressing the quantum states of individual constituent elements by using timely cold-atom technology.

Dirac fermions in an antiferromagnetic semimetal

DOE PAGES

Tang, Peizhe; Zhou, Quan; Xu, Gang; ...

2016-08-08

Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry and inversion symmetry. Here in this paper, we show that Dirac fermions can exist in one type of antiferromagnetic system, where both and are broken but their combination is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections andmore » demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.« less

Optical Selection Rule of Excitons in Gapped Chiral Fermion Systems

NASA Astrophysics Data System (ADS)

Zhang, Xiaoou; Shan, Wen-Yu; Xiao, Di

2018-02-01

We show that the exciton optical selection rule in gapped chiral fermion systems is governed by their winding number w , a topological quantity of the Bloch bands. Specifically, in a CN-invariant chiral fermion system, the angular momentum of bright exciton states is given by w ±1 +n N with n being an integer. We demonstrate our theory by proposing two chiral fermion systems capable of hosting dark s -like excitons: gapped surface states of a topological crystalline insulator with C4 rotational symmetry and biased 3 R -stacked MoS2 bilayers. In the latter case, we show that gating can be used to tune the s -like excitons from bright to dark by changing the winding number. Our theory thus provides a pathway to electrical control of optical transitions in two-dimensional material.

Role of quantum statistics in multi-particle decay dynamics

NASA Astrophysics Data System (ADS)

Marchewka, Avi; Granot, Er'el

2015-04-01

The role of quantum statistics in the decay dynamics of a multi-particle state, which is suddenly released from a confining potential, is investigated. For an initially confined double particle state, the exact dynamics is presented for both bosons and fermions. The time-evolution of the probability to measure two-particle is evaluated and some counterintuitive features are discussed. For instance, it is shown that although there is a higher chance of finding the two bosons (as oppose to fermions, and even distinguishable particles) at the initial trap region, there is a higher chance (higher than fermions) of finding them on two opposite sides of the trap as if the repulsion between bosons is higher than the repulsion between fermions. The results are demonstrated by numerical simulations and are calculated analytically in the short-time approximation. Furthermore, experimental validation is suggested.

Custodial isospin violation in the Lee-Wick standard model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chivukula, R. Sekhar; Farzinnia, Arsham; Foadi, Roshan

2010-05-01

We analyze the tension between naturalness and isospin violation in the Lee-Wick standard model (LW SM) by computing tree-level and fermionic one-loop contributions to the post-LEP electroweak parameters (S-circumflex, T-circumflex, W, and Y) and the Zb{sub L}b-bar{sub L} coupling. The model is most natural when the LW partners of the gauge bosons and fermions are light, but small partner masses can lead to large isospin violation. The post-LEP parameters yield a simple picture in the LW SM: the gauge sector contributes to Y and W only, with leading contributions arising at tree level, while the fermion sector contributes to S-circumflexmore » and T-circumflex only, with leading corrections arising at one loop. Hence, W and Y constrain the masses of the LW gauge bosons to satisfy M{sub 1}, M{sub 2} > or approx. 2.4 TeV at 95% C.L. Likewise, experimental limits on T-circumflex reveal that the masses of the LW fermions must satisfy M{sub q}, M{sub t} > or approx. 1.6 TeV at 95% C.L. if the Higgs mass is light and tend to exclude the LW SM for any LW fermion masses if the Higgs mass is heavy. Contributions from the top-quark sector to the Zb{sub L}b{sub L} coupling can be even more stringent, placing a lower bound of 4 TeV on the LW fermion masses at 95% C.L.« less

Fermionic Spinon Theory of Square Lattice Spin Liquids near the Néel State

NASA Astrophysics Data System (ADS)

Thomson, Alex; Sachdev, Subir

2018-01-01

Quantum fluctuations of the Néel state of the square lattice antiferromagnet are usually described by a CP1 theory of bosonic spinons coupled to a U(1) gauge field, and with a global SU(2) spin rotation symmetry. Such a theory also has a confining phase with valence bond solid (VBS) order, and upon including spin-singlet charge-2 Higgs fields, deconfined phases with Z2 topological order possibly intertwined with discrete broken global symmetries. We present dual theories of the same phases starting from a mean-field theory of fermionic spinons moving in π flux in each square lattice plaquette. Fluctuations about this π -flux state are described by (2 +1 )-dimensional quantum chromodynamics (QCD3 ) with a SU(2) gauge group and Nf=2 flavors of massless Dirac fermions. It has recently been argued by Wang et al. [Deconfined Quantum Critical Points: Symmetries and Dualities, Phys. Rev. X 7, 031051 (2017)., 10.1103/PhysRevX.7.031051] that this QCD3 theory describes the Néel-VBS quantum phase transition. We introduce adjoint Higgs fields in QCD3 and obtain fermionic dual descriptions of the phases with Z2 topological order obtained earlier using the bosonic CP1 theory. We also present a fermionic spinon derivation of the monopole Berry phases in the U(1) gauge theory of the VBS state. The global phase diagram of these phases contains multicritical points, and our results imply new boson-fermion dualities between critical gauge theories of these points.

Quasi-stationary states and fermion pair creation from a vacuum in supercritical Coulomb field

NASA Astrophysics Data System (ADS)

Khalilov, V. R.

2017-12-01

Creation of charged fermion pair from a vacuum in so-called supercritical Coulomb potential is examined for the case when fermions can move only in the same (one) plane. In which case, quantum dynamics of charged massive or massless fermions can be described by the two-dimensional Dirac Hamiltonians with an usual (-a/r) Coulomb potential. These Hamiltonians are singular and require the additional definition in order for them to be treated as self-adjoint quantum-mechanical operators. We construct the self-adjoint two-dimensional Dirac Hamiltonians with a Coulomb potential and determine the quantum-mechanical states for such Hamiltonians in the corresponding Hilbert spaces of square-integrable functions. We determine the scattering amplitude in which the self-adjoint extension parameter is incorporated and then obtain equations implicitly defining possible discrete energy spectra of the self-adjoint Dirac Hamiltonians with a Coulomb potential. It is shown that this quantum system becomes unstable in the presence of a supercritical Coulomb potential which manifests in the appearance of quasi-stationary states in the lower (negative) energy continuum. The energy spectrum of those states is quasi-discrete, consists of broadened levels with widths related to the inverse lifetimes of the quasi-stationary states as well as the probability of creation of charged fermion pair by a supercritical Coulomb field. Explicit analytical expressions for the creation probabilities of charged (massive or massless) fermion pair are obtained in a supercritical Coulomb field.

Transition and Damping of Collective Modes in a Trapped Fermi Gas between BCS and Unitary Limits near the Phase Transition

PubMed Central

Dong, Hang; Zhang, Wenyuan; Zhou, Li; Ma, Yongli

2015-01-01

We investigate the transition and damping of low-energy collective modes in a trapped unitary Fermi gas by solving the Boltzmann-Vlasov kinetic equation in a scaled form, which is combined with both the T-matrix fluctuation theory in normal phase and the mean-field theory in order phase. In order to connect the microscopic and kinetic descriptions of many-body Feshbach scattering, we adopt a phenomenological two-fluid physical approach, and derive the coupling constants in the order phase. By solving the Boltzmann-Vlasov steady-state equation in a variational form, we calculate two viscous relaxation rates with the collision probabilities of fermion’s scattering including fermions in the normal fluid and fermion pairs in the superfluid. Additionally, by considering the pairing and depairing of fermions, we get results of the frequency and damping of collective modes versus temperature and s-wave scattering length. Our theoretical results are in a remarkable agreement with the experimental data, particularly for the sharp transition between collisionless and hydrodynamic behaviour and strong damping between BCS and unitary limits near the phase transition. The sharp transition originates from the maximum of viscous relaxation rate caused by fermion-fermion pair collision at the phase transition point when the fermion depair, while the strong damping due to the fast varying of the frequency of collective modes from BCS limit to unitary limit. PMID:26522094

Threshold singularities in a Fermi gas with attractive potential in one dimension

DOE PAGES

Schlottmann, P.; Zvyagin, A. A.

2015-01-15

We consider the one-dimensional gas of fermions with spin S interacting via an attractive δ-function potential using the Bethe Ansatz solution. In zero magnetic field the atoms form bound states of N=2S + 1 fermions, i.e. generalized Cooper states with each atom having a different spin component. For low energy excitations the system is a Luttinger liquid and is properly described by a conformal field theory with conformal charge c=1. The linear dispersion of a Luttinger liquid is asymptotically exact in the low-energy limit where the band curvature terms in the dispersion are irrelevant. For higher energy excitations, however, themore » spectral function displays deviations in the neighborhood of the single-particle (hole) energy, which can be described by an effective X-ray edge type model. Using the Bethe Ansatz solution we obtain expressions for the critical exponents for the single-particle (hole) Green’s function. This model can be relevant in the context of ultracold atoms with effective total spin S confined to an elongated optical trap.« less

Degeneracy breakdown as a source of supernovae Ia

NASA Astrophysics Data System (ADS)

Clavelli, L.

2018-01-01

In a confined system of multiple Fermions, the particles are forced into high energy levels by the Pauli Exclusion Principle. We refer to this system as a Pauli tower. We pursue the investigation of a model for sub-Chandrasekhar supernovae Ia explosions (SNIa) in which the energy stored in the Pauli tower is released to trigger a nuclear deflagration. The simplest physical model for such a degeneracy breakdown and collapse of the Pauli tower is a phase transition to an exactly supersymmetric state in which the scalar partners of protons, neutrons, and leptons become degenerate with the familiar fermions of our world as in the supersymmetric standard model with susy breaking parameters relaxed to zero. We focus on the ability of the susy phase transition model to fit the total SNIa rate as well as the delay time distribution of SNIa after the birth of a progenitor white dwarf. We also study the ejected mass distribution and its correlation with delay time. Finally, we discuss the expected SNIa remnant in the form of a black hole of roughly Jupiter mass and the prospects for detecting such remnants.

Matter-antimatter asymmetry and dark matter from torsion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poplawski, Nikodem J.

2011-04-15

We propose a simple scenario which explains the observed matter-antimatter imbalance and the origin of dark matter in the Universe. We use the Einstein-Cartan-Sciama-Kibble theory of gravity which naturally extends general relativity to include the intrinsic spin of matter. Spacetime torsion produced by spin generates, in the classical Dirac equation, the Hehl-Datta term which is cubic in spinor fields. We show that under a charge-conjugation transformation this term changes sign relative to the mass term. A classical Dirac spinor and its charge conjugate therefore satisfy different field equations. Fermions in the presence of torsion have higher energy levels than antifermions,more » which leads to their decay asymmetry. Such a difference is significant only at extremely high densities that existed in the very early Universe. We propose that this difference caused a mechanism, according to which heavy fermions existing in such a Universe and carrying the baryon number decayed mostly to normal matter, whereas their antiparticles decayed mostly to hidden antimatter which forms dark matter. The conserved total baryon number of the Universe remained zero.« less

Physician Impact on the Total Cost of Care

PubMed Central

Taheri, Paul A.; Butz, David; Griffes, Louisa C.; Morlock, David R.; Greenfield, Lazar J.

2000-01-01

Background and Objectives Physicians’ efforts at cost containment focus on decreased resource utilization and reduced length of stay. Although these efforts appear to be appropriate, little data exist to gauge their success. As such, the goal of this study is to determine trauma service cost allocations and how this information can help physicians to contain costs. Materials and Methods The authors analyzed the costs for 696 trauma admissions at a level I trauma center for fiscal year 1997. Data were obtained from the hospital costing system. Costs analyzed were variable direct, fixed direct, and Indirect costs. Together, the fixed and indirect costs are referred to as “hospital overhead.” Total Cost equals variable direct plus fixed direct plus indirect costs. Results The mean variable, fixed, and indirect costs per patient were $7,998, $3,534, and $11,086, respectively. Mean total cost per patient was $22,618. Conclusion The 35% variable direct cost represents the percentage of total cost that is typically under the immediate influence of physicians, in contrast to the 65% of total cost over which physicians have little control. Physicians must gain a better understanding of cost drivers and must participate in the operations and allocations of institutional fixed direct and indirect costs if the overall cost of care is to be reduced. PMID:10714637

Domain wall fermion and CP symmetry breaking

NASA Astrophysics Data System (ADS)

Fujikawa, Kazuo; Suzuki, Hiroshi

2003-02-01

We examine the CP properties of chiral gauge theory defined by a formulation of the domain wall fermion, where the light field variables q and q¯ together with Pauli-Villars fields Q and Q¯ are utilized. It is shown that this domain wall representation in the infinite flavor limit N=∞ is valid only in the topologically trivial sector, and that the conflict among lattice chiral symmetry, strict locality and CP symmetry still persists for finite lattice spacing a. The CP transformation generally sends one representation of lattice chiral gauge theory into another representation of lattice chiral gauge theory, resulting in the inevitable change of propagators. A modified form of lattice CP transformation motivated by the domain wall fermion, which keeps the chiral action in terms of the Ginsparg-Wilson fermion invariant, is analyzed in detail; this provides an alternative way to understand the breaking of CP symmetry at least in the topologically trivial sector. We note that the conflict with CP symmetry could be regarded as a topological obstruction. We also discuss the issues related to the definition of Majorana fermions in connection with the supersymmetric Wess-Zumino model on the lattice.

Flavorful Z‧ signatures at LHC and ILC

NASA Astrophysics Data System (ADS)

Chen, Shao-Long; Okada, Nobuchika

2008-10-01

There are lots of new physics models which predict an extra neutral gauge boson, referred as Z‧-boson. In a certain class of these new physics models, the Z‧-boson has flavor-dependent couplings with the fermions in the Standard Model (SM). Based on a simple model in which couplings of the SM fermions in the third generation with the Z‧-boson are different from those of the corresponding fermions in the first two generations, we study the signatures of Z‧-boson at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). We show that at the LHC, the Z‧-boson with mass around 1 TeV can be produced through the Drell-Yan processes and its dilepton decay modes provide us clean signatures not only for the resonant production of Z‧-boson but also for flavor-dependences of the production cross sections. We also study fermion pair productions at the ILC involving the virtual Z‧-boson exchange. Even though the center-of-energy of the ILC is much lower than a Z‧-boson mass, the angular distributions and the forward-backward asymmetries of fermion pair productions show not only sizable deviations from the SM predictions but also significant flavor-dependences.

