Unitarity and universality in nonrelativistic conformal field theory
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Many-Body Level Statistics of Single-Particle Quantum Chaos
We consider a noninteracting many-fermion system populating levels of a unitary random matrix ensemble (equivalent to the q = 2 complex Sachdev-Ye-Kitaev model)—a generic model of single-particle quantum chaos. We study the corresponding many-particle level statistics by calculating the spectral form factor analytically using algebraic methods of random matrix theory, and match it with an exact numerical simulation. Despite the integrability of the theory, the many-body spectral rigidity is found to have a surprisingly rich landscape. In particular, we find a residual repulsion of distant many-body levels stemming from single-particle chaos, together with islands of level attraction. These results are encoded in an exponential ramp in the spectral form factor, which we show to be a universal feature of nonergodic many-fermion systems embedded in a chaotic medium.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Transverse chiral magnetic photocurrent induced by linearly polarized light in mirror-symmetric Weyl semimetals
Not Available
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Entropy excess in strongly correlated Fermi systems near a quantum critical point
A system of interacting, identical fermions described by standard Landau Fermi-liquid (FL) theory can experience a rearrangement of its Fermi surface if the correlations grow sufficiently strong, as occurs at a quantum critical point where the effective mass diverges. As yet, this phenomenon defies full understanding, but salient aspects of the non-Fermi-liquid (NFL) behavior observed beyond the quantum critical point are still accessible within the general framework of the Landau quasiparticle picture. Self-consistent solutions of the coupled Landau equations for the quasiparticle momentum distribution n(p) and quasiparticle energy spectrum {epsilon}(p) are shown to exist in two distinct classes, depending on coupling strength and on whether the quasiparticle interaction is regular or singular at zero momentum transfer. One class of solutions maintains the idempotency condition n{sup 2}(p)=n(p) of standard FL theory at zero temperature T while adding pockets to the Fermi surface. The other solutions are characterized by a swelling of the Fermi surface and a flattening of the spectrum {epsilon}(p) over a range of momenta in which the quasiparticle occupancies lie between 0 and 1 even at T=0. The latter, non-idempotent solution is revealed by analysis of a Poincare mapping associated with the fundamental Landau equation connecting n(p) and {epsilon}(p) and validated by solution of a variational condition that yields the symmetry-preserving ground state. Significantly, this extraordinary solution carries the burden of a large temperature-dependent excess entropy down to very low temperatures, threatening violation of the Nernst Theorem. It is argued that certain low-temperature phase transitions, notably those involving Cooper-pair formation, offer effective mechanisms for shedding the entropy excess. Available measurements in heavy-fermion compounds provide concrete support for such a scenario. - Highlights: Black-Right-Pointing-Pointer Extension of Landau quasiparticle theory to strongly correlated Fermi systems. Black-Right-Pointing-Pointer Analysis of associated topological phase transitions at a quantum critical point. Black-Right-Pointing-Pointer Fermi surface rearrangement featuring hole pockets or emergence of flat bands. Black-Right-Pointing-Pointer Shedding of flat-band excess entropy via pairing transition at very low temperature.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Anomalous frequency and temperature-dependent scattering and Hund's coupling in the almost quantum critical heavy-fermion system CeFe 2 Ge 2
We present THz range optical conductivity data of a thin film of the near quantum critical heavy-fermion compound CeFe 2 Ge 2 . Our complex conductivity measurements find a deviation from conventional Drude-like transport in a temperature range previously reported to exhibit unconventional behavior. We calculate the frequency-dependent effective mass and scattering rate using an extended Drude model analysis. We find the inelastic scattering rate can be described by a temperature-dependent power law ω n(T) , where n(T) approaches ~1.0 ± 0.2 at 1.5 K. This is compared to the ρ ~ T 1.5 behavior claimed in dc resistivity data and the ρ ~ T 2 expected from Fermi-liquid theory. In addition to a low-temperature mass renormalization, we find an anomalous mass renormalization that persists to high temperature. Here, we attribute this to a Hund's coupling in the Fe states in a manner similar to that recently proposed in the ferropnictides. CeFe 2 Ge 2 appears to be a very interesting system where one may study the interplay between the usual 4f lattice Kondo effect and this Hund's enhanced Kondo effect in the 3d states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Effective Hamiltonian for surface states of topological insulator thin films with hexagonal warping
