We investigate interacting spin susceptibilities in lattice models for $\mathcal{T}$-reversal symmetry-broken Weyl semimetals. We employ a random phase approximation method for the spin-SU(2)-symmetry-broken case that includes mixtures of ladder and bubble diagrams, beyond a SU(2)-symmetric case. Within this approach, the relations between the tendency towards magnetic order and the band structure tilt parameter $\gamma$ under different temperatures are explored. The critical interaction strength $U_c$ for magnetic ordering decreases as the tilt term changes from type-I Weyl semimetals to type II. The lower the temperature, the sharper is the drop in $U_c$ at the critical point between them. The variation of $U_c$ with a slight doping near half filling is also studied. It is generally found that these Weyl systems show a strongly anisotropic spin response with an enhanced doubly degenerate transverse susceptibility perpendicular to tilt direction, inherited from ${\mathcal{C}}_{4z}$ rational symmetry of a bare Hamiltonian, but with the longitudinal response suppressed with respect to that. For small tilts $\gamma$ and strong enough interaction, we find two degenerate ordering patterns with spin order orthogonal to the tilt direction but much shorter spin correlation length parallel to the spin direction. With increasing the tilt, the system develops instabilities with respect to in-plane magnetic orders with wave vector $(0,\pi ,q_z)$ and $(\pi ,0,q_z)$, with $q_z$ increasing from 0 to $\pi$ before the transition to a type-II Weyl semimetal is reached. These results indicate a greater richness of magnetic phases in correlated Weyl semimetals that also pose challenges for precise theoretical descriptions.

We investigate the quench and real-time formation of the Mott state and photoexcited carrier relaxation dynamics in the Mott insulator $\kappa \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\text{−}\mathrm{T}\mathrm{T}\mathrm{F}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Cl}\left(\kappa \text{−}\mathrm{Cl}\right)$ and the superconductor $\kappa \text{−}{\left(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\text{−}\mathrm{T}\mathrm{T}\mathrm{F}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Br}\left(\kappa \text{−}\mathrm{Br}\right)$ using three-pulse femtosecond optical spectroscopy, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. In both salts, we find that transient reflectivity amplitude gradually recovers with time after strong near-infrared pulse quench, but its relaxation time is nearly constant throughout. This indicates that in $\kappa$-Cl, the energy gap for charge excitations is filled rather than closed by photoinduced carriers of only $\sim 0.5$% per dimer. The Mott state is re-formed on a few-picosecond timescale with the disappearance of the in-gap photodoping-induced states near the Fermi energy. In $\kappa$-Br, a behavior similar to that in $\kappa$-Cl is observed and attributed to the disorder-induced phase-separated Mott insulating regions.

Antiferromagnetic or charge density wave fluctuations couple with light through the recently discovered $\pi$-ton contribution to the optical conductivity, and quite generically constitute the dominant vertex corrections in low-dimensional correlated electron systems. Here we study the arguably simplest version of these $\pi$-tons based on the semianalytical random phase approximation (RPA) ladder in the transversal particle-hole channel. The vertex corrections to the optical conductivity are calculated directly for real frequencies. We validate that the RPA qualitatively reproduces the $\pi$-ton vertex corrections to the Drude peak in the Hubbard model. Depending on the temperature we find vertex corrections to broaden or sharpen the Drude peak.