Direct observation of how the heavy-fermion state develops in CeCoIn5

NASA Astrophysics Data System (ADS)

Chen, Q. Y.; Xu, D. F.; Niu, X. H.; Jiang, J.; Peng, R.; Xu, H. C.; Wen, C. H. P.; Ding, Z. F.; Huang, K.; Shu, L.; Zhang, Y. J.; Lee, H.; Strocov, V. N.; Shi, M.; Bisti, F.; Schmitt, T.; Huang, Y. B.; Dudin, P.; Lai, X. C.; Kirchner, S.; Yuan, H. Q.; Feng, D. L.

2017-07-01

Heavy-fermion systems share some of the strange metal phenomenology seen in other unconventional superconductors, providing a unique opportunity to set strange metals in a broader context. Central to understanding heavy-fermion systems is the interplay of localization and itinerancy. These materials acquire high electronic masses and a concomitant Fermi volume increase as the f electrons delocalize at low temperatures. However, despite the wide-spread acceptance of this view, a direct microscopic verification has been lacking. Here we report high-resolution angle-resolved photoemission measurements on CeCoIn5, a prototypical heavy-fermion compound, which spectroscopically resolve the development of band hybridization and the Fermi surface expansion over a wide temperature region. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized even at the lowest temperature. These findings point to an unanticipated role played by crystal-field excitations in the strange metal behavior of CeCoIn5. Our results offer a comprehensive experimental picture of the heavy-fermion formation, setting the stage for understanding the emergent properties, including unconventional superconductivity, in this and related materials.

Inversion symmetry breaking induced triply degenerate points in orderly arranged PtSeTe family materials

NASA Astrophysics Data System (ADS)

Xiao, R. C.; Cheung, C. H.; Gong, P. L.; Lu, W. J.; Si, J. G.; Sun, Y. P.

2018-06-01

k paths exactly with symmetry allow to find triply degenerate points (TDPs) in band structures. The paths that host the type-II Dirac points in PtSe2 family materials also have the spatial symmetry. However, due to Kramers degeneracy (the systems have both inversion symmetry and time reversal symmetry), the crossing points in them are Dirac ones. In this work, based on symmetry analysis, first-principles calculations, and method, we predict that PtSe2 family materials should undergo topological transitions if the inversion symmetry is broken, i.e. the Dirac fermions in PtSe2 family materials split into TDPs in PtSeTe family materials (PtSSe, PtSeTe, and PdSeTe) with orderly arranged S/Se (Se/Te). It is different from the case in high-energy physics that breaking inversion symmetry I leads to the splitting of Dirac fermion into Weyl fermions. We also address a possible method to achieve the orderly arranged in PtSeTe family materials in experiments. Our study provides a real example that Dirac points transform into TDPs, and is helpful to investigate the topological transition between Dirac fermions and TDP fermions.

Fermion superfluid with hybridized s- and p-wave pairings

NASA Astrophysics Data System (ADS)

Zhou, LiHong; Yi, Wei; Cui, XiaoLing

2017-12-01

Ever since the pioneering work of Bardeen, Cooper and Schrieffer in the 1950s, exploring novel pairing mechanisms for fermion superfluids has become one of the central tasks in modern physics. Here, we investigate a new type of fermion superfluid with hybridized s- and p-wave pairings in an ultracold spin-1/2 Fermi gas. Its occurrence is facilitated by the co-existence of comparable s- and p-wave interactions, which is realizable in a two-component 40K Fermi gas with close-by s- and p-wave Feshbach resonances. The hybridized superfluid state is stable over a considerable parameter region on the phase diagram, and can lead to intriguing patterns of spin densities and pairing fields in momentum space. In particular, it can induce a phase-locked p-wave pairing in the fermion species that has no p-wave interactions. The hybridized nature of this novel superfluid can also be confirmed by measuring the s- and p-wave contacts, which can be extracted from the high-momentum tail of the momentum distribution of each spin component. These results enrich our knowledge of pairing superfluidity in Fermi systems, and open the avenue for achieving novel fermion superfluids with multiple partial-wave scatterings in cold atomic gases.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Levay, Peter; Nagy, Szilvia; Pipek, Janos

An elementary formula for the von Neumann and Renyi entropies describing quantum correlations in two-fermionic systems having four single-particle states is presented. An interesting geometric structure of fermionic entanglement is revealed. A connection with the generalized Pauli principle is established.

Quantum Algorithms for Computational Physics: Volume 3 of Lattice Gas Dynamics

DTIC Science & Technology

2007-01-03

time- dependent state |q(t)〉 of a two- energy level quantum mechanical system, which is a fermionic qubit and is governed by the Schroedinger wave...on-site ket of size 2B |Ψ〉 total system ket of size 2Q 2.2 The quantum state in the number representation From the previous section, a time- dependent ...duration depend on the particular experimental realization, so that the natural coupling along with the program of externally applied pulses together

Renyi entanglement entropy of interacting fermions calculated using the continuous-time quantum Monte Carlo method.

PubMed

Wang, Lei; Troyer, Matthias

2014-09-12

We present a new algorithm for calculating the Renyi entanglement entropy of interacting fermions using the continuous-time quantum Monte Carlo method. The algorithm only samples the interaction correction of the entanglement entropy, which by design ensures the efficient calculation of weakly interacting systems. Combined with Monte Carlo reweighting, the algorithm also performs well for systems with strong interactions. We demonstrate the potential of this method by studying the quantum entanglement signatures of the charge-density-wave transition of interacting fermions on a square lattice.

Resonant pairing between fermions with unequal masses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Shin-Tza; Pao, C.-H.; Yip, S.-K.

We study via mean-field theory the pairing between fermions of different masses, especially at the unitary limit. At equal populations, the thermodynamic properties are identical with the equal mass case provided an appropriate rescaling is made. At unequal populations, for sufficiently light majority species, the system does not phase separate. For sufficiently heavy majority species, the phase separated normal phase have a density larger than that of the superfluid. For atoms in harmonic traps, the density profiles for unequal mass fermions can be drastically different from their equal-mass counterparts.

Yang-Baxter and other relations for free-fermion and Ising models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davies, B.

1987-02-01

Eight-vertex, free fermion, and Ising models are formulated using a convention that emphasizes the algebra of the local transition operators that arise in the quantum inverse method. Equivalent classes of models, are investigated, with particular emphasis on the role of the star-triangle relations. Using these results, a natural and symmetrical parametrization is introduced and Yang-Baxter relations are constructed in an elementary way. The paper concludes with a consideration of duality, which links the present work to a recent paper of Baxter on the free fermion model.

The Emergence of Fermions and the E11 Content

NASA Astrophysics Data System (ADS)

Englert, François; Houart, Laurent

Claudio's warm and endearing personality adds to our admiration for his achievements in physics a sense of friendliness. His constant interest in fundamental questions motivated the following presentation of our attempt to understand the nature of fermions. This problem is an essential element of the quantum world and might be related to the quest for quantum gravity. We shall review how space-time fermions can emerge out of bosons in string theory and how this fact affects the extended Kac-Moody approach to the M-theory project.

Gauge and Non-Gauge Tensor Multiplets in 5D Conformal Supergravity

NASA Astrophysics Data System (ADS)

Kugo, T.; Ohashi, K.

2002-12-01

An off-shell formulation of two distinct tensor multiplets, a massive tensor multiplet and a tensor gauge multiplet, is presented in superconformal tensor calculus in five-dimensional space-time. Both contain a rank 2 antisymmetric tensor field, but there is no gauge symmetry in the former, while it is a gauge field in the latter. Both multiplets have 4 bosonic and 4 fermionic on-shell modes, but the former consists of 16 (boson)+16 (fermion) component fields, while the latter consists of 8 (boson)+8 (fermion) component fields.

Parametrization of fermion mixing matrices in Kobayashi-Maskawa form

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qin Nan; Ma Boqiang; Center for High Energy Physics, Peking University, Beijing 100871

2011-02-01

Recent works show that the original Kobayashi-Maskawa (KM) form of fermion mixing matrix exhibits some advantages, especially when discussing problems such as unitarity boomerangs and maximal CP violation hypothesis. Therefore, the KM form of fermion mixing matrix is systematically studied in this paper. Starting with a general triminimal expansion of the KM matrix, we discuss the triminimal and Wolfenstein-like parametrizations with different basis matrices in detail. The quark-lepton complementarity relations play an important role in our discussions on describing quark mixing and lepton mixing in a unified way.

Dark solitons with Majorana fermions in spin-orbit-coupled Fermi gases.

PubMed

Xu, Yong; Mao, Li; Wu, Biao; Zhang, Chuanwei

2014-09-26

We show that a single dark soliton can exist in a spin-orbit-coupled Fermi gas with a high spin imbalance, where spin-orbit coupling favors uniform superfluids over nonuniform Fulde-Ferrell-Larkin-Ovchinnikov states, leading to dark soliton excitations in highly imbalanced gases. Above a critical spin imbalance, two topological Majorana fermions without interactions can coexist inside a dark soliton, paving a way for manipulating Majorana fermions through controlling solitons. At the topological transition point, the atom density contrast across the soliton suddenly vanishes, suggesting a signature for identifying topological solitons.

Residual entanglement of accelerated fermions is not nonlocal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Friis, Nicolai; Koehler, Philipp; Bertlmann, Reinhold A.

2011-12-15

We analyze the operational meaning of the residual entanglement in noninertial fermionic systems in terms of the achievable violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality. We demonstrate that the quantum correlations of fermions, which were previously found to survive in the infinite acceleration limit, cannot be considered to be nonlocal. The entanglement shared by an inertial and an accelerated observer cannot be utilized for the violation of the CHSH inequality in case of high accelerations. Our results are shown to extend beyond the single-mode approximation commonly used in the literature.

Lorentz symmetry violation in the fermion number anomaly with the chiral overlap operator

NASA Astrophysics Data System (ADS)

Makino, Hiroki; Morikawa, Okuto

2016-12-01

Recently, Grabowska and Kaplan proposed a four-dimensional lattice formulation of chiral gauge theories on the basis of a chiral overlap operator. We compute the classical continuum limit of the fermion number anomaly in this formulation. Unexpectedly, we find that the continuum limit contains a term which is not Lorentz invariant. The term is, however, proportional to the gauge anomaly coefficient, and thus the fermion number anomaly in this lattice formulation automatically restores the Lorentz-invariant form when and only when the anomaly cancellation condition is met.

Majorana fermions and orthogonal complex structures

NASA Astrophysics Data System (ADS)

Calderón-García, J. S.; Reyes-Lega, A. F.

2018-05-01

Ground states of quadratic Hamiltonians for fermionic systems can be characterized in terms of orthogonal complex structures. The standard way in which such Hamiltonians are diagonalized makes use of a certain “doubling” of the Hilbert space. In this work, we show that this redundancy in the Hilbert space can be completely lifted if the relevant orthogonal structure is taken into account. Such an approach allows for a treatment of Majorana fermions which is both physically and mathematically transparent. Furthermore, an explicit connection between orthogonal complex structures and the topological ℤ2-invariant is given.

Electron-hole asymmetry, Dirac fermions, and quantum magnetoresistance in BaMnBi 2

DOE PAGES

Li, Lijun; Wang, Kefeng; Graf, D.; ...

2016-03-28

Here, we report two-dimensional quantum transport and Dirac fermions in BaMnBi 2 single crystals. BaMnBi 2 is a layered bad metal with highly anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, small cyclotron mass, and the first-principles band structure calculations indicate the presence of Dirac fermions in Bi square nets. Quantum oscillations in the Hall channel suggest the presence of both electron and hole pockets, whereas Dirac and parabolic states coexist at the Fermi level.

Aspect of Fermion Mass Hierarchy within Flavor Democracy for Yukawa Couplings

NASA Astrophysics Data System (ADS)

Higuchi, Katsuichi; Yamamoto, Katsuji

We discuss the fermion mass hierarchy by including vector-like fermions which are accommodated in E6 GUTs within flavor democracy for Yukawa couplings. In this framework, all Yukawa couplings for the standard Higgs doublet have the same strength, and all Yukawa couplings for the singlet Higgs have the same strength (New ansatz). In addition, singlet Higgs and right-handed neutrinos exist. Under this condition, the mass hierarchy mt ≫ mb ˜ mτ as well as mt ≫ mc, mu can be naturally explained.

Decoherence and Collisional Frequency Shifts of Trapped Bosons and Fermions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gibble, Kurt; LNE-SYRTE, Observatoire de Paris, 75014 Paris

2009-09-11

We perform exact calculations of collisional frequency shifts for several fermions or bosons using a singlet and triplet basis for pairs of particles. The 'factor of 2 controversy' for bosons becomes clear - the factor is always 2. Decoherence is described by singlet states and they are unaffected by spatially uniform clock fields. Spatial variations are critical, especially for fermions which were previously thought to be immune to collision shifts. The spatial variations lead to decoherence and a novel frequency shift that is not proportional to the partial density of internal states.

