We investigate the ground-state phase diagram of the frustrated transverse-field Ising (TFI) model on the checkerboard lattice (CL), which consists of Néel, collinear, quantum paramagnet, and plaquette-valence bond solid (-VBS) phases. We implement a numerical simulation that is based on the recently developed unconstrained tree tensor network ansatz, which systematically improves the accuracy over the conventional methods as it exploits the internal gauge selections. At the highly frustrated region $\left(J_2=J_1\right)$, we observe a second-order phase transition from the plaquette-VBS state to the paramagnet phase at the critical magnetic-field ${\mathrm{\Gamma }}_c=0.28$ with the associated critical exponents $\nu =1$ and $\gamma \simeq 0.4$, which are obtained within the finite-size scaling analysis on different lattice sizes $N=4\times 4,6\times 6,8\times 8$. The stability of the plaquette-VBS phase at low magnetic fields is examined by a spin-spin correlation function, which verifies the presence of plaquette-VBS at $J_2=J_1$ and rules out the existence of a Néel phase. In addition, our numerical results suggest that the transition from the Néel (for $J_2<J_1)$ to the plaquette-VBS phase is a deconfined phase transition. Moreover, we introduce a mapping, which renders the low-energy effective theory of the TFI on the CL to be the same model on the $J_1-J_2$ square lattice (SL). We show that the plaquette-VBS phase of the highly frustrated point $J_2=J_1$ on the CL is mapped to the emergent string-VBS phase on the SL at $J_2=0.5J_1$.

When superconducting (SC) and charge-density-wave (CDW) orders compete, novel low-temperature behaviors can result. From an analysis of the Landau-Ginzburg-Wilson theory of competing orders, we demonstrate the generic occurrence of a “fragile” SC phase at low temperatures and high fields in the presence of weak disorder. Here, the SC order is largely concentrated in the vicinity of dilute dislocations in the CDW order, leading to transition temperatures and critical currents that are parametrically smaller than those characterizing the zero-field SC phase. This may provide the outline of an explanation of the recently discovered “resilient” superconducting phase at high fields in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$.

Low-temperature scanning tunneling microscopy and spectroscopy has been used to image the vortex core and the vortex lattice in FeSe single crystals. The local tunneling spectra acquired at the center of elliptical vortex cores display a strong particle-hole asymmetry with spatial oscillation, characteristic of the quantum-limit vortex core. Furthermore, a quasihexagonal vortex lattice at low magnetic field undergoes noticeable rhombic distortions above a certain field $\sim 1.5$ T. This field $H^{*}$ also reveals itself as a kink in the magnetic field dependence of the specific heat. The observation of a nearly hexagonal vortex lattice at low field is very surprising for materials with an orthorhombic crystal structure and it is in apparent contradiction with the elliptical shape of the vortex cores. These observations can be directly connected to the multiband nature of superconductivity in this material, provided we attribute them to the suppression of superconducting order parameter in one of the energy bands. Above the field $H^{*}$ the superconducting coherence length for this band can well exceed the intervortex distance which strengthens the nonlocal effects. Therefore, in addition to multiple-band effects, other possible sources that can contribute to the observed evolution of the vortex-lattice structure include nonlocal effects which cause the field-dependent interplay between the symmetry of the crystal and vortex lattice or the magnetoelastic interactions due to the strain field generated by vortices.

We report strong electron-electron interactions in quantum wires etched from an InAs quantum well, a material generally expected to have strong spin-orbit interactions. We find that the current through the wires as a function of the bias voltage and temperature follows the universal scaling behavior of a Tomonaga-Luttinger liquid. Using a universal scaling formula, we extract the interaction parameter and find strong electron-electron interactions, increasing as the wires become more depleted. We establish theoretically that the spin-orbit interaction cause only minor modifications of the interaction parameter in this regime, indicating that genuinely strong electron-electron interactions are indeed achieved in the device. Our results suggest that etched InAs wires provide a platform with both strong electron-electron interactions and the strong spin-orbit interaction.