Twisted transition metal dichalcogenide (TMD) homobilayers have recently emerged as a powerful platform for studying correlated insulating states. In the strongly correlated limit, we construct an effective spin Hamiltonian on a honeycomb lattice that includes the Heisenberg interaction and nonsymmetric interactions such as a Dzyaloshinskii-Moriya interaction and a Kane-Mele coupling for the Mott-insulating phase at half-filling. For the twisted TMD homobilayers, the spin-orbit coupling in the Hubbard model, which is expected to induce the antisymmetric exchange couplings in the effective spin Hamiltonian, is a highly tunable and experimentally accessible quantity that can be tuned by an applied electric field. In this study, we investigate classical and quantum phase diagrams of the effective spin Hamiltonian using analytical and numerical methods. We show that the model exhibits a rich classical phase diagram including an antiferromagnetic (AFM) phase, a planar spiral ordered phase with high classical degeneracy, a $z$-AFM phase, a noncoplanar phase, a noncollinear phase, and a ${120}^{\circ }$-AFM phase. In the quantum treatment, we calculate low-energy magnon excitation spectrum, ground-state energy, and static spin structure factor using linear spin-wave theory and density matrix renormalization group methods to compose the quantum phase diagram of the effective spin Hamiltonian. Beyond the Heisenberg interaction, we find that the existence of these antisymmetric couplings is responsible for the quantum spin liquid, $z$-AFM, noncoplanar, and ${120}^{\circ }$ phases. Twisted TMD homobilayers, therefore, offer rich platforms for realizing rich phases of matter such as quantum spin liquid, noncoplanar, and ${120}^{\circ }$, resulting from the spin-orbit coupling.

Excellent thermoelectric performance in the out-of-layer $n$-doped SnSe has been observed experimentally [Chang et al., Science 360, 778 (2018)]. However, a first-principles investigation of the dominant scattering mechanisms governing all thermoelectric transport properties is lacking. In the present paper, by applying extensive first-principles calculations of electron-phonon coupling associated with parameterized calculation of the scattering by ionized impurities, we investigate the reasons behind the superior figure of merit as well as the enhancement of $zT$ above 600 K in $n$-doped out-of-layer SnSe, as compared to $p$-doped SnSe with similar carrier densities. For the $n$-doped case, the relaxation time is dominated by ionized impurity scattering and increases with temperature, a feature that maintains the power factor at high values at higher temperatures and simultaneously causes the carrier thermal conductivity at zero electric current $\left({\kappa }_{\mathrm{el}}\right)$ to decrease faster for higher temperatures, leading to an ultrahigh-$zT=3.1$ at 807 K. We rationalize the roles played by ${\kappa }_{\mathrm{el}}$ and ${\kappa }^0$ (the thermal conductivity due to carrier transport under isoelectrochemical conditions) in the determination of $zT$. Our results show the ratio between ${\kappa }^0$ and the lattice thermal conductivity indeed corresponds to the upper limit for $zT$, whereas the difference between calculated $zT$ and the upper limit is proportional to ${\kappa }_{\mathrm{el}}$.

The quantum spin Hall effect arises due to band inversion in topological insulators, and has the defining characteristic that it hosts helical edge channels at zero magnetic field, leading to a finite spin Hall conductivity. The spin Hall conductivity is understood as the difference of the contributions of two spin states. In the effective four-band Bernevig-Hughes-Zhang model, these two spin states appear as two uncoupled blocks in the Hamiltonian matrix. However, this idea breaks down if additional degrees of freedom are considered. The two blocks cannot be identified by proper spin $S_z$ or total angular momentum $J_z$, both not conserved quantum numbers. In this work, we discuss a notion of block structure for the more general $\mathbf{k}·\mathbf{p}$ model, defined by a conserved quantum number that we call isoparity, a combination of parity $z\to -z$ and spin. Isoparity remains a conserved quantity under a wide range of conditions, in particular in presence of a perpendicular external magnetic field. From point-group considerations, isoparity is fundamentally defined as the action of $z\to -z$ on the spatial and spinorial degrees of freedom. Since time reversal anticommutes with isoparity, the two blocks act as Kramers partners. The combination of conductivity and isoparity defines spin conductivity. This generalized notion of spin Hall conductivity uncovers the meaning of ‘spin’: It is not the proper spin $S_z$, but a crystal symmetry that is realized by a spinorial representation.