Foundations of heavy-fermion superconductivity: lattice Kondo effect and Mott physics

NASA Astrophysics Data System (ADS)

Steglich, Frank; Wirth, Steffen

2016-08-01

This article overviews the development of heavy-fermion superconductivity, notably in such rare-earth-based intermetallic compounds which behave as Kondo-lattice systems. Heavy-fermion superconductivity is of unconventional nature in the sense that it is not mediated by electron-phonon coupling. Rather, in most cases the attractive interaction between charge carriers is apparently magnetic in origin. Fluctuations associated with an antiferromagnetic (AF) quantum critical point (QCP) play a major role. The first heavy-fermion superconductor CeCu2Si2 turned out to be the prototype of a larger group of materials for which the underlying, often pressure-induced, AF QCP is likely to be of a three-dimensional (3D) spin-density-wave (SDW) variety. For UBe13, the second heavy-fermion superconductor, a magnetic-field-induced 3D SDW QCP inside the superconducting phase can be conjectured. Such a ‘conventional’, itinerant QCP can be well understood within Landau’s paradigm of order-parameter fluctuations. In contrast, the low-temperature normal-state properties of a few heavy-fermion superconductors are at odds with the Landau framework. They are characterized by an ‘unconventional’, local QCP which may be considered a zero-temperature 4 f-orbital selective Mott transition. Here, as concluded for YbRh2Si2, the breakdown of the Kondo effect concurring with the AF instability gives rise to an abrupt change of the Fermi surface. Very recently, superconductivity was discovered for this compound at ultra-low temperatures. Therefore, YbRh2Si2 along with CeRhIn5 under pressure provide a natural link between the large group of about fifty low-temperature heavy-fermion superconductors and other families of unconventional superconductors with substantially higher T c, e.g. the doped Mott insulators of the perovskite-type cuprates and the organic charge-transfer salts.

BCS to BEC evolution for mixtures of fermions with unequal masses

NASA Astrophysics Data System (ADS)

de Melo, Carlos A. R. Sa

2009-03-01

I discuss the zero and finite temperature phase diagrams of a mixture of fermions with unequal masses with and without population imbalance, which may correspond for example to mixtures of ^6Li and ^40K, ^6Li and ^87Sr, or ^40K and ^87Sr in the context of ultracold atoms. At zero temperature and when excess fermions are present, at least three phases may occur as the interaction parameter is changed from the BCS to the BEC regime. These phases correspond to normal, phase separation, or superfluid with coexistence between paired and excess fermions. The zero temperature phase diagram of population imbalance versus interaction parameter presents a remarkable asymmetry between the cases involving excess lighter or heavier fermions [1, 2], in sharp contrast with the symmetric phase diagram corresponding to the case of equal masses. At finite temperatures, the phase separation region of the phase diagram competes with superfluid regions possessing gapless elementary excitations [3] for certain ranges of the interaction parameter depending on the mass ratio. Furthermore, a phase transition may take place between two superfluid phases which are topologically distinct. The precise location of such transition is sensitive to the mass ratio between the two species of fermions. Signatures of this possible topological transition are present in the momentum distribution or structure factor, which may be measured experimentally in time-of-flight or through Bragg scattering, respectively. Lastly, throughout the evolution from BCS to BEC, I discuss the critical current and sound velocity for unequal mass systems as a function of interaction parameter and mass ratio. These quantities may also be measured via the same techniques already used in mixtures of fermions with equal masses. [1] M. Iskin, and C. A. R. Sa de Melo, Phys. Rev. Lett. 97, 100404 (2006). [2] M. Iskin and C. A. R. Sa de Melo, Phys. Rev. A 76, 013601 (2007). [3] Li Han, and C. A. R. Sa de Melo, arXiv:0812.xxxx

Fermions and Bosons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lincoln, Don

In particle physics, there are many different types of particles, mostly ending with the phrase “-on.” In this video, Fermilab’s Dr. Don Lincoln talks about fermions and bosons and what is the key difference between these two particles.

Fermion Cooper pairing with unequal masses: Standard field theory approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

He Lianyi; Jin Meng; Zhuang Pengfei

Fermion Cooper pairing with unequal masses is investigated in a standard field theory approach. We derived the superfluid density and Meissner mass squared of the U(1) gauge field in a general two-species model and found that the often used proportional relation between the two quantities is broken when the fermion masses are unequal. In the weak-coupling region, the superfluid density is always negative but the Meissner mass squared becomes mostly positive when the mass ratio between the pairing fermions is large enough. We established a proper momentum configuration of the LOFF pairing with unequal masses and showed that the LOFFmore » state is energetically favored due to the negative superfluid density. The single-plane-wave LOFF state is physically equivalent to an anisotropic state with a spontaneously generated superflow. The extension to a finite-range interaction is briefly discussed.« less

Flavor and topological current correlators in parity-invariant three-dimensional QED

NASA Astrophysics Data System (ADS)

Karthik, Nikhil; Narayanan, Rajamani

2017-09-01

We use lattice regularization to study the flow of the flavor-triplet fermion current central charge CJf from its free field value in the ultraviolet limit to its conformal value in the infrared limit of the parity-invariant three-dimensional QED with two flavors of two-component fermions. The dependence of CJf on the scale is weak with a tendency to be below the free field value at intermediate distances. Our numerical data suggest that the flavor-triplet fermion current and the topological current correlators become degenerate within numerical errors in the infrared limit, thereby supporting an enhanced O(4) symmetry predicted by strong self-duality. Further, we demonstrate that fermion dynamics is necessary for the scale-invariant behavior of parity-invariant three-dimensional QED by showing that the pure gauge theory with noncompact gauge action has a nonzero bilinear condensate.

Dark matter in E 6 Grand unification

NASA Astrophysics Data System (ADS)

Schwichtenberg, Jakob

2018-02-01

We discuss fermionic dark matter in non-supersymmetric E 6 Grand Unification. The fundamental representation of E 6 contains, in addition to the standard model fermions, exotic fermions and we argue that one of them is a viable, interesting dark matter candidate. Its stability is guaranteed by a discrete remnant symmetry, which is an unbroken subgroup of the E 6 gauge symmetry. We compute the symmetry breaking scales and the effect of possible threshold corrections by solving the renormalization group equations numerically after imposing gauge coupling unification. Since the Yukawa couplings of the exotic and the standard model fermions have a common origin, the mass of the dark matter particles is constrained. We find a mass range of 3 · 109 GeV ≲ m DM ≲ 1 · 1013 GeV for our E 6 dark matter candidate, which is within the reach of next-generation direct detection experiments.

Raman spectroscopic signature of fractionalized excitations in the harmonic-honeycomb iridates β- and γ-Li2IrO3

PubMed Central

Glamazda, A.; Lemmens, P.; Do, S. -H.; Choi, Y. S.; Choi, K. -Y.

2016-01-01

The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics. The Kitaev honeycomb spin model provides a prominent example of exotic fractionalized quasiparticles, composed of itinerant Majorana fermions and gapped gauge fluxes. However, identification of the Majorana fermions in a three-dimensional honeycomb lattice remains elusive. Here we report spectroscopic signatures of fractional excitations in the harmonic-honeycomb iridates β- and γ-Li2IrO3. Using polarization-resolved Raman spectroscopy, we find that the dynamical Raman response of β- and γ-Li2IrO3 features a broad scattering continuum with distinct polarization and composition dependence. The temperature dependence of the Raman spectral weight is dominated by the thermal damping of fermionic excitations. These results suggest the emergence of Majorana fermions from spin fractionalization in a three-dimensional Kitaev–Heisenberg system. PMID:27457278

Stationary states of fermions in a sign potential with a mixed vector–scalar coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Castilho, W.M., E-mail: castilho.w@gmail.com; Castro, A.S. de, E-mail: castro@pq.cnpq.br

2014-01-15

The scattering of a fermion in the background of a sign potential is considered with a general mixing of vector and scalar Lorentz structures with the scalar coupling stronger than or equal to the vector coupling under the Sturm–Liouville perspective. When the vector coupling and the scalar coupling have different magnitudes, an isolated solution shows that the fermion under a strong potential can be trapped in a highly localized region without manifestation of Klein’s paradox. It is also shown that the lonely bound-state solution disappears asymptotically as one approaches the conditions for the realization of spin and pseudospin symmetries. --more » Highlights: •Scattering of fermions in a sign potential assessed under a Sturm–Liouville perspective. •An isolated bounded solution. •No pair production despite the high localization. •No bounded solution under exact spin and pseudospin symmetries.« less

Experiments with Ultracold Quantum-degenerate Fermionic Lithium Atoms

NASA Technical Reports Server (NTRS)

Ketterle, Wolfgang

2003-01-01

Experimental methods of laser and evaporative cooling, used in the production of atomic Bose-Einstein condensates have recently been extended to realize quantum degeneracy in trapped Fermi gases. Fermi gases are a new rich system to explore the implications of Pauli exclusion on scattering properties of the system, and ultimately fermionic superfluidity. We have produced a new macroscopic quantum system, in which a degenerate Li-6 Fermi gas coexists with a large and stable Na-23 BEC. This was accomplished using inter-species sympathetic cooling of fermionic 6Li in a thermal bath of bosonic Na-23. We have achieved high numbers of both fermions (less than 10(exp 5) and bosons (less than 10(exp 6), and Li-6 quantum degeneracy corresponding to one half of the Fermi temperature. This is the first time that a Fermi sea was produced with a condensate as a "refrigerator".

Higher first Chern numbers in one-dimensional Bose-Fermi mixtures

NASA Astrophysics Data System (ADS)

Knakkergaard Nielsen, Kristian; Wu, Zhigang; Bruun, G. M.

2018-02-01

We propose to use a one-dimensional system consisting of identical fermions in a periodically driven lattice immersed in a Bose gas, to realise topological superfluid phases with Chern numbers larger than 1. The bosons mediate an attractive induced interaction between the fermions, and we derive a simple formula to analyse the topological properties of the resulting pairing. When the coherence length of the bosons is large compared to the lattice spacing and there is a significant next-nearest neighbour hopping for the fermions, the system can realise a superfluid with Chern number ±2. We show that this phase is stable in a large region of the phase diagram as a function of the filling fraction of the fermions and the coherence length of the bosons. Cold atomic gases offer the possibility to realise the proposed system using well-known experimental techniques.

Probing the fermionic Higgs portal at lepton colliders

DOE PAGES

Fedderke, Michael A.; Lin, Tongyan; Wang, Lian -Tao

2016-04-26

Here, we study the sensitivity of future electron-positron colliders to UV completions of the fermionic Higgs portal operator H †Hχ¯χ. Measurements of precision electroweak S and T parameters and the e +e – → Zh cross-section at the CEPC, FCC-ee, and ILC are considered. The scalar completion of the fermionic Higgs portal is closely related to the scalar Higgs portal, and we summarize existing results. We devote the bulk of our analysis to a singlet-doublet fermion completion. Assuming the doublet is sufficiently heavy, we construct the effective field theory (EFT) at dimension-6 in order to compute contributions to the observables.more » We also provide full one-loop results for S and T in the general mass parameter space. In both completions, future precision measurements can probe the new states at the (multi-)TeV scale, beyond the direct reach of the LHC.« less

Probing the fermionic Higgs portal at lepton colliders

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fedderke, Michael A.; Lin, Tongyan; Wang, Lian -Tao

Here, we study the sensitivity of future electron-positron colliders to UV completions of the fermionic Higgs portal operator H †Hχ¯χ. Measurements of precision electroweak S and T parameters and the e +e – → Zh cross-section at the CEPC, FCC-ee, and ILC are considered. The scalar completion of the fermionic Higgs portal is closely related to the scalar Higgs portal, and we summarize existing results. We devote the bulk of our analysis to a singlet-doublet fermion completion. Assuming the doublet is sufficiently heavy, we construct the effective field theory (EFT) at dimension-6 in order to compute contributions to the observables.more » We also provide full one-loop results for S and T in the general mass parameter space. In both completions, future precision measurements can probe the new states at the (multi-)TeV scale, beyond the direct reach of the LHC.« less

The massive fermion phase for the U(N) Chern-Simons gauge theory in D=3 at large N

DOE PAGES

Bardeen, William A.

2014-10-07

We explore the phase structure of fermions in the U(N) Chern-Simons Gauge theory in three dimensions using the large N limit where N is the number of colors and the fermions are taken to be in the fundamental representation of the U(N) gauge group. In the large N limit, the theory retains its classical conformal behavior and considerable attention has been paid to possible AdS/CFT dualities of the theory in the conformal phase. In this paper we present a solution for the massive phase of the fermion theory that is exact to the leading order of ‘t Hooft’s large Nmore » expansion. We present evidence for the spontaneous breaking of the exact scale symmetry and analyze the properties of the dilaton that appears as the Goldstone boson of scale symmetry breaking.« less

Phase Space Approach to Dynamics of Interacting Fermions

NASA Astrophysics Data System (ADS)

Davidson, Shainen; Sels, Dries; Kasper, Valentin; Polkovnikov, Anatoli

Understanding the behavior of interacting fermions is of fundamental interest in many fields ranging from condensed matter to high energy physics. Developing numerically efficient and accurate simulation methods is an indispensable part of this. Already in equilibrium, fermions are notoriously hard to handle due to the sign problem. Out of equilibrium, an important outstanding problem is the efficient numerical simulation of the dynamics of these systems. In this work we develop a new semiclassical phase-space approach (a.k.a. the truncated Wigner approximation) for simulating the dynamics of interacting lattice fermions in arbitrary dimensions. We demonstrate the strength of the method by comparing the results to exact diagonalization (ED) on small 1D and 2D systems. We furthermore present results on Many-Body Localized (MBL) systems in 1D and 2D, and demonstrate how the method can be used to determine the MBL transition.

Spin-polaron nature of fermion quasiparticles and their d-wave pairing in cuprate superconductors

NASA Astrophysics Data System (ADS)

Val'kov, V. V.; Dzebisashvili, D. M.; Barabanov, A. F.

