Recent studies demonstrated that novel two-dimensional (2D) triphosphide semiconductors possess high carrier mobility and show promising thermoelectric performance, but the carrier transport behaviors in 2D semimetal triphosphides have never been elucidated. Herein, using first-principles calculations and Boltzmann transport theory, we reveal that the electron-phonon coupling effect induced by increasing electron concentration can be significant and thus greatly inhibits electron and phonon transport in electron-doped ${\mathrm{BP}}_3$ and ${\mathrm{CP}}_3$. The intrinsic heat transport capacity of flexural acoustic phonon modes in the wrinkle structure is largely suppressed because of the strong phonon scatterings, leading to the low phonon thermal conductivities of 1.36 and 5.33 W/(m K) for ${\mathrm{BP}}_3$ and ${\mathrm{CP}}_3$ at room temperature, and at high doping level, the enhanced scattering from the electron diminishes the phonon thermal conductivity by 71% and 54%, respectively. Instead, electron thermal conductivity shows nonmonotonic variations with the increase of doping concentration, stemming from the competition between electron-phonon scattering rates and electron group velocity. Notably, the heavy-doping effect induced by strong scattering from the phonon largely suppresses the electron transport and reduces electron thermal conductivity to the magnitude of phonon thermal conductivity. This work sheds light on the electron and phonon transport properties in semimetal triphosphide monolayers and provides an efficient avenue for the modulation of carrier transport by the doping-induced electron-phonon coupling effect.

Classification and understanding of quantum phase transitions and critical phenomena in itinerant electron systems are outstanding questions in quantum materials research. Recent experiments on heavy fermion systems with higher-rank multipolar local moments provide a new platform to study such questions. In particular, experiments on ${\text{Ce}}_3{\text{Pd}}_{20}{(\text{Si},\text{Ge})}_6$ show novel quantum critical behaviors via two consecutive magnetic field-driven quantum phase transitions. At each transition, the derivative of the Hall resistivity jumps discontinuously, which was attributed to sequential Fermi surface reconstructions. Motivated by this discovery, we consider an effective quantum impurity model of itinerant electrons coupled to local dipolar, quadrupolar, and octupolar moments arising from ${\text{Ce}}^{3+}$ ions. Using renormalization group analyses, we demonstrate that two-stage multipolar ordering and Fermi surface reconstruction arise depending on which multipolar moments participate in the Fermi surface and which other moments are decoupled via Kondo destruction.

In the iron-based superconductors, unconventional superconductivity emerges in proximity to intertwined electronic orders consisting of an electronic nematic order and a spin density wave (SDW). Recently, ${\mathrm{BaNi}}_2{\mathrm{As}}_2$, like its well-known iron-based analog ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, has been discovered to host a symmetry-breaking structural transition but coupled to a unidirectional charge density wave (CDW) instead of SDW, providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning $B_{1}g$ uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, the structural and CDW order parameters in ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Furthermore, no nematic band splitting is resolved above the structural transition. Our measurements point to a likely lattice origin of the CDW order in ${\mathrm{BaNi}}_2{\mathrm{As}}_2$.

A strong light-matter interaction and the resulting polaritons provide a promising knob to control energy and population transfer over long length scales. Yet, existing transfer studies have been exclusively limited to a single-cavity scenario. Little is known about whether multiple cavities can facilitate new control strategies. Here, we reveal that the interplay of cavity-cavity coherence and strong vibrational light-matter coupling can induce a stationary population current between two identical, weakly coupled hybrid vibrational polaritonic systems at thermal equilibrium. Exploiting Fermi's golden rule, we qualitatively identify that the current is mediated by transitions between vibrational polariton states belonging to different hybrid systems. Our results provide insights for realizing controllable long-distance population transfer utilizing coupled light-matter hybrid systems.

Specific types of spatial defects or potentials can turn monolayer graphene into a topological material. These topological defects are classified by a spatial dimension $D$ and they are systematically obtained from the Hamiltonian by means of its symbol $\mathcal{H}(\mathit{k},\mathit{r})$, an operator which generalizes the Bloch Hamiltonian and contains all topological information. This approach, when applied to Dirac operators, allows to recover the tenfold classification of insulators and superconductors. The existence of a stable $\mathbb{Z}$ topology is predicted as a condition on the dimension $D$, similar to the classification of defects in thermodynamic phase transitions. Kekule distortions, vacancies, and adatoms in graphene are proposed as examples of such defects and their topological equivalence is discussed.