The effective Hamiltonian of the surface states on semi-infinite slabs of the topological insulators (TI) Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} require the addition of a cubic momentum hexagonal warping term on top of the usual Dirac fermion Hamiltonian in order to reproduce the experimentally measured constant energy contours at intermediate values of Fermi energy. In this work, we derive the effective Hamiltonian for the surface states of a Bi{sub 2}Se{sub 3} thin film incorporating the corresponding hexagonal warping terms. We then calculate the dispersion relation of the effective Hamiltonian and show that the hexagonal warping leads distorts the equal energy contours from the circular cross sections of the Dirac cones.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Chiral superconductivity from a parent Chern band and its non-Abelian generalization
Here, we propose a minimal model starting from a parent Chern band with quartic dispersion that can describe the spin-valley polarized electrons in rhombohedral tetralayer graphene. The interplay between repulsive and attractive interactions on top of that parent Chern band is studied. We conduct standard self-consistent mean-field calculations, and find a rich phase diagram that consists of metal, quantum anomalous Hall crystal, chiral topological superconductor, as well as trivial gapped Bose-Einstein condensate. In particular, there exists a topological phase transition from the chiral superconductor to the Bose-Einstein condensate at zero temperature. Motivated by the recent experimental and theoretical studies of composite Fermi liquid in rhombohedral stacked multilayer graphene, we further generalize the physical electron model to its composite fermion counterpart based on a field theory analysis. The chiral superconductor phase of the composite fermion becomes the non-abelian Moore-Read quantum Hall phase. We argue that a chiral (pseudo-)spin liquid phase can emerge in the vicinity of this Moore-Read quantum Hall phase. Our work suggests rhombohedral multilayer graphene as a potential platform for rich correlated topological phases.
Wang, Yan-Qi [University of Maryland, College Park↗
Fermionic tensor networks for higher-order topological insulators from charge pumping
Not Available
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Braiding statistics and link invariants of bosonic/fermionic topological quantum matter in 2+1 and 3+1 dimensions
Topological Quantum Field Theories (TQFTs) pertinent to some emergent low energy phenomena of condensed matter lattice models in 2+1 and 3+1 dimensions are explored. Many of our TQFTs are highly-interacting without free quadratic analogs. Some of our bosonic TQFTs can be regarded as the continuum field theory formulation of Dijkgraaf–Witten twisted discrete gauge theories. Other bosonic TQFTs beyond the Dijkgraaf–Witten description and all fermionic TQFTs (namely the spin TQFTs) are either higher-form gauge theories where particles must have strings attached, or fermionic discrete gauge theories obtained by gauging the fermionic Symmetry-Protected Topological states (SPTs). We analytically calculate both the Abelian and non-Abelian braiding statistics data of anyonic particle and string excitations in these theories, where the statistics data can one-to-one characterize the underlying topological orders of TQFTs. Namely, we derive path integral expectation values of links formed by line and surface operators in these TQFTs. The acquired link invariants include not only the familiar Aharonov–Bohm linking number, but also Milnor triple linking number in 3 dimensions, triple and quadruple linking numbers of surfaces, and intersection number of surfaces in 4 dimensions. We also construct new spin TQFTs with the corresponding knot/link invariants of Arf(–Brown–Kervaire), Sato–Levine and others. We propose a new relation between the fermionic SPT partition function and the Rokhlin invariant. As an example, we can use these invariants and other physical observables, including ground state degeneracy, reduced modular $S^{xy}$ and $T^{xy}$ matrices, and the partition function on $RP^3$ manifold, to identify all $\nu \in Z_8$ classes of 2+1 dimensional gauged $Z_2$-Ising-symmetric $Z^f_2$-fermionic Topological Superconductors (realized by stacking v layers of a pair of chiral and anti-chiral -wave superconductors [ p + ip and p - ip ], where boundary supports non-chiral Majorana–Weyl modes) with continuum spin-TQFTs.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Gravitational-wave-to-matter coupling of superfluid Fermi gases near unitarity