Spin-noise spectroscopy has developed into a very powerful tool to access the electron spin dynamics. While the spin-noise power spectrum in an ensemble of quantum dots in a magnetic field is essentially understood, we argue that the investigation of the higher-order cumulants promises to provide additional information not accessible by the conventional power-noise spectrum. We present a quantum-mechanical approach to the correlation function of the spin-noise power operators at two different frequencies for small spin bath sizes and compare the results with a simulation obtained from the classical spin dynamics for large number of nuclear spins. This bispectrum is defined as a two-dimensional frequency cut in the parameter space of the fourth-order spin correlation function. It reveals information on the influence of the nuclear-electric quadrupolar interactions on the long-time electron spin dynamics dominated by a magnetic field. For large bath sizes and spin lengths the quantum-mechanical spectra converge to those of the classical simulations. The broadening of the bispectrum across the diagonal in the frequency space is a direct measure of the quadrupolar interaction strength. A narrowing is found with increasing magnetic field indicating a suppression of the influence of quadrupolar interactions in favor of the nuclear Zeeman effect.

Linearly polarized microphotoluminescence ($\mu$-PL) measurements of strained ${\mathrm{WSe}}_2$ monolayers in out-of-plane high magnetic fields are presented. At low temperature, a splitting of the bright exciton emission into two exciton components is observed, which is attributed to an in-plane uniaxial strain based on the full polarization dependence of the photoluminescence spectrum. High magnetic field measurements directly reveal a distinct evolution of the linear polarization and allows us to extract the valley coherence time constants ($T_{s2}^{*}$) for both exciton components. For the high-energy transition of the exciton, the valley coherence time $\simeq 0.45$ ps, closely matching $T_{s2}^{*}$ of an unstrained monolayer ($\simeq 0.34$ ps). For the low-energy exciton, however, $T_{s2}^{*}$ is four times larger, $\simeq 1.97$ ps. This valley coherence time observed here may be explored for future valleytronic applications.

${\mathrm{PdSb}}_2$ is a candidate for hosting sixfold-degenerate exotic fermions (beyond Dirac and Weyl fermions). The nontrivial band crossing protected by the nonsymmorphic symmetry plays a crucial role in physical properties. We have grown high-quality single crystals of ${\mathrm{PdSb}}_2$ and characterized their physical properties under several stimuli (temperature, magnetic field, and pressure). While it is a diamagnetic Fermi-liquid metal under ambient pressure, ${\mathrm{PdSb}}_2$ exhibits a large magnetoresistance with continuous increase up to 14 T, which follows Kohler's scaling law at all temperatures. This implies one-band electrical transport, although multiple bands are predicted by first-principles calculations. By applying magnetic field along the [111] direction, de Haas–van Alphen oscillations are observed with frequency of 102 T. The effective mass is nearly zero ($0.045m_0$) with the Berry phase close to $\pi$, confirming that the band close to the $R$ point has a nontrivial character. Under quasihydrostatic pressure ($p$), evidence for superconductivity is observed in the resistivity below the critical temperature $T_c$. The dome-shaped $T_c$ versus $p$ is obtained with maximum $T_{\mathrm{c}}^{max}\sim 2.9\mathrm{K}$. We argue that the formation of Cooper pairs (bosons) is the consequence of the redistribution of the sixfold-degenerate fermions under pressure.

We report large enhancement of upper critical field $H_{\mathrm{c}2}$ observed in superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ thin films. Through dimensional crossover approaching two dimensions, $H_{\mathrm{c}2}$ except the in-plane field direction is dramatically enhanced compared to bulks, following a definite relation distinct from bulk one between $H_{\mathrm{c}2}$ and the transition temperature. The anomalous enhancement of $H_{\mathrm{c}2}$ is highly suggestive of important changes of the superconducting properties, possibly accompanied with rotation of the triplet $d$ vector. Our findings will become a crucial step to further explore exotic properties by employing ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ thin films.

We study the polarization dependence of optical absorption for a modified Haldane model, which exhibits antichiral edge modes in the presence of sample boundaries and has been argued to be realizable in transition metal dichalcogenides or Weyl semimetals. A rich optical phase diagram is unveiled, in which the correlations between perfect circular dichroism, pseudospin andvalley polarization can be tuned independently upon varying the Fermi energy. In particular, perfect circular dichroism and valley polarization are achieved simultaneously. This combination of optical properties suggests some interesting photonic device functionality (e.g., light polarizer) which could be combined with valleytronics applications (e.g., generation of valley currents).

We develop and benchmark a technique for simulating excitation spectra of generic two-dimensional quantum lattice systems using the framework of projected entangled-pair states (PEPS). The technique relies on a variational ansatz for capturing quasiparticle excitations on top of a PEPS ground state. Our method perfectly captures the quasiparticle dispersion relation of the square-lattice transverse-field Ising model and reproduces the spin-wave velocity and the spin-wave anomaly in the square-lattice Heisenberg model with high precision.