Yttrium iron garnet (YIG) has been a prototype material for the study of the basic physics in magnonic and a key material for spintronics and some technological applications. For this reason, its detailed characterization is crucial for a more complete knowledge of its properties and for the understanding of phenomena in which it is involved. Here, we report a study of magnon energy renormalization on YIG utilizing Brillouin light-scattering (BLS) technique to observe the temperature dependence of the acoustic magnon mode frequency. A numerical calculation performed considering the magnon energy renormalization due to four-magnon exchange scattering shows a quite good fit to the experimental data obtained with BLS.

Allosterism traditionally refers to local changes in an extended object, for instance the binding of a ligand to a macromolecule, leading to a localized response at some other, possibly quite remote position. Here, we show that such fascinating effects may already occur in very simple and common quantum many-body systems, such as an anisotropic Heisenberg spin chain: Introducing an impurity at one end of a sufficiently long chain may lead to quite significant changes of the observable behavior near the other end, but not in the much larger region in between. Specifically, spin autocorrelation functions at thermal equilibrium are found to exhibit a pronounced allosterism of this type.

We demonstrate the existence of an intermediate superexponential localization regime for eigenstates of the Aubry-André chain. In this regime, the eigenstates localize similar to the eigenstates of the Wannier-Stark ladder. The superexponential decay emerges on intermediate length scales for large values of the winding length—the quasiperiod of the Aubry-André potential. This intermediate length scale localization is present both in the metallic and insulating phases of the system. In the insulating phase, the superexponential localization is periodically interrupted by weaker decaying tails to form the conventional asymptotic exponential decay predicted for the Aubry-André model. In the metallic phase, the superexponential localization is only a transient phenomena and happens for states with energies away from the center of the spectrum and is followed by a superexponential growth into the next peak of the extended eigenstate. By adjusting the parameters, it is possible to extend the range of the superexponential localization to arbitrarily low values of the eigenfunction's amplitude. A similar intermediate superexponential localization regime is demonstrated in quasiperiodic discrete-time unitary maps.

We study the intrinsic spin Hall effect of a Dirac Hamiltonian system with ferromagnetic exchange coupling, a minimal model combining relativistic spin-orbit interaction and ferromagnetism. The energy bands of the Dirac Hamiltonian are split after introducing a Stoner-type ferromagnetic ordering, which breaks the spherical symmetry of pristine Dirac model. The totally antisymmetric spin Hall conductivity (SHC) tensor becomes axially anisotropic along the direction of external electric field. Interestingly, the anisotropy does not vanish in the asymptotic limit of zero magnetization. We show that the ferromagnetic ordering breaks the spin degeneracy of the eigenfunctions and modifies the selection rules of the interband transitions for the intrinsic spin Hall effect. The difference in the selection rule between the pristine and the ferromagnetic Dirac phases causes the anisotropy of the SHC, resulting in a discontinuity of the SHC as the magnetization, directed orthogonal to the electric field, is reduced to zero in the ferromagnetic Dirac phase and enters the pristine Dirac phase.

Recent theoretical studies have suggested that the low-energy Hamiltonian of honeycomb cobaltate systems could be dominated by anisotropic Kitaev interactions. Motivated by the theory, a honeycomb layered material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ with a hexagonal unit cell has been studied and found to exhibit antiferromagnetic (AFM) ordering at 27 K with two spin reorientation transitions at 15 and 5 K. Here we report a monoclinic polymorph of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, also with honeycomb layered structure but with a single AFM transition at 9.6 K and without spin reorientation transitions at lower temperatures. Using neutron diffraction, we identify an in-plane zigzag AFM order in the ground state with the spins canted out of the honeycomb planes and ferromagnetically coupled between them. The zigzag order is suppressed by a magnetic field of 6 T. The lower critical temperature and field, positive Curie-Weiss temperature, and out-of-plane canting of spins in the monoclinic ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ suggest enhanced frustration in this polymorph compared to the hexagonal one.