A modified $GW$ approximation to many-body systems is developed. The approximation has the same computational complexity as the traditional $GW$ approach, but uses a different truncation scheme. This scheme neglects the high-order connected correlation functions. A covariant (preserving the Ward identities due to the charge conservation) scheme for the two-body correlators is employed, which holds the relation between the charge correlator and the charge susceptibility. The method is tested on the two-dimensional one-band Hubbard model. The results are compared with exact diagonalization, the $GW$ approximation, the fluctuation-exchange (FLEX) theory, and determinantal Monte Carlo approach. The comparison for the (one-body) Green's function demonstrates that it is more precise in the strong-coupling regime (especially away from half filling) than the $GW$ and FLEX approximations, which have a similar complexity. More importantly, this method indicates a Mott-Hubbard gap as the Hubbard $U$ increases, whereas the $GW$ and FLEX methods fail. In addition, the charge correlator obtained from the covariant scheme not only holds the consistency of the static charge susceptibility, but also makes a significant improvement over the random phase approximation calculations.

Particle-hole symmetry breaking in the fractional quantum Hall effect has recently been studied both theoretically and experimentally with most works focusing on non-Abelian states in the second electronic Landau level. In this work, we theoretically investigate particle-hole symmetry breaking of incompressible fractional quantum Hall states in the lowest Landau level under the influence of the realistic effect of a finite magnetic-field strength. A finite magnetic field induces Landau-level and sub-band mixing which are known to break particle-hole symmetry at the level of the Hamiltonian. We analyze the Haldane pseudopotentials, energy spectra and energy gaps, and the wave functions themselves, under realistic conditions. We find that particle-hole symmetry is broken, as determined by energy gaps, between states related via particle-hole conjugation, however, we find that particle-hole symmetry is largely maintained as determined by the effective mass of composite fermions. Finally, we comment and make connection to recent experimental observations regarding particle-hole symmetry in the lowest Landau-level fractional quantum Hall effect [Phys. Rev. Lett. 124, 156801 (2020)].

We demonstrate room temperature (RT) polariton lasing in an all-dielectric microcavity containing non-polar III-nitride quantum wells (QWs) as active media. The microcavity is fabricated using the photo-electrochemical etching method, by which an optimally grown $m$-plane III-nitride active region is detached from the substrate in the form of a membrane, which is subsequently inserted between two $\mathrm{Si}{\mathrm{O}}_2/{\mathrm{Ta}}_2{\mathrm{O}}_5$ distributed Bragg reflectors, with 4 and 10 pairs for the top and bottom mirrors, respectively. The active region consists of 25 $\mathrm{GaN}/{\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{N}$ (5 nm/3 nm) QWs. The produced microcavities exhibit two closely spaced polarization-resolved lower polariton branches at RT, in line with the selection rules of the non-polar orientation, having a Rabi splitting of 62 and 72 meV in the $E\parallel a$ and $E\parallel c$ polarizations, respectively. In a positively detuned 3λ/2-thick microcavity, polariton lasing is observed at ambient conditions in the $E\parallel a$ polarization, with a threshold ∼ 3 times lower than previous state of the art, despite the use of a relatively weak top reflector.

Silicon (Si) donors in GaAs have been the topic of extensive studies since Si is the most common and well understood $n$-type dopant in III-V semiconductor devices and substrates. The indirect band gap of AlAs compared to the direct one of GaAs leads to interesting effects when introducing Si dopants. Here we present a study of cross-sectional scanning tunneling microscopy and density functional theory (DFT) calculations to study Si donors in AlAs at the atomic scale. Based on their crystal symmetry and contrast strengths, we identify Si donors up to four layers below the (110) surface of AlAs. Interestingly, their short-range local density of states (LDOS) is very similar to Si atoms in the (110) surface of GaAs. Additionally, we show high-resolution images of Si donors in all these layers. For empty state imaging, the experimental and simulated STM images based on DFT show excellent agreement for Si donors up to two layers below the surface.