2016-11-01

In the framework of the spin-fermion model, to which the Emery model is reduced in the limit of strong electron correlations, it is shown that the fermion quasiparticles in cuprate high- T c superconductors (HTSCs) arise under a strong effect of exchange coupling between oxygen holes and spins of copper ions. This underlies the spin-polaron nature of fermion quasiparticles in cuprate HTSCs. The Cooper instability with respect to the d-wave symmetry of the order parameter is revealed for an ensemble of such quasiparticles. For the normal phase, the spin-polaron concept allows us to reproduce the fine details in the evolution of the Fermi surface with the changes in the doping level x observed in experiment for La2-xSrxCuO4. The calculated T-x phase diagram correlates well with the available experimental data for cuprate HTSCs.

Interface and phase transition between Moore-Read and Halperin 331 fractional quantum Hall states: Realization of chiral Majorana fermion

NASA Astrophysics Data System (ADS)

Yang, Kun

2017-12-01

We consider an interface separating the Moore-Read state and Halperin 331 state in a half-filled Landau level, which can be realized in a double quantum well system with varying interwell tunneling and/or interaction strengths. In the presence of electron tunneling and strong Coulomb interactions across the interface, we find that all charge modes localize and the only propagating mode left is a chiral Majorana fermion mode. Methods to probe this neutral mode are proposed. A quantum phase transition between the Moore-Read and Halperin 331 states is described by a network of such Majorana fermion modes. In addition to a direct transition, they may also be separated by a phase in which the Majorana fermions are delocalized, realizing an incompressible state which exhibits quantum Hall charge transport and bulk heat conduction.

Exploring triplet-quadruplet fermionic dark matter at the LHC and future colliders

NASA Astrophysics Data System (ADS)

Wang, Jin-Wei; Bi, Xiao-Jun; Xiang, Qian-Fei; Yin, Peng-Fei; Yu, Zhao-Huan

2018-02-01

We study the signatures of the triplet-quadruplet dark matter model at the LHC and future colliders, including the 100 TeV Super Proton-Proton Collider and the 240 GeV Circular Electron Positron Collider. The dark sector in this model contains one fermionic electroweak triplet and two fermionic quadruplets, which have two kinds of Yukawa couplings to the Higgs doublet. Electroweak production signals of the dark sector fermions in the monojet+ ET, disappearing track, and multilepton+ET channels at the LHC and the Super Proton-Proton Collider are investigated. Moreover, we study the loop effects of this model on the Circular Electron Positron Collider precision measurements of e+e-→Z h and h →γ γ . We find that most of the parameter regions allowed by the observed dark matter relic density will be well explored by such direct and indirect searches at future colliders.

Cooling a band insulator with a metal: fermionic superfluid in a dimerized holographic lattice.

PubMed

Haldar, Arijit; Shenoy, Vijay B

2014-10-17

A cold atomic realization of a quantum correlated state of many fermions on a lattice, eg. superfluid, has eluded experimental realization due to the entropy problem. Here we propose a route to realize such a state using holographic lattice and confining potentials. The potentials are designed to produces a band insulating state (low heat capacity) at the trap center, and a metallic state (high heat capacity) at the periphery. The metal "cools" the central band insulator by extracting out the excess entropy. The central band insulator can be turned into a superfluid by tuning an attractive interaction between the fermions. Crucially, the holographic lattice allows the emergent superfluid to have a high transition temperature - even twice that of the effective trap temperature. The scheme provides a promising route to a laboratory realization of a fermionic lattice superfluid, even while being adaptable to simulate other many body states.

Peccei-Quinn symmetry for Dirac seesaw and leptogenesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gu, Pei-Hong

We extend the DFSZ invisible axion model to simultaneously explain small Dirac neutrino masses and cosmic matter-antimatter asymmetry. After the Peccei-Quinn and electroweak symmetry breaking, the effective Yukawa couplings of the Dirac neutrinos to the standard model Higgs scalar can be highly suppressed by the ratio of the vacuum expectation value of an iso-triplet Higgs scalar over the masses of some heavy gauge-singlet fermions, iso-doublet Higgs scalars or iso-triplet fermions. The iso-triplet fields can carry a zero or nonzero hypercharge. Through the decays of the heavy gauge-singlet fermions, iso-doublet scalars or iso-triplet fermions, we can obtain a lepton asymmetry inmore » the left-handed leptons and an opposite lepton asymmetry in the right-handed neutrinos. Since the right-handed neutrinos do not participate in the sphaleron processes, the left-handed lepton asymmetry can be partially converted to a baryon asymmetry.« less

Two Higgs doublets with fourth-generation fermions: Models for TeV-scale compositeness

DOE Office of Scientific and Technical Information (OSTI.GOV)

Soni A.; Bar-Shalom, S.; Nandi, S.

2011-09-21

We construct a class of two Higgs doublets models with a 4th sequential generation of fermions that may effectively accommodate the low-energy characteristics and phenomenology of a dynamical electroweak symmetry breaking scenario which is triggered by the condensates of the 4th family fermions. In particular, we single out the heavy quarks by coupling the heavier Higgs doublet ({Phi}{sub h}) which possesses a much larger VEV only to them while the lighter doublet ({Phi}{sub {ell}) couples only to the light fermions. We study the constraints on these models from precision electroweak data as well as from flavor data. We also discussmore » some distinct new features that have direct consequences on the production and decays of the 4th family quarks and leptons in high-energy colliders; in particular, the conventional search strategies for t{prime} and b{prime} may need to be significantly revised.« less

Correlated spin currents generated by resonant-crossed Andreev reflections in topological superconductors

PubMed Central

He, James J.; Wu, Jiansheng; Choy, Ting-Pong; Liu, Xiong-Jun; Tanaka, Y.; Law, K. T.

2014-01-01

Topological superconductors, which support Majorana fermion excitations, have been the subject of intense studies due to their novel transport properties and their potential applications in fault-tolerant quantum computations. Here we propose a new type of topological superconductors that can be used as a novel source of correlated spin currents. We show that inducing superconductivity on a AIII class topological insulator wire, which respects a chiral symmetry and supports protected fermionic end states, will result in a topological superconductor. This topological superconductor supports two topological phases with one or two Majorana fermion end states, respectively. In the phase with two Majorana fermions, the superconductor can split Cooper pairs efficiently into electrons in two spatially separated leads due to Majorana-induced resonant-crossed Andreev reflections. The resulting currents in the leads are correlated and spin-polarized. Importantly, the proposed topological superconductors can be realized using quantum anomalous Hall insulators in proximity to superconductors. PMID:24492649

Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions.

PubMed

Masuda, Hidetoshi; Sakai, Hideaki; Tokunaga, Masashi; Yamasaki, Yuichi; Miyake, Atsushi; Shiogai, Junichi; Nakamura, Shintaro; Awaji, Satoshi; Tsukazaki, Atsushi; Nakao, Hironori; Murakami, Youichi; Arima, Taka-hisa; Tokura, Yoshinori; Ishiwata, Shintaro

2016-01-01

For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm(2)/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.

Three-component fermions with surface Fermi arcs in tungsten carbide

NASA Astrophysics Data System (ADS)

Ma, J.-Z.; He, J.-B.; Xu, Y.-F.; Lv, B. Q.; Chen, D.; Zhu, W.-L.; Zhang, S.; Kong, L.-Y.; Gao, X.; Rong, L.-Y.; Huang, Y.-B.; Richard, P.; Xi, C.-Y.; Choi, E. S.; Shao, Y.; Wang, Y.-L.; Gao, H.-J.; Dai, X.; Fang, C.; Weng, H.-M.; Chen, G.-F.; Qian, T.; Ding, H.

2018-04-01

Topological Dirac and Weyl semimetals not only host quasiparticles analogous to the elementary fermionic particles in high-energy physics, but also have a non-trivial band topology manifested by gapless surface states, which induce exotic surface Fermi arcs1,2. Recent advances suggest new types of topological semimetal, in which spatial symmetries protect gapless electronic excitations without high-energy analogues3-11. Here, using angle-resolved photoemission spectroscopy, we observe triply degenerate nodal points near the Fermi level of tungsten carbide with space group P 6 ¯m 2 (no. 187), in which the low-energy quasiparticles are described as three-component fermions distinct from Dirac and Weyl fermions. We further observe topological surface states, whose constant-energy contours constitute pairs of `Fermi arcs' connecting to the surface projections of the triply degenerate nodal points, proving the non-trivial topology of the newly identified semimetal state.

Entanglement negativity bounds for fermionic Gaussian states

NASA Astrophysics Data System (ADS)

Eisert, Jens; Eisler, Viktor; Zimborás, Zoltán

2018-04-01

The entanglement negativity is a versatile measure of entanglement that has numerous applications in quantum information and in condensed matter theory. It can not only efficiently be computed in the Hilbert space dimension, but for noninteracting bosonic systems, one can compute the negativity efficiently in the number of modes. However, such an efficient computation does not carry over to the fermionic realm, the ultimate reason for this being that the partial transpose of a fermionic Gaussian state is no longer Gaussian. To provide a remedy for this state of affairs, in this work, we introduce efficiently computable and rigorous upper and lower bounds to the negativity, making use of techniques of semidefinite programming, building upon the Lagrangian formulation of fermionic linear optics, and exploiting suitable products of Gaussian operators. We discuss examples in quantum many-body theory and hint at applications in the study of topological properties at finite temperature.

Observation of Landau levels in potassium-intercalated graphite under a zero magnetic field

PubMed Central

Guo, Donghui; Kondo, Takahiro; Machida, Takahiro; Iwatake, Keigo; Okada, Susumu; Nakamura, Junji

2012-01-01

The charge carriers in graphene are massless Dirac fermions and exhibit a relativistic Landau-level quantization in a magnetic field. Recently, it has been reported that, without any external magnetic field, quantized energy levels have been also observed from strained graphene nanobubbles on a platinum surface, which were attributed to the Landau levels of massless Dirac fermions in graphene formed by a strain-induced pseudomagnetic field. Here we show the generation of the Landau levels of massless Dirac fermions on a partially potassium-intercalated graphite surface without applying external magnetic field. Landau levels of massless Dirac fermions indicate the graphene character in partially potassium-intercalated graphite. The generation of the Landau levels is ascribed to a vector potential induced by the perturbation of nearest-neighbour hopping, which may originate from a strain or a gradient of on-site potentials at the perimeters of potassium-free domains. PMID:22990864

Supersymmetry breaking and Nambu-Goldstone fermions with cubic dispersion

NASA Astrophysics Data System (ADS)

Sannomiya, Noriaki; Katsura, Hosho; Nakayama, Yu

2017-03-01

We introduce a lattice fermion model in one spatial dimension with supersymmetry (SUSY) but without particle number conservation. The Hamiltonian is defined as the anticommutator of two nilpotent supercharges Q and Q†. Each supercharge is built solely from spinless fermion operators and depends on a parameter g . The system is strongly interacting for small g , and in the extreme limit g =0 , the number of zero-energy ground states grows exponentially with the system size. By contrast, in the large-g limit, the system is noninteracting and SUSY is broken spontaneously. We study the model for modest values of g and show that under certain conditions spontaneous SUSY breaking occurs in both finite and infinite chains. We analyze the low-energy excitations both analytically and numerically. Our analysis suggests that the Nambu-Goldstone fermions accompanying the spontaneous SUSY breaking have cubic dispersion at low energies.

Nonperturbative quantization of the electroweak model's electrodynamic sector

NASA Astrophysics Data System (ADS)

Fry, M. P.

2015-04-01

Consider the Euclidean functional integral representation of any physical process in the electroweak model. Integrating out the fermion degrees of freedom introduces 24 fermion determinants. These multiply the Gaussian functional measures of the Maxwell, Z , W , and Higgs fields to give an effective functional measure. Suppose the functional integral over the Maxwell field is attempted first. This paper is concerned with the large amplitude behavior of the Maxwell effective measure. It is assumed that the large amplitude variation of this measure is insensitive to the presence of the Z , W , and H fields; they are assumed to be a subdominant perturbation of the large amplitude Maxwell sector. Accordingly, we need only examine the large amplitude variation of a single QED fermion determinant. To facilitate this the Schwinger proper time representation of this determinant is decomposed into a sum of three terms. The advantage of this is that the separate terms can be nonperturbatively estimated for a measurable class of large amplitude random fields in four dimensions. It is found that the QED fermion determinant grows faster than exp [c e2∫d4x Fμν 2] , c >0 , in the absence of zero mode supporting random background potentials. This raises doubt on whether the QED fermion determinant is integrable with any Gaussian measure whose support does not include zero mode supporting potentials. Including zero mode supporting background potentials can result in a decaying exponential growth of the fermion determinant. This is prima facie evidence that Maxwellian zero modes are necessary for the nonperturbative quantization of QED and, by implication, for the nonperturbative quantization of the electroweak model.

Relativistic Fermions Generated by Square Lattices in Layered Compounds

NASA Astrophysics Data System (ADS)

Mao, Zhiqiang

Recent discoveries of topological semimetals have generated immense interests since they represent new topological states of quantum matters. In this talk, I will present our recent studies on topological semimetals, which are focused on Dirac/Weyl fermions generated by square lattices in layered compounds. I will first report on our discoveries of two new Dirac materials Sr1-yMn1-zSb2 and BaMnSb2 in which nearly massless Dirac fermions are generated by 2D Sb layers. In Sr1-yMn1-zSb2, Dirac fermions are found to coexist with ferromagnetism, offering a rare opportunity to investigate the interplay between relativistic fermions and spontaneous time reversal symmetry breaking and explore a possible magnetic Weyl state. Then I will show our quantum oscillation studies on two new Dirac nodal line semimetals - ZrSiSe and ZrSiTe. We have not only revealed their signatures of nodal-line fermions, but also demonstrated that their atomically thin crystals are accessible via mechanical exfoliation, raising the possibility of realizing the theoretically predicted 2D topological insulators. Finally I will discuss exotic quantum transport behavior arising from the zeroth Landau level in Weyl semimetal YbMnBi2. This work is supported by the U.S. DOE under Grant No. DE-SC0014208 (support for the work on ZrSiSe and ZrSiTe) and DOE-EPSCoR Grant No. DE-SC0012432 with additional support from the Louisiana BoR (support for the work on (Sr/Ba)MnSb2 and YbMnBi2).

Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Hyunsoo; Wang, Kefeng; Nakajima, Yasuyuki

In all known fermionic super fluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is xed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or even septet pairs. Here, we report evidence of unconventional superconductivity emerging from a spin-3/2 quasiparticle electronic structure in the half-Heuslermore » semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ 8 band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic super fluid state. We propose a k ∙ p model of the j = 3/2 fermions to explain how a dominant J=3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically non-trivial band structure, the unconventional pairing in this system represents a truly novel form of super fluidity that has strong potential for leading the development of a new generation of topological superconductors.« less

Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal

DOE PAGES

Kim, Hyunsoo; Wang, Kefeng; Nakajima, Yasuyuki; ...

2018-04-06

In all known fermionic super fluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is xed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or even septet pairs. Here, we report evidence of unconventional superconductivity emerging from a spin-3/2 quasiparticle electronic structure in the half-Heuslermore » semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ 8 band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic super fluid state. We propose a k ∙ p model of the j = 3/2 fermions to explain how a dominant J=3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically non-trivial band structure, the unconventional pairing in this system represents a truly novel form of super fluidity that has strong potential for leading the development of a new generation of topological superconductors.« less

Ward identities and chiral anomalies for coupled fermionic chains

DOE Office of Scientific and Technical Information (OSTI.GOV)

Costa, L. C.; Ferraz, A.; Mastropietro, Vieri

2013-12-15

Coupled fermionic chains are usually described by an effective model written in terms of bonding and anti-bonding fermionic fields with linear dispersion in the vicinities of the respective Fermi points. We derive for the first time exact Ward Identities (WI) for this model, proving the existence of chiral anomalies which verify the Adler-Bardeen non-renormalization property. Such WI are expected to play a crucial role in the understanding of the thermodynamic properties of the system. Our results are non-perturbative and are obtained analyzing Grassmann functional integrals by means of constructive quantum field theory methods.

On the effective operators for Dark Matter annihilations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simone, Andrea De; Thamm, Andrea; Monin, Alexander

2013-02-01

We consider effective operators describing Dark Matter (DM) interactions with Standard Model fermions. In the non-relativistic limit of the DM field, the operators can be organized according to their mass dimension and their velocity behaviour, i.e. whether they describe s- or p-wave annihilations. The analysis is carried out for self-conjugate DM (real scalar or Majorana fermion). In this case, the helicity suppression at work in the annihilation into fermions is lifted by electroweak bremsstrahlung. We construct and study all dimension-8 operators encoding such an effect. These results are of interest in indirect DM searches.

Atomic quantum simulation of dynamical gauge fields coupled to fermionic matter: from string breaking to evolution after a quench.

PubMed

Banerjee, D; Dalmonte, M; Müller, M; Rico, E; Stebler, P; Wiese, U-J; Zoller, P

2012-10-26

Using a Fermi-Bose mixture of ultracold atoms in an optical lattice, we construct a quantum simulator for a U(1) gauge theory coupled to fermionic matter. The construction is based on quantum links which realize continuous gauge symmetry with discrete quantum variables. At low energies, quantum link models with staggered fermions emerge from a Hubbard-type model which can be quantum simulated. This allows us to investigate string breaking as well as the real-time evolution after a quench in gauge theories, which are inaccessible to classical simulation methods.

Composite fermion theory for bosonic quantum Hall states on lattices.

PubMed

Möller, G; Cooper, N R

2009-09-04

We study the ground states of the Bose-Hubbard model in a uniform magnetic field, motivated by the physics of cold atomic gases on lattices at high vortex density. Mapping the bosons to composite fermions (CF) leads to the prediction of quantum Hall fluids that have no counterpart in the continuum. We construct trial states for these phases and test numerically the predictions of the CF model. We establish the existence of strongly correlated phases beyond those in the continuum limit and provide evidence for a wider scope of the composite fermion approach beyond its application to the lowest Landau level.

Heavy Fermions

NASA Astrophysics Data System (ADS)

Sugiyama, K.; Ónuki, Y.

Recent experimental results of high-field magnetization and de Haas-van Alphen experiments in f-electron systems are presented. The magnetic moment and the electronic state are simultaneously discussed because both properties are connected with each other. The first example is a drastic change of the Fermi surface in the antiferromagnet NdIn3. The second is the metamagnetic transition based on the quadrupolar interaction in PrCu2. The third is the metamagnetic transition in a typical heavy fermion compound CeRu2Si2, together with the heavy fermion uranium compounds such as UPd2Al3, URu2Si2 and UPt3.

Minimally doubled fermions at one loop

NASA Astrophysics Data System (ADS)

Capitani, Stefano; Weber, Johannes; Wittig, Hartmut

2009-10-01

Minimally doubled fermions have been proposed as a cost-effective realization of chiral symmetry at non-zero lattice spacing. Using lattice perturbation theory at one loop, we study their renormalization properties. Specifically, we investigate the consequences of the breaking of hyper-cubic symmetry, which is a typical feature of this class of fermionic discretizations. Our results for the quark self-energy indicate that the four-momentum undergoes a renormalization which is linearly divergent. We also compute renormalization factors for quark bilinears, construct the conserved vector and axial-vector currents and verify that at one loop the renormalization factors of the latter are equal to one.

The fermionic projector in a time-dependent external potential: Mass oscillation property and Hadamard states

NASA Astrophysics Data System (ADS)

Finster, Felix; Murro, Simone; Röken, Christian

2016-07-01

We give a non-perturbative construction of the fermionic projector in Minkowski space coupled to a time-dependent external potential which is smooth and decays faster than quadratically for large times. The weak and strong mass oscillation properties are proven. We show that the integral kernel of the fermionic projector is of the Hadamard form, provided that the time integral of the spatial sup-norm of the potential satisfies a suitable bound. This gives rise to an algebraic quantum field theory of Dirac fields in an external potential with a distinguished pure quasi-free Hadamard state.

Duality and topology

NASA Astrophysics Data System (ADS)

Sacramento, P. D.; Vieira, V. R.

2018-04-01

Mappings between models may be obtained by unitary transformations with preservation of the spectra but in general a change in the states. Non-canonical transformations in general also change the statistics of the operators involved. In these cases one may expect a change of topological properties as a consequence of the mapping. Here we consider some dualities resulting from mappings, by systematically using a Majorana fermion representation of spin and fermionic problems. We focus on the change of topological invariants that results from unitary transformations taking as examples the mapping between a spin system and a topological superconductor, and between different fermionic systems.

Production of a Scalar Boson and a Fermion Pair in Arbitrarily Polarized e - e + Beams

NASA Astrophysics Data System (ADS)

Abdullayev, S. K.; Gojayev, M. Sh.; Nasibova, N. A.

2018-05-01

Within the framework of the Standard Model (Minimal Supersymmetric Standard Model) we consider the production of the scalar boson HSM (h; H) and a fermion pair ff- in arbitrarily polarized, counterpropagating electron-positron beams e - e + ⇒ HSM (h; H) ff-. Characteristic features of the behavior of the cross sections and polarization characteristics (right-left spin asymmetry, degree of longitudinal polarization of the fermion, and transverse spin asymmetry) are investigated and elucidated as functions of the energy of the electron-positron beams and the mass of the scalar boson.

Singlet fermionic dark matter with Veltman conditions

NASA Astrophysics Data System (ADS)

Kim, Yeong Gyun; Lee, Kang Young; Nam, Soo-hyeon

2018-07-01

We reexamine a renormalizable model of a fermionic dark matter with a gauge singlet Dirac fermion and a real singlet scalar which can ameliorate the scalar mass hierarchy problem of the Standard Model (SM). Our model setup is the minimal extension of the SM for which a realistic dark matter (DM) candidate is provided and the cancellation of one-loop quadratic divergence to the scalar masses can be achieved by the Veltman condition (VC) simultaneously. This model extension, although renormalizable, can be considered as an effective low-energy theory valid up to cut-off energies about 10 TeV. We calculate the one-loop quadratic divergence contributions of the new scalar and fermionic DM singlets, and constrain the model parameters using the VC and the perturbative unitarity conditions. Taking into account the invisible Higgs decay measurement, we show the allowed region of new physics parameters satisfying the recent measurement of relic abundance. With the obtained parameter set, we predict the elastic scattering cross section of the new singlet fermion into target nuclei for a direct detection of the dark matter. We also perform the full analysis with arbitrary set of parameters without the VC as a comparison, and discuss the implication of the constraints by the VC in detail.

Multiscale Monte Carlo equilibration: Two-color QCD with two fermion flavors

DOE PAGES

Detmold, William; Endres, Michael G.

2016-12-02

In this study, we demonstrate the applicability of a recently proposed multiscale thermalization algorithm to two-color quantum chromodynamics (QCD) with two mass-degenerate fermion flavors. The algorithm involves refining an ensemble of gauge configurations that had been generated using a renormalization group (RG) matched coarse action, thereby producing a fine ensemble that is close to the thermalized distribution of a target fine action; the refined ensemble is subsequently rethermalized using conventional algorithms. Although the generalization of this algorithm from pure Yang-Mills theory to QCD with dynamical fermions is straightforward, we find that in the latter case, the method is susceptible tomore » numerical instabilities during the initial stages of rethermalization when using the hybrid Monte Carlo algorithm. We find that these instabilities arise from large fermion forces in the evolution, which are attributed to an accumulation of spurious near-zero modes of the Dirac operator. We propose a simple strategy for curing this problem, and demonstrate that rapid thermalization--as probed by a variety of gluonic and fermionic operators--is possible with the use of this solution. Also, we study the sensitivity of rethermalization rates to the RG matching of the coarse and fine actions, and identify effective matching conditions based on a variety of measured scales.« less

Amplitudes in the N=4 supersymmetric Yang-Mills theory from quantum geometry of momentum space

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gorsky, A.

We discuss multiloop maximally helicity violating amplitudes in the N=4 supersymmetric Yang-Mills theory in terms of effective gravity in the momentum space with IR regulator branes as degrees of freedom. Kinematical invariants of external particles yield the moduli spaces of complex or Kahler structures which are the playgrounds for the Kodaira-Spencer or Kahler type gravity. We suggest fermionic representation of the loop maximally helicity violating amplitudes in the N=4 supersymmetric Yang-Mills theory assuming the identification of the IR regulator branes with Kodaira-Spencer fermions in the B model and Lagrangian branes in the A model. The two-easy mass box diagram ismore » related to the correlator of fermionic currents on the spectral curve in the B model or hyperbolic volume in the A model and it plays the role of a building block in the whole picture. The Bern-Dixon-Smirnov-like ansatz has the interpretation as the semiclassical limit of a fermionic correlator. It is argued that fermionic representation implies a kind of integrability on the moduli spaces. We conjecture the interpretation of the reggeon degrees of freedom in terms of the open strings stretched between the IR regulator branes.« less

Dark matter direct detection of a fermionic singlet at one loop

NASA Astrophysics Data System (ADS)

Herrero-García, Juan; Molinaro, Emiliano; Schmidt, Michael A.

2018-06-01

The strong direct detection limits could be pointing to dark matter - nucleus scattering at loop level. We study in detail the prototype example of an electroweak singlet (Dirac or Majorana) dark matter fermion coupled to an extended dark sector, which is composed of a new fermion and a new scalar. Given the strong limits on colored particles from direct and indirect searches we assume that the fields of the new dark sector are color singlets. We outline the possible simplified models, including the well-motivated cases in which the extra scalar or fermion is a Standard Model particle, as well as the possible connection to neutrino masses. We compute the contributions to direct detection from the photon, the Z and the Higgs penguins for arbitrary quantum numbers of the dark sector. Furthermore, we derive compact expressions in certain limits, i.e., when all new particles are heavier than the dark matter mass and when the fermion running in the loop is light, like a Standard Model lepton. We study in detail the predicted direct detection rate and how current and future direct detection limits constrain the model parameters. In case dark matter couples directly to Standard Model leptons we find an interesting interplay between lepton flavor violation, direct detection and the observed relic abundance.

Grassmann phase space methods for fermions. I. Mode theory

NASA Astrophysics Data System (ADS)

Dalton, B. J.; Jeffers, J.; Barnett, S. M.

2016-07-01

In both quantum optics and cold atom physics, the behaviour of bosonic photons and atoms is often treated using phase space methods, where mode annihilation and creation operators are represented by c-number phase space variables, with the density operator equivalent to a distribution function of these variables. The anti-commutation rules for fermion annihilation, creation operators suggest the possibility of using anti-commuting Grassmann variables to represent these operators. However, in spite of the seminal work by Cahill and Glauber and a few applications, the use of Grassmann phase space methods in quantum-atom optics to treat fermionic systems is rather rare, though fermion coherent states using Grassmann variables are widely used in particle physics. The theory of Grassmann phase space methods for fermions based on separate modes is developed, showing how the distribution function is defined and used to determine quantum correlation functions, Fock state populations and coherences via Grassmann phase space integrals, how the Fokker-Planck equations are obtained and then converted into equivalent Ito equations for stochastic Grassmann variables. The fermion distribution function is an even Grassmann function, and is unique. The number of c-number Wiener increments involved is 2n2, if there are n modes. The situation is somewhat different to the bosonic c-number case where only 2 n Wiener increments are involved, the sign of the drift term in the Ito equation is reversed and the diffusion matrix in the Fokker-Planck equation is anti-symmetric rather than symmetric. The un-normalised B distribution is of particular importance for determining Fock state populations and coherences, and as pointed out by Plimak, Collett and Olsen, the drift vector in its Fokker-Planck equation only depends linearly on the Grassmann variables. Using this key feature we show how the Ito stochastic equations can be solved numerically for finite times in terms of c-number stochastic quantities. Averages of products of Grassmann stochastic variables at the initial time are also involved, but these are determined from the initial conditions for the quantum state. The detailed approach to the numerics is outlined, showing that (apart from standard issues in such numerics) numerical calculations for Grassmann phase space theories of fermion systems could be carried out without needing to represent Grassmann phase space variables on the computer, and only involving processes using c-numbers. We compare our approach to that of Plimak, Collett and Olsen and show that the two approaches differ. As a simple test case we apply the B distribution theory and solve the Ito stochastic equations to demonstrate coupling between degenerate Cooper pairs in a four mode fermionic system involving spin conserving interactions between the spin 1 / 2 fermions, where modes with momenta - k , + k-each associated with spin up, spin down states, are involved.