${\mathrm{YbInCu}}_4$, which is an excellent material for understanding first-order valence transitions, has been studied for over three decades. However, direct evidence for the evolution of the Fermi surface at the valence transition temperature $T_{\mathrm{V}}$ has not been obtained owing to the complicated nature of the electronic band structure near the Fermi level $E_{\mathrm{F}}$. In this study, we use angle-resolved photoemission spectroscopy measurements and local density approximation calculations to reveal the temperature-dependent Fermi surface evolution in ${\mathrm{YbInCu}}_4$. We experimentally identify the topology of the Fermi surface and find that the area of the Fermi surface increases with an accompanying decrease in the Fermi velocity below $T_{\mathrm{V}}$. These observations demonstrate a reconstruction from a small to a large Fermi surface. Moreover, we theoretically reveal that the conduction bands of Cu $s, p$, and $d$ and In $s$ and $p$ states hybridize with the $f$ bands ($c-f$ hybridization), thereby inducing a dramatic change in the band structure near $E_{\mathrm{F}}$. Interestingly, a Van Hove singularity with the saddle point moves to $E_{\mathrm{F}}$ owing to $c-f$ hybridization. We propose that the $c-f$ hybridization and the enhancement in the density of states at $E_{\mathrm{F}}$ should be incorporated in the scenarios of the first-order valence transition of ${\mathrm{YbInCu}}_4$.

By first-principles calculations, we study the magnetic and topological properties of $\mathit{XY}{\mathrm{Bi}}_2{\mathrm{Te}}_5$-family $(X, Y=\mathrm{Mn},\mathrm{Ni},\mathrm{V},\mathrm{Eu})$ compounds. The strongly coupled double magnetic atom-layers can significantly enhance the magnetic ordering temperature while keeping the topologically nontrivial properties. Particularly, ${\mathrm{NiVBi}}_2{\mathrm{Te}}_5$ is found to be a magnetic Weyl semimetal in bulk and a Chern insulator in thin film with both the Curie temperature ($\sim 150$ K) and full gap well above 77 K. ${\mathrm{Ni}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5, {\mathrm{MnNiBi}}_2{\mathrm{Te}}_5, {\mathrm{NiVBi}}_2{\mathrm{Te}}_5$, and ${\mathrm{NiEuBi}}_2{\mathrm{Te}}_5$ exhibit intrinsic dynamic axion state. Among them, ${\mathrm{MnNiBi}}_2{\mathrm{Te}}_5$ has a Néel temperature over 200 K and ${\mathrm{Ni}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ even demonstrates antiferromagnetic order above room temperature. These results indicate an approach to realize high-temperature quantum anomalous Hall effect and other topological quantum effects for practical applications.

We investigate the nature of quantum phases arising in chiral interacting Hamiltonians recently realized in Rydberg atom arrays. We classify all possible fermionic chiral spin liquids with $\mathrm{U}\left(1\right)$ global symmetry using parton construction on the honeycomb lattice. The resulting classification includes six distinct classes of gapped quantum spin liquids: the corresponding variational wavefunctions obtained from two of these classes accurately describe the Rydberg many-body ground state at $1/2$ and $1/4$ particle density. Complementing this analysis with tensor network simulations, we conclude that both particle filling sectors host a spin liquid with the same topological order of a $\nu =1/2$ fractional quantum Hall effect. At density $1/2$, our results clarify the phase diagram of the model, while at density $1/4$, they provide an explicit construction of the ground-state wavefunction with almost unit overlap with the microscopic one. These findings pave the way to the use of parton wavefunctions to guide the discovery of quantum spin liquids in chiral Rydberg models.

The Holstein Hamiltonian describes itinerant electrons whose site density couples to local phonon degrees of freedom. In the single-site limit, at half filling, the electron-phonon coupling results in a double-well structure for the lattice displacement, favoring empty or doubly occupied sites. In two dimensions and on a bipartite lattice in $d\ge 2$, intersite hopping causes these doubly occupied and empty sites to alternate in a charge density wave (CDW) pattern when the temperature is lowered. Because a discrete symmetry is broken, this occurs in a conventional second-order transition at finite $T_{\mathrm{cdw}}$. In this paper, we investigate the effect of changing the phonon potential energy to one with an intrinsic double-well structure even in the absence of electron-phonon coupling. While this aids in the initial process of pair formation, the implications for subsequent CDW order are nontrivial. One expects that, when the electron-phonon coupling is too strong, the double wells become deep and the polaron mass gets large, an effect which reduces $T_{\mathrm{cdw}}$. We show here the existence of regions of parameter space where the double-well potential, while aiding local pair formation, does so in a way which also substantially enhances long-range CDW order.