It is well known that gravitational waves distort equilibrium matter globally, making them amenable to detection with laser interferometers. Less well known is the fact that gravitational waves create local nonequilibrium stresses inside matter, which could conceivably lead to alternative detection methods. The gravitational wave-to-matter coupling κ is a transport coefficient depending on the material and is poorly known for most substances. Here, in the present work, we calculate κ for a superfluid Fermi gas near unitarity using large-N techniques, finding κ= $\frac{n}{12m}$, with n the number density and m the mass of the fermion, matching the result for free Dirac fermions at zero temperature. Our prediction is amenable to nonperturbative theoretical as well as experimental tests.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Interplay between Superconductivity and Magnetism in Fe1-xPdxTe
The love/hate relationship between superconductivity and magnetic ordering has fascinated the condensed matter physics community for a century. In the early days, magnetic impurities doped into a superconductor were found to quickly suppress superconductivity. Later, a variety of systems, such as cuprates, heavy fermions and Fe pnictides, show superconductivity in a narrow region near the border to antiferromagnetism (AFM) as a function of pressure or doping. On the other hand, the coexistence of superconductivity and ferromagnetic (FM) or AFM ordering is found in a few compounds (RRh4B4 (R = Nd, Sm, Tm, Er), R'Mo6X8 (R' = Tb, Dy, Er, Ho, and X = S, Se), UMGe (M = Ge, Rh, Co), CeCoIn5, EuFe2(As1-xPx)2 etc.), providing evidence for their compatibility. Here, we present a third situation, where superconductivity coexists with FM and near the border of AFM in Fe1-xPdxTe. The doping of Pd for Fe gradually suppresses the first-order AFM ordering at temperature TN/S, and turns into short-range (SR) AFM correlation with a characteristic peak in magnetic susceptibility at T'N. Superconductivity sets in when T'N reaches zero. However, there is a gigantic ferromagnetic dome imposed in the superconducting-AFM (SR) crossover regime. Such a system is ideal for studying the interplay between superconductivity and two types of magnetic interactions (FM and AFM).
Karki, A B↗
Investigation of the physical properties of the tetragonal CeMAl 4 Si 2 (M = Rh, Ir, Pt) compounds
The synthesis, crystal structure and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl 4 Si 2 (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for LaMAl 4 Si 2 (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl 4 blocks, separated by MAl 2 units, stacked along the c-axis. Both CeRhAl 4 Si 2 and CeIrAl 4 Si 2 order antiferromagnetically below T N1 = 14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature (T N2 = 9 and 14 K, respectively). CePtAl 4 Si 2 orders ferromagnetically below T C = 3 K with an ordered moment of μ sat = 0.8 μ B for a magnetic field applied perpendicular to the c-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Chiral kinematic theory and converse vortical effects
Response theories in condensed matter typically describe the response of an electron fluid to external electromagnetic fields, while perturbations on neutral particles are often designed to mimic such fields. Here, we study the response of fermions to a space-time-dependent velocity field, thereby sidestepping the issue of gauge charge. First, we use a semiclassical chiral kinematic theory to obtain the local density of current and extract the orbital magnetization. The theory immediately predicts a "converse vortical effect," defined as an orbital magnetization driven by linear velocity. It receives contributions from magnetic moments on the Fermi surface and the Berry curvature of the occupied bands. Then, transcending semiclassics via a complementary Kubo formalism reveals that the uniform limit of a clean system receives only the Berry curvature contribution while other limits sense the Fermi surface magnetic moments too. We propose CoSi as a candidate material and suggest magnetometry of a sample under a thermal gradient to detect the effect. Overall, our study sheds light on the effects of a space-time-dependent velocity field on electron fluids and paves the way for exploring quantum materials using new probes and perturbations.
Chen, Kai↗
Time-resolved plasma measurements in Ge-doped silica exposed to infrared femtosecond laser
Using a time-resolved interferometric technique, we study the laser-induced carrier-trapping dynamics in SiO{sub 2} and Ge-doped SiO{sub 2}. The fast trapping of electrons in the band gap is associated with the formation of self-trapped excitons (STE). The STE trapping is doping dependent in SiO{sub 2}. The mean trapping time of electrons excited in the conduction band was found to be significantly lower in Ge-doped silica (75 {+-} 5 fs) when compared to pure silica (155 {+-} 5 fs). At our concentration level, this indicates that the plasma properties are determined by the presence of easily ionizable states such as the presence of Ge atoms in the glass network. Therefore, we suggest that in Ge-doped silica there exist an additional trapping pathway that leads to a significantly faster excitons trapping and a higher plasma density when compared to undoped silica.
