The chiral anomaly effect has been regarded as a hallmark of Weyl semimetals. While most research focuses on the change of electrical conductivity due to the chiral charge-pumping effect, these transport measurements are accompanied by other effects such as weak antilocalization which are difficult to exclude. Also, the magnetic field intensity at which the energy dispersion of the Landau level starts to display the chiral anomaly effect is hard to identify. Here, we report the evolutionary process of the energy dispersion of the zeroth Landau level under increasing magnetic fields, and we elucidate the relationship between the chiral Landau level and the Fermi level via magneto-optical measurements in Weyl semimetal TaP. Two series of Landau-level transitions have been extracted and fitted well to a massless Weyl fermion model and a classical semiconductor model, respectively. The field-induced chiral Landau-level transition is obtained from a comparison between the spectra under Voigt geometry and Faraday geometry in an increasing magnetic field. The chiral nature of the zeroth Landau level is found to enhance with the increase of magnetic field and clearly manifests in the magneto-optical spectrum above 13 T. The state transition of the zeroth Landau level can be explained by the strength of the field-induced chiral anomaly effect and the position of the Fermi level. These magneto-optical results unveil the chiral Landau-level transitions in TaP, which serves as an ideal material platform to analyze the chiral anomaly effect.

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Tensor networks provide a useful tool to describe low-dimensional complex many-body systems. Finding efficient algorithms to use these methods for finite-temperature simulations in two dimensions is a continuing challenge. Here, we use the class of recently introduced isometric tensor network states, which can also be directly realized with unitary gates on a quantum computer. We utilize a purification ansatz to efficiently represent thermal states of the transverse field Ising model. By performing an imaginary-time evolution starting from infinite temperature, we find that this approach offers a different way with low computational complexity to represent states at finite temperatures.

Recent ab initio studies of electron transport in GaAs have reported that electron-phonon (e-ph) interactions beyond the lowest order play a fundamental role in charge transport and noise phenomena. Inclusion of the next-leading-order process in which an electron scatters with two phonons was found to yield good agreement for the high-field drift velocity, but the characteristic nonmonotonic trend of the power spectral density of current fluctuations (PSD) with electric field was not predicted. The high computational cost of the ab initio approach necessitated various approximations to the two-phonon scattering term, which were suggested as possible origins of the discrepancy. Here we report a semianalytical transport model of two-phonon electron scattering via the Fröhlich mechanism, allowing a number of the approximations in the ab initio treatment to be lifted while retaining the accuracy to within a few percent. We compare the calculated and experimental transport and noise properties as well as scattering rates measured by photoluminescence experiments. We find quantitative agreement within 15% for the drift velocity and $25\%$ for the $\mathrm{\Gamma }$ valley scattering rates, and agreement with the $\mathrm{\Gamma }\text{−}\mathrm{L}$ intervalley scattering rates within a factor of two. Considering these results and prior studies of current noise in GaAs, we conclude that the most probable origin of the nonmonotonic PSD trend versus electric field is the formation of space-charge domains rather than intervalley scattering as has been assumed.

The Chern insulator, as the most prominent topological state of matter, is characterized by the Chern number $\mathcal{C}$, which in principle can take any integer values. So far, most realizations of the Chern insulator are limited to $\left|\mathcal{C}\right|=1$, and achieving higher Chern numbers remains challenging. Here, we propose a general scheme to realize the Chern state with arbitrarily large Chern number in an electric circuit. Our proposal is based on the possibility of precise control on long-range couplings, which is a unique advantage of circuit systems. Besides simulations, we present an experimental demonstration of a $\mathcal{C}=3$ state fabricated on a printed circuit board and directly image its topological chiral edge modes via AC sweep analysis. Our work opens a horizon for exploring exotic topological states of matter in classical systems.