Topics in elementary particle physics

NASA Astrophysics Data System (ADS)

Jin, Xiang

The author of this thesis discusses two topics in elementary particle physics: n-ary algebras and their applications to M-theory (Part I), and functional evolution and Renormalization Group flows (Part II). In part I, Lie algebra is extended to four different n-ary algebraic structure: generalized Lie algebra, Filippov algebra, Nambu algebra and Nambu-Poisson tensor; though there are still many other n-ary algebras. A natural property of Generalized Lie algebras — the Bremner identity, is studied, and proved with a totally different method from its original version. We extend Bremner identity to n-bracket cases, where n is an arbitrary odd integer. Filippov algebras do not focus on associativity, and are defined by the Fundamental identity. We add associativity to Filippov algebras, and give examples of how to construct Filippov algebras from su(2), bosonic oscillator, Virasoro algebra. We try to include fermionic charges into the ternary Virasoro-Witt algebra, but the attempt fails because fermionic charges keep generating new charges that make the algebra not closed. We also study the Bremner identity restriction on Nambu algebras and Nambu-Poisson tensors. So far, the only example 3-algebra being used in physics is the BLG model with 3-algebra A4, describing two M2-branes interactions. Its extension with Nambu algebra, BLG-NB model, is believed to describe infinite M2-branes condensation. Also, there is another propose for M2-brane interactions, the ABJM model, which is constructed by ordinary Lie algebra. We compare the symmetry properties between them, and discuss the possible approaches to include these three models into a grand unification theory. In Part II, we give an approximate solution for Schroeder's equations, based on series and conjugation methods. We use the logistic map as an example, and demonstrate that this approximate solution converges to known analytical solutions around the fixed point, around which the approximate solution is constructed. Although the closed-form solutions for Schroeder's equations can not always be approached analytically, by fitting the approximation solutions, one can still obtain closed-form solutions sometimes. Based on Schroeder's theory, approximate solutions for trajectories, velocities and potentials can also be constructed. The approximate solution is significantly useful to calculate the beta function in renormalization group trajectory. By "wrapping" the series solutions with the conjugations from different inverse functions, we generate different branches of the trajectory, and construct a counterexample for a folk theorem about limited cycles.

B-Parameters of 4-Fermion Operators from Lattice QCD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gupta, Rajan

1997-12-31

This talk summarizes the status of the calculations of B{sub K}, B{sub 7}, B{sub 8}, and B{sub s}, done in collaboration with T. Bhattacharya, C. Kilcup, and S. Sharpe. Results for staggered, Wilson, and Clover fermions are presented.

Simultaneous mobile- and fixed-bearing total knee replacement in the same patients. A prospective comparison of mid-term outcomes using a similar design of prosthesis.

PubMed

Kim, Y-H; Kim, D-Y; Kim, J-S

2007-07-01

We conducted a randomised prospective study to evaluate the clinical and radiological results of a mobile- and fixed-bearing total knee replacement of similar design in 174 patients who had bilateral simultaneous knee replacement. The mean follow-up was for 5.6 years (5.2 to 6.1). The total knee score, pain score, functional score and range of movement were not statistically different (p > 0.05) between the two groups. Osteolysis was not seen in any knee in either group. Two knees (1%) in the mobile-bearing group required revision because of infection; none in the fixed-bearing group needed revision. Excellent results can be achieved with both mobile- and fixed-bearing prostheses of similar design at mid-term follow-up. We could demonstrate no significant clinical advantage for a mobile bearing.

Universal relations with fermionic dark matter

NASA Astrophysics Data System (ADS)

Krut, A.; Argüelles, C. R.; Rueda, J. A.; Ruffini, R.

2018-01-01

We have recently introduced a new model for the distribution of dark matter (DM) in galaxies, the Ruffini-Argüelles-Rueda (RAR) model, based on a self-gravitating system of massive fermions at finite temperatures. The RAR model, for fermion masses above keV, successfully describes the DM halos in galaxies, and predicts the existence of a denser quantum core towards the center of each configuration. We demonstrate here, for the first time, that the introduction of a cutoff in the fermion phase-space distribution, necessary to account for galaxies finite size and mass, defines a new solution with a compact quantum core which represents an alternative to the central black hole (BH) scenario for SgrA*. For a fermion mass in the range 48keV ≤ mc2 ≤ 345keV, the DM halo distribution fulfills the most recent data of the Milky Way rotation curves while harbors a dense quantum core of 4×106M⊙ within the S2 star pericenter. In particular, for a fermion mass of mc2 ˜ 50keV the model is able to explain the DM halos from typical dwarf spheroidal to normal elliptical galaxies, while harboring dark and massive compact objects from ˜ 103M⊙ tp to 108M⊙ at their respective centers. The model is shown to be in good agreement with different observationally inferred universal relations, such as the ones connecting DM halos with supermassive dark central objects. Finally, the model provides a natural mechanism for the formation of supermassive BHs as heavy as few ˜ 108M⊙. We argue that larger BH masses (few ˜ 109-10M⊙) may be achieved by assuming subsequent accretion processes onto the above heavy seeds, depending on accretion efficiency and environment.

(1) Majorana fermions in pinned vortices; (2) Manipulating and probing Majorana fermions using superconducting circuits; and (3) Controlling a nanowire spin-orbit qubit via electric-dipole spin resonance

NASA Astrophysics Data System (ADS)

Nori, Franco

2014-03-01

We study a heterostructure which consists of a topological insulator and a superconductor with a hole. This system supports a robust Majorana fermion state bound to the vortex core. We study the possibility of using scanning tunneling spectroscopy (i) to detect the Majorana fermion in this setup and (ii) to study excited states bound to the vortex core. The Majorana fermion manifests itself as an H-dependent zero-bias anomaly of the tunneling conductance. The excited states spectrum differs from the spectrum of a typical Abrikosov vortex, providing additional indirect confirmation of the Majorana state observation. We also study how to manipulate and probe Majorana fermions using super-conducting circuits. In we consider a semiconductor nanowire quantum dot with strong spin-orbit coupling (SOC), which can be used to achieve a spin-orbit qubit. In contrast to a spin qubit, the spin-orbit qubit can respond to an external ac electric field, i.e., electric-dipole spin resonance. We develop a theory that can apply in the strong SOC regime. We find that there is an optimal SOC strength ηopt = √ 2/2, where the Rabi frequency induced by the ac electric field becomes maximal. Also, we show that both the level spacing and the Rabi frequency of the spin-orbit qubit have periodic responses to the direction of the external static magnetic field. These responses can be used to determine the SOC in the nanowire. FN is partly supported by the RIKEN CEMS, iTHES Project, MURI Center for Dynamic Magneto-Optics, JSPS-RFBR Contract No. 12-02-92100, Grant-in-Aid for Scientific Research (S), MEXT Kakenhi on Quantum Cybernetics, and the JSPS via its FIRST program.

Causal fermion systems as a candidate for a unified physical theory

NASA Astrophysics Data System (ADS)

Finster, Felix; Kleiner, Johannes

2015-07-01

The theory of causal fermion systems is an approach to describe fundamental physics. Giving quantum mechanics, general relativity and quantum field theory as limiting cases, it is a candidate for a unified physical theory. We here give a non-technical introduction.

Entanglement Entropy of the ν=1/2 Composite Fermion Non-Fermi Liquid State.

PubMed

Shao, Junping; Kim, Eun-Ah; Haldane, F D M; Rezayi, Edward H

2015-05-22

The so-called "non-Fermi liquid" behavior is very common in strongly correlated systems. However, its operational definition in terms of "what it is not" is a major obstacle for the theoretical understanding of this fascinating correlated state. Recently there has been much interest in entanglement entropy as a theoretical tool to study non-Fermi liquids. So far explicit calculations have been limited to models without direct experimental realizations. Here we focus on a two-dimensional electron fluid under magnetic field and filling fraction ν=1/2, which is believed to be a non-Fermi liquid state. Using a composite fermion wave function which captures the ν=1/2 state very accurately, we compute the second Rényi entropy using the variational Monte Carlo technique. We find the entanglement entropy scales as LlogL with the length of the boundary L as it does for free fermions, but has a prefactor twice that of free fermions.

Theory of a peristaltic pump for fermionic quantum fluids

NASA Astrophysics Data System (ADS)

Romeo, F.; Citro, R.

2018-05-01

Motivated by the recent developments in fermionic cold atoms and in nanostructured systems, we propose the model of a peristaltic quantum pump. Differently from the Thouless paradigm, a peristaltic pump is a quantum device that generates a particle flux as the effect of a sliding finite-size microlattice. A one-dimensional tight-binding Hamiltonian model of this quantum machine is formulated and analyzed within a lattice Green's function formalism on the Keldysh contour. The pump observables, as, e.g., the pumped particles per cycle, are studied as a function of the pumping frequency, the width of the pumping potential, the particles mean free path, and system temperature. The proposed analysis applies to arbitrary peristaltic potentials acting on fermionic quantum fluids confined to one dimension. These confinement conditions can be realized in nanostructured systems or, in a more controllable way, in cold atoms experiments. In view of the validation of the theoretical results, we describe the outcomes of the model considering a fermionic cold atoms system as a paradigmatic example.

Dirac Fermions in an Antiferromagnetic Semimetal

NASA Astrophysics Data System (ADS)

Tang, Peizhe; Zhou, Quan; Xu, Gang; Zhang, Shou-Cheng; Shou-Cheng Zhang's Group Team, Prof.

Analogues of the elementary particles have been extensively searched for in condensed matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low energy excitations in materials now known as Dirac semimetals. All the currently known Dirac semimetals are nonmagnetic with both time-reversal symmetry  and inversion symmetry "". Here we show that Dirac fermions can exist in one type of antiferromagnetic systems, where both  and "" are broken but their combination "" is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyze the robustness of the Dirac points under symmetry protections, and demonstrate its distinctive bulk dispersions as well as the corresponding surface states by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism. We acknowledge the DOE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515, NSF under Grant No.DMR-1305677 and FAME, one of six centers of STARnet.

Strongly-correlated crystal-field approach to heavy-fermion compounds and to 3d oxides

NASA Astrophysics Data System (ADS)

Radwanski, Ryszard; Ropka, Zofia

2005-03-01

The description of electronic and magnetic properties of real compounds like LaMnO3, LaCoO3, Na2V3O7, FeO, NdAl2 and ErNi5 as well as heavy-fermion superconductor UPd2Al3 and heavy-fermion metal YbRh2Si2, both zero-temperature ground state properties and thermodynamics, will be presented pointing out the existence of a discrete atomic-like low-energy, in the meV scale, electronic structure. This low-energy many-electron discrete atomic-like electronic structure is governed by very strong electron correlations, predominantly on-site, by the intra-atomic spin-orbit coupling and by details of the local surrounding (crystal-field interactions), but later is modified by inter-site interactions. Our studies indicate that there is the highest time to ``unquench'' the orbital moment in solid state physics in description of 3d-/4f-/5f-atom containing compounds and that heavy-fermion phenomena are of the relativistic origin.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Loh, Yen Lee; Randeria, Mohit; Trivedi, Nandini

The direct transition from an insulator to a superconductor (SC) in Fermi systems is a problem of long-standing interest, which necessarily goes beyond the standard BCS paradigm of superconductivity as a Fermi surface instability. We introduce here a simple, translationally invariant lattice fermion model that undergoes a SC-insulator transition (SIT) and elucidate its properties using analytical methods and quantum Monte Carlo simulations. We show that there is a fermionic band insulator to bosonic insulator crossover in the insulating phase and a BCS-to-BEC crossover in the SC. The SIT is always found to be from a bosonic insulator to a BEC-likemore » SC, with an energy gap for fermions that remains finite across the SIT. Hence, the energy scales that go critical at the SIT are the gap to pair excitations in the insulator and the superfluid stiffness in the SC. In addition to giving insight into important questions about the SIT in solid-state systems, our model should be experimentally realizable using ultracold fermions in optical lattices.« less

Pseudoscalar portal dark matter and new signatures of vector-like fermions

DOE PAGES

Fan, JiJi; Koushiappas, Savvas M.; Landsberg, Greg

2016-01-19

Fermionic dark matter interacting with the Standard Model sector through a pseudoscalar portal could evade the direct detection constraints while preserving a WIMP miracle. Here, we study the LHC constraints on the pseudoscalar production in simplified models with the pseudoscalar either dominantly coupled to b quarks ormore » $${{\\tau}}$$ leptons and explore their implications for the GeV excesses in gamma ray observations. We also investigate models with new vector-like fermions that could realize the simplfied models of pseudoscalar portal dark matter. Furthermore, these models yield new decay channels and signatures of vector-like fermions, for instance, bbb; b$${{\\tau}}$$ $${{\\tau}}$$, and $${{\\tau}}$$ $${{\\tau}}$$ $${{\\tau}}$$ resonances. Some of the signatures have already been strongly constrained by the existing LHC searches and the parameter space fitting the gamma ray excess is further restricted. Conversely, the pure $${{\\tau}}$$-rich final state is only weakly constrained so far due to the small electroweak production rate.« less

Superconductor-insulator transition and Fermi-Bose crossovers

DOE PAGES

Loh, Yen Lee; Randeria, Mohit; Trivedi, Nandini; ...