Herein we report the formation energies of native point defects in $1T\text{−}{\mathrm{TiS}}_2$ material. In the present study we used one of the most refined models, e.g., a global hybrid density functional with dispersion corrections for all the calculations. The stability domain of $1T\text{−}{\mathrm{TiS}}_2$ was thoroughly investigated. We demonstrate unambiguously that the most stable defect is titanium interstitial in octahedral site in the van der Waals gap. Indeed, this simple defect can explain the distinguishable electric behavior of ${\mathrm{TiS}}_2$ reported in the litterature, i.e., nondegenerated semiconductor (SC) or semimetal (SM) depending on the synthesis conditions. Simulated defect concentrations vs synthesis temperature are also provided and the SC-SM transition is discussed.

Identifying variational wave functions that efficiently parametrize the physically relevant states in the exponentially large Hilbert space is one of the key tasks towards solving the quantum many-body problem. Powerful tools in this context such as tensor network states have recently been complemented by states derived from artificial neural networks (ANNs). Here, we propose and investigate a new family of quantum states, coined generalized transfer matrix states (GTMS), which bridges between the two mentioned approaches in the framework of deep ANNs. In particular, we show by means of a constructive embedding that the class of GTMS contains generic matrix product states while at the same time being capable of capturing more long-ranged quantum correlations that go beyond the area-law entanglement properties of tensor networks. While the state amplitude of generic deep ANNs cannot be exactly evaluated, that of a GTMS network can be analytically computed using transfer matrix methods. With numerical simulations, we demonstrate how the GTMS network learns random matrix product states in a supervised learning scheme, and how augmenting the network by long-ranged couplings leads to the onset of volume-law entanglement scaling. By means of an explicit example using variational Monte Carlo, we also show that GTMS can parametrize critical quantum many-body ground states to a good accuracy. Our findings suggest that GTMS are a promising candidate for the study of critical and dynamical quantum many-body systems.

Using time- and angle-resolved photoemission spectroscopy, we study the response of metallic single-layer ${\mathrm{TaS}}_2$ in the 1H structural modification to the generation of excited carriers by a femtosecond laser pulse. A complex interplay of band structure modifications and electronic temperature increase is observed and analyzed by direct fits of model spectral functions to the two-dimensional (energy and $k$-dependent) photoemission data. Upon excitation, the partially occupied valence band is found to shift to higher binding energies by up to $\approx 100\mathrm{meV}$, accompanied by electronic temperatures exceeding 3000 K. These observations are explained by a combination of temperature-induced shifts of the chemical potential, as well as temperature-induced changes in static screening. Both contributions are evaluated in a semiempirical tight-binding model. The shift resulting from a change in the chemical potential is found to be dominant.

Compounds with a tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure are characterized by the possibility of the isostructural phase transition from the tetragonal phase to the collapsed tetragonal phase induced by the external pressure. An example of a compound with such a phase transition is ${\mathrm{KFe}}_2{\mathrm{As}}_2$, which belongs to the family of the iron-based superconductors. In this paper, we investigate the effects of the phase transition on the structural, electronic, and dynamical properties of this compound. Performing the ab initio calculations, we reproduce the dependence of the lattice constants on pressure and analyze the changes of the interatomic distances in the tetragonal and collapsed tetragonal phases. Using the tight binding model with maximally localized Wannier orbitals, we calculate and discuss the influence of pressure on the electronic band structure as well as on the shape of the Fermi surface. We found a precursor of the phase transition in the form of enhancement of overlapping between two Wannier orbitals of As atoms. In order to better understand the superconducting properties of ${\mathrm{KFe}}_2{\mathrm{As}}_2$, we study the orbital-projected Cooper pairs susceptibility as a function of pressure. We found a decrease of susceptibility with the increasing pressure in a good qualitative agreement with experimental observation. The structural transition also influences the phonon spectrum of ${\mathrm{KFe}}_2{\mathrm{As}}_2$, which exhibits pronounced changes induced by pressure. Some phonon modes related with the vibrations of Fe and As atoms show an anomalous, nonmonotonic dependence on pressure close to the phase transition.