36 MATERIALS SCIENCE↗
Multichannel charge Kondo effect and non-Fermi-liquid fixed points in conventional and topological superconductor islands
We study multiterminal Majorana and conventional superconducting islands in the vicinity of the charge degeneracy point using bosonization and the numerical renormalization group. Both models map to the multichannel charge Kondo problem, but for noninteracting normal leads they flow to different non-Fermi-liquid fixed points at low temperatures. We compare and contrast both cases by numerically obtaining the full crossover to the low-temperature regime, and we predict distinctive transport signatures. Here, the differences between the two types of islands result from a crucial distinction between charge-2e and charge-e transfer in the conventional and topological case, respectively. In the conventional case, our results establish s-wave islands as a platform to study the intermediate multichannel Kondo fixed point. In the topological setup, the crossover temperature to non-Fermi-liquid behavior is relatively high as it is proportional to the level broadening, and the transport results are not sensitive to channel coupling anisotropy, moving away from the charge degeneracy point or including a small Majorana hybridization, which makes our proposal experimentally feasible.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Suppression of hybridization by Cd doping in CeCoIn 5
The interaction between superconductivity and magnetism in heavy fermion compounds has remained a challenge ever since the discovery of unconventional superconductivity in such systems. The heavy fermion Ce T In 5 ( T = Co,Rh,Ir) family offers a good platform to study the relationship between superconductivity and magnetism, whose ground state can be easily tuned by chemical doping, making the ambient-pressure experimental techniques like angle-resolved photoemission spectroscopy possible to address this old but engaging question. By Cd doping, the superconducting state of CeCoIn 5 can be smoothly tuned to an antiferromagnetic ground state, which can be reversed by applying pressure. In this study, we present an electronic structure study of CeCo ( In 0.85 Cd 0.15 ) 5 with an antiferromagnetic ground state and the results are compared with CeCoIn 5 . We found that the hybridization strength between the f electrons and conduction electrons in CeCoIn 5 has been suppressed by Cd doping, but still the Fermi surface includes a significant contribution from the Ce f electrons. Our results find that the electronic tuning effect by Cd doping arises from the change in the hybridization strength between the Ce f electrons and conduction electrons caused by the Cd dopant, and the subtle interplay between itinerancy and localization of the f electrons determines the ground state.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface
Borophene, a crystalline monolayer boron sheet, is a new two-dimensional (2D) quantum material, predicted to feature tunable structure, intriguing physics and to find applications in flexible electronics, energy storage and catalysis. Nanoscale borophene flakes have been synthesized on noble-metal surfaces, but for device fabrication, one needs large single-crystal domains. In this work, we report the synthesis of borophene on a square lattice Cu(100) surface and show that incommensurate coordinations could reduce the borophene-substrate interactions and alter the borophene structures in interesting ways. Micrometer-scale single-crystal domains can form as isolated faceted islands or merge together to achieve full monolayer coverage. We have discovered a new crystal structure of borophene, with ten boron atoms and two hexagonal vacancies in the unit cell. First-principle calculations indicate that charge transfer rather than covalent bonding binds 2D boron to the copper surface, and confirm its integrity and uniformity The electronic band structure features multiple anisotropic tilted Dirac cones, heralding emergent quantum fermions.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Semi-Dirac semimetal in silicene oxide
A semi-Dirac semimetal is a material that exhibits linear band dispersion in one direction and quadratic band dispersion in the orthogonal direction and, therefore, hosts massless and massive fermions at the same point in the momentum space. While a number of interesting physical properties have been predicted in semi-Dirac semimetals, it has been rare to realize such materials in condensed matter. Based on the fact that some honeycomb materials are easily oxidized or chemically absorb other atoms, here, we theoretically propose an approach of modifying their band structures by covalent addition of group-VI elements and strain engineering. We predict a silicene oxide with the chemical formula of Si 2 O to be a candidate semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of graphene-like materials.
Zhong, Chengyong↗