A recent experiment has unveiled an anomalously strong electron-electron attraction in the one-dimensional copper-oxide chain ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{3+\delta }$. While the effect of the near-neighbor electron attraction $V$ in the one-dimensional extended Hubbard chain has been examined recently, its effect in the Hubbard model beyond the one-dimensional chain remains unclear. Here, we report a density matrix renormalization group study of the extended Hubbard model on long four-leg cylinders on the square lattice. We find that the near-neighbor electron attraction $V$ can notably enhance the long-distance superconducting correlations while simultaneously suppressing the charge-density-wave (CDW) correlations. Specifically, for a modestly strong electron attraction, the superconducting correlations become dominant over the CDW correlations with a Luttinger exponent $K_{sc}\sim 1$ and strong divergent superconducting susceptibility. Our results provide a promising way to realize long-range superconductivity in the doped Hubbard model. The relevance of our numerical results to cuprate materials is also discussed.

In this paper we calculate the exchange interaction constant between the manganese ion inner electronic $d$-shell and GaAs valence band bound hole using their microscopic multiparticle wave functions. We reveal its parametric dependence on crystal lattice deformations and find that it could be about and even more than dozens of a percent when the strain tensor reaches values of ${10}^{-3}-{10}^{-2}$. This fact is in accordance with the previous hypothesis of deformation dependence of Mn acceptors in a GaAs fine energy structure obtained from Raman spectroscopy; we show that this dependence has the same magnitude. Also, we resolve here the problem of a substantial high-temperature mismatch between well-developed theory and experimental data for the static magnetic susceptibility of Mn ions in GaAs. We show by numerical estimates and calculations that quite a strong parametric dependence of the exchange coupling value on GaAs lattice expansion determines the high-temperature (above 50 K) magnetic susceptibility reduction as well.

Heusler alloys have attracted interest in various fields of functional materials since their properties can quite easily be tuned by composition. Here, we have investigated the relatively new class of all-$3d$ Heusler alloys in view of their potential as permanent magnets. To identify suitable candidates, we performed a high-throughput study using an electronic structure database to search for $X_{2}YZ$-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys which passed our selection filters, we have used a combination of density functional theory calculations and spin dynamics modeling to investigate their magnetic properties including the magnetocrystalline anisotropy energy and exchange interactions. The candidates which fulfilled all the search criteria served as input for the investigation of the temperature dependence of the magnetization and determination of the Curie temperature. Based on our results, we suggest that ${\mathrm{Fe}}_2\mathrm{NiZn}$, ${\mathrm{Fe}}_2\mathrm{NiTi}$, and ${\mathrm{Ni}}_2\mathrm{CoFe}$ are potential candidates for permanent magnets with large out-of-plane magnetic anisotropy (1.23, 0.97, and 0.82 MJ/${\mathrm{m}}^3$, respectively) and high Curie temperatures lying more than 200 K above the room temperature. We further show that the magnitude and direction of anisotropy are very sensitive to the strain by calculating the values of anisotropy energy for several tetragonal phases. Thus application of strain can be used to tune the anisotropy in these compounds.

Flow of the normal fluid component of He II about a cylinder and flat plate in a channel with a square cross-section was observed by tracking clouds of ${\text{He}}_2^{*}$ excimers. Flow was produced using a lithographically patterned heater. The direction and speed of flow projected onto a plane parallel to the axis of the channel and perpendicular to the heater exhibited significant change with time even after the heater was turned off. Flow was recorded in movies. Excimers moved at least as fast as could be recorded by the camera. Velocity vector field maps of flow suggest formation of structures downstream of the cylinder object that are consistent with parts of eddies. In this paper, we establish a foundation to observe normal component flow of a quantum fluid over centimeters.