2016-05-31

The direct transition from an insulator to a superconductor (SC) in Fermi systems is a problem of long-standing interest, which necessarily goes beyond the standard BCS paradigm of superconductivity as a Fermi surface instability. We introduce here a simple, translationally invariant lattice fermion model that undergoes a SC-insulator transition (SIT) and elucidate its properties using analytical methods and quantum Monte Carlo simulations. We show that there is a fermionic band insulator to bosonic insulator crossover in the insulating phase and a BCS-to-BEC crossover in the SC. The SIT is always found to be from a bosonic insulator to a BEC-likemore » SC, with an energy gap for fermions that remains finite across the SIT. Hence, the energy scales that go critical at the SIT are the gap to pair excitations in the insulator and the superfluid stiffness in the SC. In addition to giving insight into important questions about the SIT in solid-state systems, our model should be experimentally realizable using ultracold fermions in optical lattices.« less

Strongly Correlated Metal Built from Sachdev-Ye-Kitaev Models

NASA Astrophysics Data System (ADS)

Song, Xue-Yang; Jian, Chao-Ming; Balents, Leon

2017-11-01

Prominent systems like the high-Tc cuprates and heavy fermions display intriguing features going beyond the quasiparticle description. The Sachdev-Ye-Kitaev (SYK) model describes a (0 +1 )D quantum cluster with random all-to-all four-fermion interactions among N fermion modes which becomes exactly solvable as N →∞ , exhibiting a zero-dimensional non-Fermi-liquid with emergent conformal symmetry and complete absence of quasiparticles. Here we study a lattice of complex-fermion SYK dots with random intersite quadratic hopping. Combining the imaginary time path integral with real time path integral formulation, we obtain a heavy Fermi liquid to incoherent metal crossover in full detail, including thermodynamics, low temperature Landau quasiparticle interactions, and both electrical and thermal conductivity at all scales. We find linear in temperature resistivity in the incoherent regime, and a Lorentz ratio L ≡(κ ρ /T ) varies between two universal values as a function of temperature. Our work exemplifies an analytically controlled study of a strongly correlated metal.

Fulde-Ferrell-Larkin-Ovchinnikov correlation and free fluids in the one-dimensional attractive Hubbard model

NASA Astrophysics Data System (ADS)

Cheng, Song; Yu, Yi-Cong; Batchelor, M. T.; Guan, Xi-Wen

2018-03-01

In this Rapid Communication, we show that low-energy macroscopic properties of the one-dimensional (1D) attractive Hubbard model exhibit two fluids of bound pairs and of unpaired fermions. Using the thermodynamic Bethe ansatz equations of the model, we first determine the low-temperature phase diagram and analytically calculate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing correlation function for the partially polarized phase. We then show that for such an FFLO-like state in the low-density regime the effective chemical potentials of bound pairs and unpaired fermions behave like two free fluids. Consequently, the susceptibility, compressibility, and specific heat obey simple additivity rules, indicating the "free" particle nature of interacting fermions on a 1D lattice. In contrast to the continuum Fermi gases, the correlation critical exponents and thermodynamics of the attractive Hubbard model essentially depend on two lattice interacting parameters. Finally, we study scaling functions, the Wilson ratio and susceptibility, which provide universal macroscopic properties and dimensionless constants of interacting fermions at low energy.

Δmix parameter in the overlap on domain-wall mixed action

NASA Astrophysics Data System (ADS)

Lujan, M.; Alexandru, A.; Chen, Y.; Draper, T.; Freeman, W.; Gong, M.; Lee, F. X.; Li, A.; Liu, K. F.; Mathur, N.

2012-07-01

A direct calculation of the mixed action parameter Δmix with valence overlap fermions on a domain-wall fermion sea is presented. The calculation is performed on four ensembles of the 2+1 flavor domain-wall gauge configurations: 243×64 (aml=0.005, a=0.114fm) and 323×64 (aml=0.004, 0.006, 0.008, a=0.085fm). For pion masses close to 300 MeV we find Δmix=0.030(6)GeV4 at a=0.114fm and Δmix=0.033(12)GeV4 at a=0.085fm. The results are quite independent of the lattice spacing and they are significantly smaller than the results for valence domain-wall fermions on asqtad sea or those of valence overlap fermions on clover sea. Combining the results extracted from these two ensembles, we get Δmix=0.030(6)(5)GeV4, where the first error is statistical and the second is the systematic error associated with the fitting method.

Topological superconductivity in monolayer transition metal dichalcogenides.

PubMed

Hsu, Yi-Ting; Vaezi, Abolhassan; Fischer, Mark H; Kim, Eun-Ah

2017-04-11

Theoretically, it has been known that breaking spin degeneracy and effectively realizing spinless fermions is a promising path to topological superconductors. Yet, topological superconductors are rare to date. Here we propose to realize spinless fermions by splitting the spin degeneracy in momentum space. Specifically, we identify monolayer hole-doped transition metal dichalcogenide (TMD)s as candidates for topological superconductors out of such momentum-space-split spinless fermions. Although electron-doped TMDs have recently been found superconducting, the observed superconductivity is unlikely topological because of the near spin degeneracy. Meanwhile, hole-doped TMDs with momentum-space-split spinless fermions remain unexplored. Employing a renormalization group analysis, we propose that the unusual spin-valley locking in hole-doped TMDs together with repulsive interactions selectively favours two topological superconducting states: interpocket paired state with Chern number 2 and intrapocket paired state with finite pair momentum. A confirmation of our predictions will open up possibilities for manipulating topological superconductors on the device-friendly platform of monolayer TMDs.

Baryon bags in strong coupling QCD

NASA Astrophysics Data System (ADS)

Gattringer, Christof

2018-04-01

We discuss lattice QCD with one flavor of staggered fermions and show that in the path integral the baryon contributions can be fully separated from quark and diquark contributions. The baryonic degrees of freedom (d.o.f.) are independent of the gauge field, and the corresponding free fermion action describes the baryons through the joint propagation of three quarks. The nonbaryonic dynamics is described by quark and diquark terms that couple to the gauge field. When evaluating the quark and diquark contributions in the strong coupling limit, the partition function completely factorizes into baryon bags and a complementary domain. Baryon bags are regions in space-time where the dynamics is described by a single free fermion made out of three quarks propagating coherently as a baryon. Outside the baryon bags, the relevant d.o.f. are monomers and dimers for quarks and diquarks. The partition sum is a sum over all baryon bag configurations, and for each bag, a free fermion determinant appears as a weight factor.

Dynamics of Fermionic Impurity in One Dimension

NASA Astrophysics Data System (ADS)

Guan, Huijie; Andrei, Natan

2014-03-01

We study the dynamics of a fermionic impurity propagating in a one dimensional infinite line. The system is described by the Gaudin-Yang Model and is exactly solvable by the Nested Bethe Ansatz. Starting from a generic initial state, we obtain the time evolution of the wavefunction by the Yudson Approach in which we expand the initial state with the Nested Bethe Ansatz solutions. One situation that we are interested in is where, initially, the impurity is embedded in host fermions with a lattice configuration and one remove the periodic potential at time zero. We calculate the density profile and correlation functions at a later time. Another situation is to shoot an impurity into a cloud of fermions and calculate the probability for it to pass through. While the repulsive case has been studied already[1], we extend it to the attractive case and study the role of bound states in the evolution. We are also interested in boson impurity problem, where not only impurity interacts with host particles, all host particles interact with each other.

Magnetoinfrared spectroscopy of Landau levels and Zeeman splitting of three-dimensional massless Dirac Fermions in ZrTe 5

DOE PAGES

R. Y. Chen; Gu, G. D.; Chen, Z. G.; ...

2015-10-22

We present a magnetoinfrared spectroscopy study on a newly identified three-dimensional (3D) Dirac semimetal ZrTe 5. We observe clear transitions between Landau levels and their further splitting under a magnetic field. Both the sequence of transitions and their field dependence follow quantitatively the relation expected for 3D massless Dirac fermions. The measurement also reveals an exceptionally low magnetic field needed to drive the compound into its quantum limit, demonstrating that ZrTe 5 is an extremely clean system and ideal platform for studying 3D Dirac fermions. The splitting of the Landau levels provides direct, bulk spectroscopic evidence that a relatively weakmore » magnetic field can produce a sizable Zeeman effect on the 3D Dirac fermions, which lifts the spin degeneracy of Landau levels. As a result, our analysis indicates that the compound evolves from a Dirac semimetal into a topological line-node semimetal under the current magnetic field configuration.« less

Quantum computing with Majorana fermion codes

NASA Astrophysics Data System (ADS)

Litinski, Daniel; von Oppen, Felix

2018-05-01

We establish a unified framework for Majorana-based fault-tolerant quantum computation with Majorana surface codes and Majorana color codes. All logical Clifford gates are implemented with zero-time overhead. This is done by introducing a protocol for Pauli product measurements with tetrons and hexons which only requires local 4-Majorana parity measurements. An analogous protocol is used in the fault-tolerant setting, where tetrons and hexons are replaced by Majorana surface code patches, and parity measurements are replaced by lattice surgery, still only requiring local few-Majorana parity measurements. To this end, we discuss twist defects in Majorana fermion surface codes and adapt the technique of twist-based lattice surgery to fermionic codes. Moreover, we propose a family of codes that we refer to as Majorana color codes, which are obtained by concatenating Majorana surface codes with small Majorana fermion codes. Majorana surface and color codes can be used to decrease the space overhead and stabilizer weight compared to their bosonic counterparts.

Atiyah-Patodi-Singer index from the domain-wall fermion Dirac operator

NASA Astrophysics Data System (ADS)

Fukaya, Hidenori; Onogi, Tetsuya; Yamaguchi, Satoshi

2017-12-01

The Atiyah-Patodi-Singer (APS) index theorem attracts attention for understanding physics on the surface of materials in topological phases. The mathematical setup for this theorem is, however, not directly related to the physical fermion system, as it imposes on the fermion fields a nonlocal boundary condition known as the "APS boundary condition" by hand, which is unlikely to be realized in the materials. In this work, we attempt to reformulate the APS index in a "physicist-friendly" way for a simple setup with U (1 ) or S U (N ) gauge group on a flat four-dimensional Euclidean space. We find that the same index as APS is obtained from the domain-wall fermion Dirac operator with a local boundary condition, which is naturally given by the kink structure in the mass term. As the boundary condition does not depend on the gauge fields, our new definition of the index is easy to compute with the standard Fujikawa method.

Z'→ggg decay in left-right symmetric models with three and four fermion families

NASA Astrophysics Data System (ADS)

Montaño, J.; Napsuciale, M.; Vaquera-Araujo, C. A.

2011-12-01

We study the Z'→q¯q,ggg decays in the context of a manifest left-right symmetric gauge theory with three and four generations. The Z' couplings to quarks are fixed essentially by the parameters of the standard model and we obtain Γ(Z'→qq¯)≈14GeV for MZ'≈1TeV. For the Z'→ggg decay and three families we obtain a branching ratio BR(Z'→ggg)=(Γ(Z'→ggg))/(Γ(Z'→qq¯))=1.2-2.8×10-5 for mZ'=700-1500GeV. The fourth generation produces an enhancement in the branching ratio for Z' masses close to the b¯'b' threshold and a dip for Z' masses close to the t¯'t' threshold. Using the values of the fourth-generation quark masses allowed by electroweak precision data, we obtain a branching ratio BR(Z'→ggg)=(1-6)×10-5 for mZ'=(700-1500)GeV.

Solving general gauge theories on inner product spaces

NASA Astrophysics Data System (ADS)

Batalin, Igor; Marnelius, Robert

1995-02-01

By means of a generalized quartet mechanism we show in a model independent way that a BRST quantization on an inner product space leads to physical states of the form ph> = exp [ Q, ψ]ph> 0 where Q is the nilpotent BRST operator, ψ a hermitian fermionic gauge-fixing operator, and ph> o BRST invariant states determined by a hermitian set of BRST doublets in involution. ph> 0 does not belong to an inner product space although ph> does. Since the BRST quartets are split into two sets of hermitian BRST doublets there are two choices for ph> 0 and the corresponding ψ. When applied to general, both irreducible and reducible, gauge theories of arbitrary rank within the BFV formulation we find that ph> 0 are trivial BRST invariant states which only depend on the matter variables for one set of solutions, and for the other set ph> 0 are solutions of a Dirac quantization. This generalizes previous Lie group solutions obtained by means of a bigrading.

Absence of thermalization in finite isolated interacting Floquet systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seetharam, Karthik; Titum, Paraj; Kolodrubetz, Michael

Conventional wisdom suggests that the long time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy non-thermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit non-thermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even withmore » no static component of the interaction, the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have non-thermal average doublon densities. Finally, we show that this non-thermal behavior arises due to emergent integrability at large interaction strength and discuss how the integrability breaks down with power-law dependence on system size.« less

Solutions of the two-dimensional Hubbard model: Benchmarks and results from a wide range of numerical algorithms

DOE PAGES

LeBlanc, J. P. F.; Antipov, Andrey E.; Becca, Federico; ...

2015-12-14

Numerical results for ground-state and excited-state properties (energies, double occupancies, and Matsubara-axis self-energies) of the single-orbital Hubbard model on a two-dimensional square lattice are presented, in order to provide an assessment of our ability to compute accurate results in the thermodynamic limit. Many methods are employed, including auxiliary-field quantum Monte Carlo, bare and bold-line diagrammatic Monte Carlo, method of dual fermions, density matrix embedding theory, density matrix renormalization group, dynamical cluster approximation, diffusion Monte Carlo within a fixed-node approximation, unrestricted coupled cluster theory, and multireference projected Hartree-Fock methods. Comparison of results obtained by different methods allows for the identification ofmore » uncertainties and systematic errors. The importance of extrapolation to converged thermodynamic-limit values is emphasized. Furthermore, cases where agreement between different methods is obtained establish benchmark results that may be useful in the validation of new approaches and the improvement of existing methods.« less

Absence of thermalization in finite isolated interacting Floquet systems

NASA Astrophysics Data System (ADS)

Seetharam, Karthik; Titum, Paraj; Kolodrubetz, Michael; Refael, Gil

2018-01-01

Conventional wisdom suggests that the long-time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal-entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy nonthermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit nonthermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even with no static component of the interaction, the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have nonthermal average doublon densities. We show that this nonthermal behavior arises due to emergent integrability at large interaction strength and discuss how the integrability breaks down with power-law dependence on system size.