We develop a quantum theory of plasmon polaritons in chains of metallic nanoparticles, describing both near- and far-field interparticle distances, by including plasmon–photon umklapp processes. Taking into account the retardation effects of the long-range dipole–dipole interaction between the nanoparticles, which are induced by the coupling of the plasmonic degrees of freedom to the photonic continuum, we reveal the polaritonic nature of the normal modes of the system. We compute the dispersion relation and radiative linewidth, as well as the group velocities of the eigenmodes, and compare our numerical results to classical electrodynamic calculations within the point-dipole approximation. Interestingly, the group velocities of the polaritonic excitations present an almost periodic sign change and are found to be highly tunable by modifying the spacing between the nanoparticles. We show that, away from the intersection of the plasmonic eigenfrequencies with the free photon dispersion, an analytical perturbative treatment of the light-matter interaction is in excellent agreement with our fully retarded numerical calculations. We further study quantitatively the hybridization of light and matter excitations through an analysis of Hopfield's coefficients. Finally, we consider the limit of infinitely spaced nanoparticles and discuss some recent results on single nanoparticles that can be found in the literature.

Majorana fermions are promising building blocks of forthcoming technology in quantum computing. However, their nonambiguous identification has remained a difficult issue because of the concomitant competition with other topologically trivial fermionic states, which poison their detection in most spectroscopic probes. By employing numerical and analytical methods, here we show that the Fano factor tomography is a key distinctive feature of a Majorana bound state, displaying a spatially constant Poissonian value equal to one. In contrast, the Fano factor of other trivial fermionic states, like the Yu-Shiba-Rusinov or Andreev ones, is strongly spatially dependent and exceeds one as a direct consequence of the local particle-hole symmetry breaking.

Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic-scale devices. Employing two isomers of bipyridine, $4,4^{\prime }$ bipyridine and $2,2^{\prime }$ bipyridine between gold electrodes, here, we investigate the formation of a metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4,4^{\prime }$ bipyridine via a conductance jump from the tunneling regime, referred to as “jump to molecular contact,” being absent for $2,2^{\prime }$ bipyridine. Through statistical analysis of the data, along with molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during the breaking process, providing important insights for using a single molecule in an electronic device.

We investigate the fate of coherence in the dynamical evolution of symmetry protected quantum systems. Under the formalism of system plus bath for open quantum systems, antiunitary symmetries exhibit significant differences from the unitary ones in protecting initial coherence. Specifically, taking advantage of the Lindblad master equation, we find that a pure state in the symmetry protected degenerate subspace will be decoherent even though both the system Hamiltonian and system-environment interaction respect the same antiunitary symmetry. In contrast, the coherence will persist when the protecting symmetry is unitary. We provide an elaborate classification table to illustrate what kinds of symmetry combinations are able to preserve the coherence of the initial state, which is confirmed in several concrete models of single spin-$3/2$ systems. It can also be applied to the Haldane phase in interacting topological systems. Our results could be helpful in exploring possible experimental realization of stable time-reversal symmetric topological states.

In heat conduction through a homogenous nanomaterial, various phonons may exhibit diverse temperatures even at the same location at a steady state, known as the local phonon nonequilibrium phenomenon. Different phonons are often considered to behave independently, and the phonons with longer mean free paths (MFPs) have smaller temperature gradients. That is, the temperature gradient exhibits the following order: ballistic phonons < semiballistic phonons ≈ lattice (average) temperature gradient < diffusive phonons, where ballistic phonons have MFPs much larger than the characteristic length, semiballistic phonons have MFPs like the characteristic length, and diffusive phonons have MFPs much smaller than the characteristic length. However, in this paper, we reveal that the effect of phonon-phonon coupling leads nonequilibrium phonon temperature gradients to the following trend: diffusive phonon temperature gradients will decrease to the lattice temperature, and temperature gradients of some semiballistic phonons even surpass that of diffusive phonons. The diffusive phonon temperature is merged onto the lattice temperature since they have large scattering rates and can be equilibrated quickly to the lattice temperature after traveling for a short distance away from the boundaries into the nanomaterial. The semiballistic phonons have large scattering rates but not large enough to bring them down to the lattice temperature. To obtain a further understanding of the nonequilibrium phonon temperatures, we have also derived a simple analytical model which can accurately predict the temperature profiles of all individual phonons given their MFPs. Using this model, we find that, near the boundary, phonon temperatures decay with position exponentially (instead of linearly), with a rate inversely proportional to their MFPs. Our findings offer insight for the understanding and prediction of phonon nonequilibrium temperatures within nanodevices.