We carry out molecular-dynamics simulation of shear-induced martensitic phase transitions between the orthorhombic and nonorthorhombic (triclinic and monoclinic) phases of crystalline polyethylene (PE) in the framework of a realistic all atom model of the polymer. We show that the variation of the shear rate allows observing on a nanosample both a strongly nonequilibrium phase transition occurring by random nucleation and irregular growth of a new phase (“civilian” way, for rapid deformations) and the coherent, or “military,” kinetics (generally considered as usual for martensitic transformations). We induce transitions from the orthorhombic to the triclinic phase according to two transformation modes observed in experiment on PE single crystals. Rapid deformation favors the transition directly to the triclinic phase, slow deformation—first to the intermediate monoclinic, and only then—to the triclinic phase. The second way corresponds to the experiment on extended chain PE. We explain this result and analyze the competition between different transformation and plastic deformation modes. Rotations of PE chains around their axes necessary for the transition between the orthorhombic and nonorthorhombic phases are executed by short twist defects diffusing along the chains. The transition between the monoclinic and triclinic phases occurs through half-chain-period translations of the chains along their axes, mostly collectively, as crystallographic slips.

Using the Feynman diagram techniques, we theoretically obtain the quantum conductivity correction for a two-dimensional pseudospin-1 electron system in the presence of long-range diagonal disorder. Theoretical results clearly reveal that the quantum correction depends on the sublattice correlation properties of disorder potential. The sublattice correlated disorder gives rise to normal weak localization, while the sublattice uncorrelated impurity potential leads to the absence of logarithmic term in quantum conductivity correction. Remarkably, those results cannot be understood by conventional symmetry classification. An additional symmetry operator involving internal sublattice degrees of freedom, analogous with the time reversal symmetry operator, enables us to clarify our findings from symmetry consideration.

Using the density-matrix renormalization group technique, we study a one-dimensional spin-$\frac{1}{2}$ Heisenberg chain consisting of coupled tetramers as an effective spin model for the copper vanadate ${\mathrm{CuInVO}}_5$. We obtain the ground-state phase diagram as a function of intratetramer and intertetramer exchange interactions, exhibiting two multimerized singlet phases: one is characterized by the formation of tetramer-singlet units, and the other is characterized by the formation of dimer-singlet pairs. We show that the finite spin gaps in both the singlet phases smoothly vanish at the phase boundary: a second-order phase transition defining a quantum critical point (QCP). The phase boundary is also captured by the fact that the central charge is unity at the phase boundary and zero otherwise in the thermodynamic limit. It is interesting that the dimer-singlet state is interpreted as a Haldane state with hidden ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry breaking. We further demonstrate that the experimental magnetization curve (which starts increasing with zero or tiny field) can be reasonably explained only by assuming the exchange parameters of ${\mathrm{CuInVO}}_5$ to be very close to the phase boundary. Thus, we argue that ${\mathrm{CuInVO}}_5$ may be a first example material which at ambient pressure stands near a QCP between two singlet phases. By varying the balance of exchange interactions with pressure, a transition from the Néel phase to either of the singlet phases could be observed.

We studied the spin torque efficiency and the Dzyaloshinskii-Moriya interaction (DMI) of heterostructures that contain interface(s) of Ir and Co. The current-induced shifts of the anomalous Hall loops were used to determine the spin torque efficiency and DMI of $[\text{Pt}/\text{Co}/X]$ multilayers $(X=\mathrm{Ir},\text{Cu})$ as well as Ir/Co and Pt/Ir/Co reference films. We find the effective spin Hall angle and the spin-diffusion length of Ir to be ∼0.01 and less than ∼1 nm, respectively. The short spin-diffusion length and the high conductivity make Ir an efficient spin sink layer. Such spin sink layer can be used to control the flow of spin current in heterostructures and to induce sufficient spin-orbit torque on the magnetic layer. The DMI of the Ir and Co interface is found to be in the range of ∼1.4 to $\sim 2.2\mathrm{mJ}/{\mathrm{m}}^2$, similar in magnitude to that of the Pt and Co interface. The Ir/Co and Pt/Co interfaces possess the same sign of DMI, resulting in a reduced DMI for the [Pt/Co/Ir] multilayers compared to that of the [Pt/Co/Cu] multilayers. These results show the unique role that the Ir layer plays in defining spin-orbit torque and chiral magnetism in thin-film heterostructures.

Phonon measurements in the $A15$-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of ${\mathrm{Nb}}_3\mathrm{Sn}$, or unfavorable neutron scattering properties in the case of ${\mathrm{V}}_3\mathrm{Si}$. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature $T_c$. Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in ${\mathrm{V}}_3\mathrm{Si}$ with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at $T_s=18.9\mathrm{K}$, i.e., just above the measured superconducting phase transition at $T_c=16.8\mathrm{K}$. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of $T_c$. Furthermore, we explain superconductivity-induced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al, [Phys. Rev. B 56, 5552 (1997)].