Binary metal sulfides are a large material family with distinct physical behavior. Here, we report the superconductivity observed in PbS by means of pressure engineering. PbS undergoes a series of electronic phase transitions upon compression, and superconductivity occurs above 23.9 GPa in the high-pressure CsCl-type cubic phase. Further compression suppresses the superconductivity while the $T_c$ is reduced from $\sim 12.0$ K at 23.9 GPa to 6.8 K at 44.3 GPa. Marginally higher $T_c\approx 12.3$ K is observed at 19.1 GPa during pressure release. The magnetic field effect verifies the superconductivity with possible multiband features. It is noted that PbS is one of the few binary metal sulfide superconductors with a $T_c$ above 10 K. This work demonstrates that PbS has a high $T_c$ under a moderate pressure due to the chemical “precompression” effect, which is very close to the $T_c$ in pure sulfur at $\sim 100$ GPa. PbS also shows unexpected multiband behavior in the cubic structure, benefiting the study of pressure-induced superconductivity and the underlying mechanism in metal sulfide compounds.

Unlike ferromagnetism, antiferromagnetism cannot readily be included in the quasiclassical Keldysh theory because of the rapid spatial variation in the directions of of the magnetic moments. The quasiclassical framework is useful because it separates the quantum effects occurring at length scales comparable to the Fermi wavelength from other length scales, and has successfully been used to study a wide range of phenomena involving both superconductivity and ferromagnetism. Starting from a tight-binding Hamiltonian, we develop general quasiclassical equations of motion and boundary conditions, which can be used to describe two-sublattice metallic antiferromagnets in the dirty limit. The boundary conditions are applicable also for spin-active boundaries that can be either compensated or uncompensated. Additionally, we show how nonuniform or dynamic magnetic textures influence the equations and we derive a general expression for observables within this framework.

Germanium telluride (GeTe) and its variants are promising compounds as high figure of merit thermoelectric materials due to their low lattice thermal conductivity. The strong anharmonicity and the intrinsic Ge vacancies are shown to be the origin of the low thermal conductivity. While the anharmonic effect on the lattice thermal conductivity has been systematically studied using the perturbation theory, the vacancy disorder has been treated perturbatively mostly as an extreme case of phonon-isotope scattering. This simplification ignores realistic features such as the nonbonding character and the detailed local environments near the vacancies. In this study, we calculate the lattice thermal conductivity of the cubic GeTe by considering the anharmonicity and the vacancy disorder on the same footing using the machine-learning potential molecular dynamics. We obtain the spectral function via the nonperturbative approaches, the velocity autocorrelation function, and the phonon unfolding scheme to investigate the effect of the vacancies on the lattice thermal conductivity. We find that the reduction in the lattice thermal conductivity by the vacancies originates from the momentum-dependent scattering of the acoustic phonon modes and the momentum-independent scattering of the optical phonon modes as determined by the strength of the anharmonicity.

We investigate the experimental signatures of Raman spectroscopy of bimagnon and trimagnon excitations in the distorted triangular lattice antiferromagnets $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. Motivated by Raman scattering experiments, we utilize spin wave theory to analyze the nearly ${120}^{\circ }$ spin-3/2 spiral-ordered antiferromagnetic ground state to compute the single-magnon density of states, single-magnon dispersion, and bimagnon and trimagnon Raman spectra (polarized and unpolarized). We perform calculations of the Heisenberg antiferromagnetic Hamiltonian that incorporates magnetic interactions (exchange, anisotropy, and interlayer coupling) and lattice distortion within a four-sublattice unit cell. We investigate the Hamiltonian for both model parameter sets and experimentally proposed magnetic interactions for $\alpha \text{−}L{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(L=\mathrm{Sr}, \mathrm{Ca})$. It is found that Raman scattering is capable of capturing the effect of the rotonlike $M$ and $M^{\prime }$ points on the bimagnon Raman spectrum. Our calculation confirms the connection between single-magnon rotonlike excitation energy and the bimagnon Raman excitation spectrum observed experimentally. The roton energy minimum in momentum space is half of the energy of a bimagnon excitation signal. The experimental magnetic Raman scattering result displays two peaks which have Raman shifts of 15 and 40 meV. Theoretical modeling and analysis of the experimental spectrum of $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$ within our distorted Heisenberg Hamiltonian lattice suggest that the low-energy peak at 15 meV is associated with the bimagnon excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon excitation. Based on our fitting procedure we propose a set of magnetic interaction parameters for $\alpha \text{−}{\mathrm{SrCr}}_2{\mathrm{O}}_4$. These parameters reproduce not only the experimental Raman spectrum but also the inelastic neutron scattering response (including capturing high-energy magnon branches). We also compute the unpolarized bimagnon and trimagnon Raman spectra for $\alpha \text{−}{\mathrm{CaCr}}_2{\mathrm{O}}_4$. In contrast to its Sr cousin the Ca-based material has an enhanced bimagnon response, with the high-energy peak still dominated by the trimagnon excitation. Furthermore, the polarization sensitivity of the Raman spectrum can be utilized to distinguish the bimagnon and trimagnon excitation channels.