Thermal modifications of charmonia and bottomonia from spatial correlation functions

NASA Astrophysics Data System (ADS)

Ding, Heng-Tong; Kaczmarek, Olaf; Kruse, Anna-lena; Mukherjee, Swagato; Ohno, Hiroshi; Sandmeyer, Hauke; Shu, Hai-Tao

2018-03-01

We present our study on the thermal modifications of charmonia and bottomonia from spatial correlation functions at zero and nonzero momenta in quenched QCD. To accommodate the heavy quarks on the lattice we performed simulations on very fine lattices at a fixed beta value corresponding to a lattice spacing a-1 = 22:8 GeV on 1923×32, 1923 × 48, 1923 × 56, 1923 × 64 and 1923 × 96 lattices using clover-improved Wilson fermions. These lattices correspond to temperatures of 2.25Tc, 1.50Tc, 1.25Tc, 1.10Tc and 0.75Tc. To increase the signal to noise ratio in the axial-vector and scalar channels we used multi-sources for the measurement of spatial correlation functions. By investigating on the differences between spatial and temporal correlators as well as the temperature dependence of screening masses we will discuss the thermal effects in different channels of quarkonium states. Besides this the dispersion relation of the screening mass at different momenta is also discussed.

Finite-size effects in Luther-Emery phases of Holstein and Hubbard models

NASA Astrophysics Data System (ADS)

Greitemann, J.; Hesselmann, S.; Wessel, S.; Assaad, F. F.; Hohenadler, M.

2015-12-01

The one-dimensional Holstein model and its generalizations have been studied extensively to understand the effects of electron-phonon interaction. The half-filled case is of particular interest, as it describes a transition from a metallic phase with a spin gap due to attractive backscattering to a Peierls insulator with charge-density-wave order. Our quantum Monte Carlo results support the existence of a metallic phase with dominant power-law charge correlations, as described by the Luther-Emery fixed point. We demonstrate that for Holstein and also for purely fermionic models the spin gap significantly complicates finite-size numerical studies, and explains inconsistent previous results for Luttinger parameters and phase boundaries. On the other hand, no such complications arise in spinless models. The correct low-energy theory of the spinful Holstein model is argued to be that of singlet bipolarons with a repulsive, mutual interaction. This picture naturally explains the existence of a metallic phase, but also implies that gapless Luttinger liquid theory is not applicable.

Absence of thermalization in finite isolated interacting Floquet systems

DOE PAGES

Seetharam, Karthik; Titum, Paraj; Kolodrubetz, Michael; ...

2018-01-29

Conventional wisdom suggests that the long time behavior of isolated interacting periodically driven (Floquet) systems is a featureless maximal entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to realize a Floquet many-body localized phase or working in a narrow region of drive frequencies to achieve glassy non-thermal behavior at long time. Here we show that in clean systems the Floquet eigenstates can exhibit non-thermal behavior due to finite system size. We consider a one-dimensional system of spinless fermions with nearest-neighbor interactions where the interaction term is driven. Interestingly, even withmore » no static component of the interaction, the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have non-thermal average doublon densities. Finally, we show that this non-thermal behavior arises due to emergent integrability at large interaction strength and discuss how the integrability breaks down with power-law dependence on system size.« less

[Effects of different vegetation restoration patterns on the diversity of soil nitrogen-fixing microbes in Hulunbeier sandy land, Inner Mongolia of North China].

PubMed

Li, Gang; Wang, Li-Juan; Li, Yu-Jie; Qiao, Jiang; Zhang, Hai-Fang; Song, Xiao-Long; Yang, Dian-Lin

2013-06-01

By using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and sequence analysis, this paper studied the nifH gene diversity and community structure of soil nitrogen-fixing microbes in Hulunbeier sandy land of Inner Mongolia under four years management of five vegetation restoration modes, i. e., mixed-planting of Agropyron cristatum, Hedysarum fruticosum, Caragana korshinskii, and Elymus nutans (ACHE) and of Agropyron cristatum and Hedysarum fruticosum (AC), and mono-planting of Caragana korshinskii (UC), Agropyron cristatum (UA), and Hedysarum fruticosum (UH), taking the bare land as the control (CK). There existed significant differences in the community composition of nitrogen-fixing microbes among the five vegetation restoration patterns. The Shannon index of the nifH gene was the highest under ACHE, followed by under AC, UC, UA, and UH, and the lowest in CK. Except that UH and CK had less difference in the Shannon index, the other four vegetation restoration modes had a significantly higher Shannon index than CK (P < 0.05). The phylogenetic analysis showed that the soil nitrogen-fixing microbes under UA, UH, and UC were mainly of cyanobacteria, but the soil nitrogen-fixing microbes under AC and ACHE changed obviously, mainly of proteobacteria, and also of cyanobacteria. The canonical correlation analysis showed that the soil total phosphorus, available phosphorus, total nitrogen, and nitrate nitrogen contents under the five vegetation restoration modes had significant effects on the nitrogen-fixing microbial communities, and there existed significant correlations among the soil total phosphorus, available phosphorus, total nitrogen, and nitrate nitrogen. It was suggested that the variations of the community composition of soil nitrogen-fixing microbes under the five vegetation restoration modes were resulted from the interactive and combined effects of the soil physical and chemical factors.

Vacuum Cherenkov radiation for Lorentz-violating fermions

NASA Astrophysics Data System (ADS)

Schreck, M.

2017-11-01

The current work focuses on the process of vacuum Cherenkov radiation for Lorentz-violating fermions that are described by the minimal standard-model extension (SME). To date, most considerations of this important hypothetical process have been restricted to Lorentz-violating photons, as the necessary theoretical tools for the SME fermion sector have not been available. With their development in a very recent paper, we are now in a position to compute the decay rates based on a modified Dirac theory. Two realizations of the Cherenkov process are studied. In the first scenario, the spin projection of the incoming fermion is assumed to be conserved, and in the second, the spin projection is allowed to flip. The first type of process is shown to be still forbidden for the dimensionful a and b coefficients where there are strong indications that it is energetically disallowed for the H coefficients, as well. However, it is rendered possible for the dimensionless c , d , e , f , and g coefficients. For large initial fermion energies, the decay rates for the c and d coefficients were found to grow linearly with momentum and to be linearly suppressed by the smallness of the Lorentz-violating coefficient where for the e , f , and g coefficients this suppression is even quadratic. The decay rates vanish in the vicinity of the threshold, as expected. The decay including a fermion spin-flip plays a role for the spin-nondegenerate operators and it was found to occur for the dimensionful b and H coefficients as well as for the dimensionless d and g . The characteristics of this process differ much from the properties of the spin-conserving one, e.g., there is no threshold. Based on experimental data of ultra-high-energy cosmic rays, new constraints on Lorentz violation in the quark sector are obtained from the thresholds. However, it does not seem to be possible to derive bounds from the spin-flip decays. This work reveals the usefulness of the quantum field theoretic methods recently developed to study the phenomenology of high-energy fermions within the framework of the SME.

Scaled lattice fermion fields, stability bounds, and regularity

NASA Astrophysics Data System (ADS)

O'Carroll, Michael; Faria da Veiga, Paulo A.

2018-02-01

We consider locally gauge-invariant lattice quantum field theory models with locally scaled Wilson-Fermi fields in d = 1, 2, 3, 4 spacetime dimensions. The use of scaled fermions preserves Osterwalder-Seiler positivity and the spectral content of the models (the decay rates of correlations are unchanged in the infinite lattice). In addition, it also results in less singular, more regular behavior in the continuum limit. Precisely, we treat general fermionic gauge and purely fermionic lattice models in an imaginary-time functional integral formulation. Starting with a hypercubic finite lattice Λ ⊂(aZ ) d, a ∈ (0, 1], and considering the partition function of non-Abelian and Abelian gauge models (the free fermion case is included) neglecting the pure gauge interactions, we obtain stability bounds uniformly in the lattice spacing a ∈ (0, 1]. These bounds imply, at least in the subsequential sense, the existence of the thermodynamic (Λ ↗ (aZ ) d) and the continuum (a ↘ 0) limits. Specializing to the U(1) gauge group, the known non-intersecting loop expansion for the d = 2 partition function is extended to d = 3 and the thermodynamic limit of the free energy is shown to exist with a bound independent of a ∈ (0, 1]. In the case of scaled free Fermi fields (corresponding to a trivial gauge group with only the identity element), spectral representations are obtained for the partition function, free energy, and correlations. The thermodynamic and continuum limits of the free fermion free energy are shown to exist. The thermodynamic limit of n-point correlations also exist with bounds independent of the point locations and a ∈ (0, 1], and with no n! dependence. Also, a time-zero Hilbert-Fock space is constructed, as well as time-zero, spatially pointwise scaled fermion creation operators which are shown to be norm bounded uniformly in a ∈ (0, 1]. The use of our scaled fields since the beginning allows us to extract and isolate the singularities of the free energy when a ↘ 0.

Liquid-gas phase transitions and C K symmetry in quantum field theories

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nishimura, Hiromichi; Ogilvie, Michael C.; Pangeni, Kamal

A general field-theoretic framework for the treatment of liquid-gas phase transitions is developed. Starting from a fundamental four-dimensional field theory at nonzero temperature and density, an effective three-dimensional field theory is derived. The effective field theory has a sign problem at finite density. Although finite density explicitly breaks charge conjugation C , there remains a symmetry under C K , where K is complex conjugation. Here, we consider four models: relativistic fermions, nonrelativistic fermions, static fermions and classical particles. The interactions are via an attractive potential due to scalar field exchange and a repulsive potential due to massive vector exchange.more » The field-theoretic representation of the partition function is closely related to the equivalence of the sine-Gordon field theory with a classical gas. The thermodynamic behavior is extracted from C K -symmetric complex saddle points of the effective field theory at tree level. In the cases of nonrelativistic fermions and classical particles, we find complex saddle point solutions but no first-order transitions, and neither model has a ground state at tree level. The relativistic and static fermions show a liquid-gas transition at tree level in the effective field theory. The liquid-gas transition, when it occurs, manifests as a first-order line at low temperature and high density, terminated by a critical end point. The mass matrix controlling the behavior of correlation functions is obtained from fluctuations around the saddle points. Due to the C K symmetry of the models, the eigenvalues of the mass matrix are not always real but can be complex. This then leads to the existence of disorder lines, which mark the boundaries where the eigenvalues go from purely real to complex. The regions where the mass matrix eigenvalues are complex are associated with the critical line. In the case of static fermions, a powerful duality between particles and holes allows for the analytic determination of both the critical line and the disorder lines. Depending on the values of the parameters, either zero, one, or two disorder lines are found. Our numerical results for relativistic fermions give a very similar picture.« less

Liquid-gas phase transitions and C K symmetry in quantum field theories

DOE PAGES

Nishimura, Hiromichi; Ogilvie, Michael C.; Pangeni, Kamal

2017-04-04

A general field-theoretic framework for the treatment of liquid-gas phase transitions is developed. Starting from a fundamental four-dimensional field theory at nonzero temperature and density, an effective three-dimensional field theory is derived. The effective field theory has a sign problem at finite density. Although finite density explicitly breaks charge conjugation C , there remains a symmetry under C K , where K is complex conjugation. Here, we consider four models: relativistic fermions, nonrelativistic fermions, static fermions and classical particles. The interactions are via an attractive potential due to scalar field exchange and a repulsive potential due to massive vector exchange.more » The field-theoretic representation of the partition function is closely related to the equivalence of the sine-Gordon field theory with a classical gas. The thermodynamic behavior is extracted from C K -symmetric complex saddle points of the effective field theory at tree level. In the cases of nonrelativistic fermions and classical particles, we find complex saddle point solutions but no first-order transitions, and neither model has a ground state at tree level. The relativistic and static fermions show a liquid-gas transition at tree level in the effective field theory. The liquid-gas transition, when it occurs, manifests as a first-order line at low temperature and high density, terminated by a critical end point. The mass matrix controlling the behavior of correlation functions is obtained from fluctuations around the saddle points. Due to the C K symmetry of the models, the eigenvalues of the mass matrix are not always real but can be complex. This then leads to the existence of disorder lines, which mark the boundaries where the eigenvalues go from purely real to complex. The regions where the mass matrix eigenvalues are complex are associated with the critical line. In the case of static fermions, a powerful duality between particles and holes allows for the analytic determination of both the critical line and the disorder lines. Depending on the values of the parameters, either zero, one, or two disorder lines are found. Our numerical results for relativistic fermions give a very similar picture.« less

Exact Boson-Fermion Duality on a 3D Euclidean Lattice

DOE PAGES

Chen, Jing-Yuan; Son, Jun Ho; Wang, Chao; ...

2018-01-05

The idea of statistical transmutation plays a crucial role in descriptions of the fractional quantum Hall effect. However, a recently conjectured duality between a critical boson and a massless two-component Dirac fermion extends this notion to gapless systems. This duality sheds light on highly nontrivial problems such as the half-filled Landau level, the superconductor-insulator transition, and surface states of strongly coupled topological insulators. Although this boson-fermion duality has undergone many consistency checks, it has remained unproven. Here, we describe the duality in a nonperturbative fashion using an exact UV mapping of partition functions on a 3D Euclidean lattice.