A quantum mechanical theory which describes the exciton-acoustic phonon interaction in plane semiconductor AlN/AlGaN nanosystems at nonzero temperatures has been developed. The theory of exciton states is constructed by applying the effective mass model to an electron and a hole, taking into account internal electric fields due to the macroscopic value of polarization, which is a significant factor affecting the states of quasiparticles in nitride nanosystems. The theory of acoustic phonons was developed on the basis of the elastic continuum model by analytically obtained solutions of the equations describing the components of the deformation field of a semiconductor medium. We have considered both relevant cases, when the semiconductors forming the studied nanosystem have a wurtzite-type crystal lattice and a zinc-blende-type lattice. In addition, we obtained analytical expressions that specify the deformation and piezoelectric potential created by acoustic phonons for these cases. For each type of crystal lattice, the cases of exciton-phonon interaction through the deformation and piezoelectric potentials have been separately and sufficiently investigated. The theory of exciton-phonon interaction was developed and characteristics of the renormalized exciton states were investigated using the method of temperature Green's functions using and the Dyson equation. We found that for wurtzite nanosystems, the mechanisms of interaction through the piezoelectric and deformation potentials contribute to the exciton-phonon interaction, while for zinc-blende nanosystems, the main contribution to this interaction is due to the deformation potential. It is found that the temperature shifts of the exciton spectrum are of negative sign for all temperatures, and the absolute values of these shifts increase with the temperature increase. An analysis of the calculated exciton basic band shape function and decay rates made it possible to establish that, in the case of the zinc-blende nanosystem, the contribution of acoustic phonons to the exciton-phonon interaction is smaller than that in the case of the wurtzite nanosystem. Calculations of the correlated exciton mass testify that the interaction of an exciton with acoustic phonons at all temperatures results in its increase by maximum $8\%$ in the case of wurtzite nanosystems and by $5\%$ in the case of zinc-blende nanosystems. We believe that the obtained theory and results of calculations will be useful both for a more complete and broader understanding of exciton-phonon interaction processes in nitride nanosystems as active elements of THz optoelectronic devices and for the further development of the theory of exciton-phonon interaction in geometric nanosystems of different symmetries.

A variety of analytical approaches have been developed for the study of quantum spin systems in two dimensions, the notable ones being spin-waves, slave boson/fermion parton constructions, and for lattices with one-to-one local correspondence of faces and vertices, the two-dimensional (2D) Jordan-Wigner (JW) fermionization. Field-theoretically, JW fermionization is implemented through Chern-Simons (CS) flux attachment. For a correct fermionization of lattice quantum spin-$1/2$ magnets, it is necessary that the fermions obey mutual bosonic (anyonic) statistics under exchange—this is not possible to implement on arbitrary 2D lattices if fermionic matter couples only to the lattice gauge fields. Enlarging the gauge degrees of freedom to include the dual lattice allows the construction of consistent mutual Chern-Simons field theories. Here we propose a mutual CS theory where the microscopic (spin) degrees of freedom are represented as lattice fermionic matter additionally coupled to specific combinations of dual lattice gauge fields that depend on the local geometry. We illustrate the use of this method for understanding the properties of a honeycomb Kitaev model subjected to a strong Zeeman field in the $z$-direction. Our CS gauge theory framework provides an understanding of why the topological phase is degraded at lower (higher) critical fields for the ferromagnetic (antiferromagnetic) Kitaev interaction. Additionally, we observe an effectively one-dimensional character of the low excitations at higher fields in the $z$-direction, which we also confirm by spin-wave calculations.