Promoting the Curie temperature and tunning the magnetocrystalline anisotropy have been key issues in two-dimensional (2D) magnetic materials. Here, we address this issue by studying $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructures using first-principles calculations. Results show that the easy-magnetization direction rotates from the easy axis in the ${\mathrm{CrI}}_3$ monolayer to the easy plane in the $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructure. A built-in electric field in ${\mathrm{CrI}}_3$ introduced by the coupling between the ${\mathrm{CrI}}_3$ and Bi monolayers is found responsible for this spin reorientation. The Curie temperature of the $\mathrm{Bi}/{\mathrm{CrI}}_3$ heterostructure reaches 107 K, significantly larger than the Curie temperature (45 K) of the pristine ${\mathrm{CrI}}_3$ monolayer. The metal property of the heterostructure introduces a double exchangelike ferromagnetic (FM) coupling which promotes the Curie temperature. Moreover, in the Bi/bilayer ${\mathrm{CrI}}_3$ heterostructure, the coupling between ${\mathrm{CrI}}_3$ and Bi makes the interlayer ${\mathrm{CrI}}_3-{\mathrm{CrI}}_3$ exchange interaction become FM, and the ${\mathrm{CrI}}_3$ bilayer has a FM ground state. Our work provides a way to modulate the magnetic properties of 2D materials and may be used for designing 2D spintronic devices.

Two-band electronic structures with a valence and a conduction band separated by a tunable energy gap and with pairing of electrons in different channels can be relevant to investigate the properties of two-dimensional multiband superconductors and electron-hole superfluids, such as monolayer FeSe, recently discovered superconducting bilayer graphene, and double-bilayer graphene electron-hole systems. This electronic configuration also allows us to study the coexistence of superconductivity and charge-density waves in connection with underdoped cuprates and transition metal dichalcogenides. By using a mean-field approach to study the system mentioned above, we have obtained numerical results for superconducting gaps, chemical potential, condensate fractions, coherence lengths, and superconducting mean-field critical temperature, considering a tunable band gap and different fillings of the conduction band, for a parametric choice of the pairing interactions. By tuning these quantities, the electrons redistribute among valence and conduction band in a complex way, leading to a new physics with respect to single-band superconductors, such as density-induced and band-selective BCS-BEC crossover, quantum phase transitions, and hidden criticalities. At finite temperature, this phenomenon is also responsible for the nonmonotonic behavior of the superconducting gaps resulting in a superconducting-normal state reentrant transition, without the need of disorder or magnetic effects.

We show that nitrogen-vacancy (NV) centers could provide distinct signatures of emergent gauge fields in certain phase transitions in spin liquids. We consider the relaxation rate $1/T_1$ of a diamond NV center placed a distance $z_0$ above a crystal consisting of two-dimensional layers which hosts a U(1) quantum spin liquid with spinon Fermi surface. We show that, during a spinon pairing transition from a U(1) to a Z2 spin liquid, the relaxation rate exhibits a rapid fall just below the pairing transition. The drop is much faster than what can be accounted for by the opening of the energy gap. Instead, the rapid fall with a unique $z_0$ dependence of its height and width is the signature of the Meissner effect of the gauge field. We identify the organic ET compound which has a phase transition at 6 K as a candidate for the experimental observation of this phenomenon.