By employing the exact diagonalization method in and out of equilibrium, we investigate the half-filled spinless Haldane model on the honeycomb lattice with nearest-neighbor Coulomb repulsion. In equilibrium, the ground states with small and large interactions are in Chern insulator and charge-density-wave phase, respectively. Motivated by the recent study reporting a topologically nontrivial excited state in the strong coupling regime, we propose that such an underlying state can be detected by using optical conductivity in the open boundary condition. Specifically, we find that the dominant peak of the optical conductivity on edge is inside the bulk gap and its characteristic frequency is associated with the second excited state. Further analyses on the Drude peak indicate that the second excited state preserves a metallic edge even in the system away from the critical point. Meanwhile, it can be excited largely by not only circularly but also linearly polarized pump pulse. The latter is mostly overlooked by previous studies and implies that the polarization of the driving field is not so crucial to our system.

Strong coupling between cavity photons and elementary excitations in condensed matters boosts the field of light-matter interaction and generates several exciting subfields, such as cavity optomechanics and cavity magnon polariton. Axion quasiparticles, emerging in topological insulators, were predicted to strongly couple with the light and generate the so-called axion polariton. Here we demonstrate that there arises a gapless level attraction (LA) of cavity axion polariton in an antiferromagnetic topological insulator, which originates from the high-order interaction between the axion and the odd-order resonance of a cavity. Such a novel coupling mechanism is essentially different from conventional LAs with the linear-order interaction. Our results open up promising roads for exploring the axion polariton with cavity technologies.

We investigate two holes in the $t_{2g}$ levels of a square-lattice Mott insulator with strong spin-orbit coupling. Exact diagonalization of a spin-orbital model, with arbitrary spin-orbit coupling and crystal field (valid for strong onsite interactions) is complemented by an effective triplon model (valid for strong spin-orbit coupling) and by a semiclassical Monte Carlo variant of the model. We provide the magnetic phase diagram depending on crystal field and spin-orbit coupling, which largely agrees for the semiclassical Monte Carlo and quantum models, as well as excitation spectra characterizing the various phases.

The dynamical behavior of a quantum many-particle system is characterized by the lifetime of its excitations. When the system is perturbed, observables of any nonconserved quantity decay exponentially, but those of a conserved quantity relax to equilibrium with a power law. Such processes are associated with a dynamical exponent $z$ that relates the spread of correlations in space and time. We present numerical results for the Fredkin model, a quantum spin chain with a three-body interaction term, which exhibits an unusually large dynamical exponent. We discuss our efforts to produce a reliable estimate $z\doteq 3.16\left(1\right)$ through direct simulation of the quantum evolution and to explain the slow dynamics in terms of an excited bond that executes a constrained random walk in Monte Carlo time.

Building on previous work [N. Read, Phys. Rev. B 58, 16262 (1998); Z. Dong and T. Senthil, Phys. Rev. B 102, 205126 (2020)] on the system of bosons at filling factor $\nu =1$, we derive the Dirac composite fermion theory for a half-filled Landau level from first principles and apply the Hartree-Fock approach in a preferred representation. On the basis of the microscopic formulation, in the long-wavelength limit, we propose a noncommutative field-theoretical description, which in a commutative limit reproduces the Son's theory, with additional terms that may be expected on physical grounds. The microscopic representation of the problem is also used to discuss pairing instabilities of composite fermions. We find that a presence of a particle-hole symmetry breaking leads to a weak (BCS) coupling $p$-wave pairing in the lowest Landau level, and strong coupling $p$-wave pairing in the second Landau level that occurs in a band with nearly flat dispersion, a third power function of momentum.