The effect of the energy valley on interlayer charge transfer in transition metal dichalcogenide (TMD) heterostructures is studied by transient absorption spectroscopy and density functional theory. First-principles calculations confirm that the ${\mathrm{\Lambda }}_{\text{min}}$ valley in the conduction band of few-layer ${\mathrm{WSe}}_2$ evolves from above its K valley in the monolayer (1L) to below it in 4L. Heterostructure samples of $n\mathrm{L}\text{−}{\mathrm{WSe}}_2/1\mathrm{L}\text{−}{\mathrm{MoS}}_2$, where $n=1,2,3$, and 4, are obtained by mechanical exfoliation and dry transfer. Photoluminescence spectroscopy reveals a thickness-dependent ${\mathrm{WSe}}_2$ band structure and efficient interlayer charge transfer. Transient absorption measurements show that the electron transfer time from the ${\mathrm{\Lambda }}_{\text{min}}$ valley of 4L ${\mathrm{WSe}}_2$ to the K valley of ${\mathrm{MoS}}_2$ is on the order of 30 ps. This process is much slower than the K-K charge transfer in 1L/1L TMD heterostructures. The momentum-indirect interlayer excitons formed after charge transfer have lifetimes >1 ns.

Two-dimensional (2D) semiconductor materials offer a platform for unconventional applications such as valleytronics, flexible nanoelectronics, and hosts of quantum emitters. Many of these materials and their electronic properties remain to be explored. Using ab initio simulations based on the density functional theory, we investigate group-IV monochalcogenides $MQ$ ($M=\mathrm{Si}, \mathrm{Ge}, \mathrm{Sn}$ and $Q=\mathrm{S}, \mathrm{Se}, \mathrm{Te}$), an emerging class of 2D materials, with two competing crystal structures: (i) phosphorenelike ($Pmn{2}_1$), which has already been synthesized, and (ii) $\mathrm{SiTe}$-type ($P\overline{3}m1$), which has been much less explored. Except for $\mathrm{SnS}$, we find that the $\mathrm{SiTe}$ type is the lowest-energy structure and has higher structural stability, motivating efforts to synthesize this less explored $P\overline{3}m1$ phase. Regarding the optoelectronic properties of these two phases, in the $P\overline{3}m1$ phase, $MQ$ compounds have band gaps around the sunlight spectrum peak and show narrower variations in band gap with the composition and higher absorption coefficients for lighter chalcogens. In contrast, in the $Pmn{2}_1$ phase, $MQ$ compounds have wider band gaps and show a band gap variation of up to 72% with composition, higher absorption coefficients with $\mathrm{Te}$ atoms, and potential for valleytronics. In particular, $\mathrm{SiS}$ shows interesting high optical anisotropy among all the investigated materials. Furthermore, the optical spectra present peaks that are particular to each phase or composition, making the refractive index a distinguishing parameter for identifying the different $MQ$ compounds. Finally, a phase transition from monolayer to bulk due to an interaction between the layers is observed. Thus, the present results straighten out the role of the crystalline phase in the optoelectronic properties of these monochalcogenides.

We develop a length-gauge formalism for treating one-dimensional periodic lattice systems in the presence of a photon cavity inducing light-matter interaction. The purpose of the formalism is to remove mathematical ambiguities that occur when defining the position operator in the context of the Power-Zienau-Woolley Hamiltonian. We then use a diagrammatic approach to analyze perturbatively the interaction between an electronic quantum system and a photonic cavity mode of long wavelength. We illustrate the versatility of the formalism by studying the cavity-induced electric charge imbalance and polarization in the Rice-Mele model with broken inversion symmetry.

Ferroelectric memories experienced a revival in the last decade due to the discovery of ferroelectricity in ${\mathrm{HfO}}_2$-based nanometer-thick thin films. These films exhibit exceptional silicon compatibility, overcoming the scaling and integration obstacles that impeded perovskite ferroelectrics' use in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films remains debated with no single factor identified that could stabilize the ferroelectric phase thermodynamically. Here, supported by density functional theory (DFT) high-throughput (HT) calculations that screen a broad range of epitaxial conditions, we demonstrate conclusively that specific epitaxial conditions achievable with common substrates such as yttria-stabilized zirconia (YSZ) and ${\mathrm{SrTiO}}_3$ can favor the polar $Pca{2}_1$ phase thermodynamically over other polar phases such as $R3m$ and $Pmn{2}_1$ and nonpolar $P{2}_1/c$ phase. The substrate's symmetry constraint-induced shear strain is crucial for the preference of $Pca{2}_1$. The strain-stability phase diagrams resolve experiment-theory discrepancies and can guide the improvement of ferroelectric properties of epitaxial hafnia thin films.