Owning to their high controllability, laser pulses have contributed greatly to our understanding of strongly correlated electron systems. However, typical multicycle pulses do not control the symmetry of systems, which plays an important role in the emergence of novel quantum phases. Here, we demonstrate that subcycle pulses whose oscillation is less than one period within a pulse envelope can control inversion and time-reversal symmetries in the electronic states of the one-dimensional extended Hubbard model. Using an ultrashort subcycle pulse, one can generate a steady electric current (SEC) in a photoexcited state due to an Aharonov-Bohm flux instantaneously introduced through the phase of an electric field. Consequently, time-reversal symmetry is broken. In contrast, a broad subcycle pulse does not induce SEC but instead generates electric polarization, thus breaking inversion symmetry. Both symmetry breakings in a photoexcited state can be monitored by second-harmonic generation. These findings provide a new methodology for designing the symmetries of electronic states and open up a new field of subcycle-pulse engineering.

Strong coupling between matter and quantized electromagnetic fields in a cavity has emerged as a possible route toward controlling the phase of matter in the absence of an external drive. We develop a faithful and efficient theoretical framework to analyze quantum geometry and topology in materials ultrastrongly coupled to cavity electromagnetic fields in two dimensions. The formalism allows us to accurately evaluate geometrical and topological quantities, such as Berry phase and Chern number, in ultrastrong and deep strong-coupling regimes. We apply our general framework to analyze a concrete model of massive Dirac fermions coupled to a circularly polarized cavity mode. Surprisingly, in addition to an ordinary transition to the topological phase, our analysis reveals a unique feature in deep strong-coupling regimes, namely the emergence of a reentrant transition to the topologically trivial phase. We also demonstrate its intuitive understanding by showing the unitary mapping between the low-energy effective theory of strongly coupled light-matter systems and the Haldane honeycomb model.

The layered transition metal dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$ has evoked great interest owing to its particularly rich electronic phase diagram including different charge density wave (CDW) phases. However, few studies have focused on its hysteretic electronic phase transitions based on the in-depth discussion of the delicate interplay among temperature-dependent electronic interactions. Here, we report a sequence of spatial electronic phase transitions in the hysteresis temperature range (160–230 K) of $1T\text{−}{\mathrm{TaS}}_2$ via variable-temperature scanning tunneling microscopy. Several emergent electronic states are investigated at multiscale during the commensurate CDW–triclinic CDW (CCDW-TCDW) phase transitions: a spotty-CDW state above $\sim 160\mathrm{K}$, a network-CDW (NCDW) state above $\sim 180\mathrm{K}$ during the warmup process, a belt-TCDW state below $\sim 230\mathrm{K}$, a NCDW state below $\sim 200\mathrm{K}$, and finally a mosaic-CDW state below $\sim 160\mathrm{K}$ during cooldown from the TCDW phase. These emergent electronic states are associated with the delicate temperature-dependent competition and/or cooperation of stacking-dependent interlayer interactions, intralayer electron-electron correlations, and electron-phonon ($e\text{−}ph$) coupling of $1T\text{−}{\mathrm{TaS}}_2$. Our results not only provide insight to understand the hysteretic electronic phase transitions in the correlated CDW state, but also pave a way to realize more exotic quantum states by accurately and effectively tuning various interior interactions in correlated materials.

We report $c$-axis transport studies on magnetic topological insulators $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$. We performed systematic $c$-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find that the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the Néel temperature $\left(T_{\mathrm{N}}\right)$ where the magnetoconductivity is proportional to $B^2$. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above $T_{\mathrm{N}}$. These results demonstrate that in the $c$-axis magnetotransport of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$.