Type-I superconductivity was recently reported for the Dirac semimetal ${\mathrm{PdTe}}_2$ ($T_c\approx 1.6$ K) with, remarkably, multiple critical fields and a complex phase diagram. Here, measurements of the specific heat utilizing a thermal relaxation technique are presented. Conventional weak-coupling BCS superconductivity is confirmed by examining the temperature dependence of the specific heat in zero field. By probing the latent heat accompanying the superconducting transition, thermodynamic evidence for type-I superconductivity is attained. The presence of the intermediate state is observed as a significant broadening of the superconducting transition onto lower temperatures at high fields as well as irreversibility in the specific heat in zero-field-cooled data at 8.5 mT.

Two-dimensional (2D) topological superconductors are highly desired because they not only offer opportunities for exploring novel exotic quantum physics but also possess potential applications in quantum computation. However, there are few reports about 2D superconductors, let alone topological superconductors. Here, we find a 2D monolayer ${\mathrm{W}}_2{\mathrm{N}}_3$, which can be exfoliated from its real van der Waals bulk material with much lower exfoliation energy than ${\mathrm{MoS}}_2$, to be a topological metal with exotic topological states at different energy levels. Owing to the Van Hove singularities, the density of states near the Fermi level are high, making the monolayer a compensate metal. Moreover, the monolayer ${\mathrm{W}}_2{\mathrm{N}}_3$ is unveiled to be a superconductor with the superconducting transition temperature $T_C$ $\sim$ 22 K and a superconducting gap of about 5 meV based on the anisotropic Migdal-Eliashberg formalism, arising from the strong electron-phonon coupling around the $\mathrm{\Gamma }$ point, and the 2D superconductor is phonon mediated and fits the BCS mechanism with an Ising-type pairing. Because of the strong electron and lattice coupling, the monolayer displays a non-Fermi liquid behavior in its normal states at temperatures lower than 80 K, where the specific heat exhibits $T^3$ behavior and the Wiedemann-Franz law is dramatically violated. Our findings not only provide a platform to study the emergent phenomena in 2D topological superconductors, but also open a door to discover more 2D high-temperature topological superconductors in van der Waals materials.

Inspired by the recent discovery of superconductivity in the nickelate ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$, we study a generalized $t\text{−}J$ model to investigate the correlated phases induced by doping spin-1 ${\mathrm{Ni}}^{2+}$ into a spin-$1/2$ Mott insulator formed by ${\mathrm{Ni}}^{1+}$. Based on a three-fermion parton mean-field analysis, we identify a robust fractional Fermi liquid (FL*) phase for almost every doping level. The FL* state is characterized by a small Fermi pocket on top of a spin liquid, which violates the Luttinger theorem of a conventional Fermi liquid and is an example of a symmetric pseudogap metal. Furthermore, we verify our theory in one dimension through density matrix renormalization group simulations on both the generalized $t\text{−}J$ model and a two-orbital Hubbard model. The fractional Fermi liquid reduces to a fractional Luttinger liquid (LL*) in one dimension, which is connected to the conventional Luttinger liquid through a continuous quantum phase transition by tuning interaction strength. Our findings offer insights into correlated electron phenomena in nickelate superconductors and other multiorbital transition-metal oxides with a spin-triplet $d^8$ state.

We motivate a close look on the usefulness of the Gaffnian and Haffnian quasihole manifold (null spaces of the respective model Hamiltonians) for well-known gapped fractional quantum Hall phases. The conformal invariance of these subspaces is derived explicitly from microscopic many-body states. The resultant conformal field theory (CFT) description leads to an intriguing emergent primary field with $h=2,c=0$, and we argue the quasihole manifolds are quantum mechanically well defined and well behaved. Focusing on the incompressible phases at $\nu =\frac{1}{3}$ and $\frac{2}{5}$, we show the low-lying excitations of the Laughlin phase are quantum fluids of Gaffnian and Haffnian quasiholes, and give a microscopic argument showing that the Haffnian model Hamiltonian is gapless against Laughlin quasielectrons. We discuss the thermal Hall conductance and shot-noise measurements at $\nu =\frac{2}{5}$, and argue that the experimental observations can be understood from the dynamics within the Gaffnian quasihole manifold. A number of detailed predictions on these experimental measurements are proposed, and we discuss their relationships to the conventional CFT arguments and the composite fermion descriptions.

In this paper, we make a comprehensive study of the properties of a gapped Dirac semimetal model, which was originally proposed in the magnetoinfrared spectroscopy measurement of $\mathrm{Ze}{\mathrm{Te}}_5$, and includes both the linear and parabolic dispersions in all three directions. We find that, depending on the band inversion parameters, ${\zeta }^{\prime }$ and ${\zeta }_z^{\prime }$, the model can support three different phases: the single Dirac point (DP) phase, the double DPs phase, and the Dirac ring phase. The three different phases can be distinguished by their low-energy features in the density of states (DOS) and optical conductivity. At high energy, both the DOS and optical conductivity exhibit power-law-like behaviors, with the asymptotic exponents depending heavily on the signs of ${\zeta }^{\prime }$ and ${\zeta }_z^{\prime }$. Moreover, the thumb-of-rule formula between the DOS and optical conductivity is satisfied only when $({\zeta }^{\prime },{\zeta }_z^{\prime })>0$. The implications of our results for experiments are discussed.

Thin transition metal dichalcogenides sustain superconductivity at large in-plane magnetic fields due to Ising spin-orbit protection, which locks their spins in an out-of-plane orientation. Here we use thin $\mathrm{Nb}{\mathrm{Se}}_2$ as superconducting electrodes laterally coupled to graphene, making a planar, all van der Waals two-dimensional Josephson junction (2DJJ). We map out the behavior of these novel devices with respect to temperature, gate voltage, and both out-of-plane and in-plane magnetic fields. Notably, the 2DJJs sustain supercurrent up to parallel fields as high as 8.5 T, where the Zeeman energy $E_Z$ rivals the Thouless energy $E_{Th}$, a regime hitherto inaccessible in graphene. As the parallel magnetic field $H_{\parallel }$ increases, the 2DJJ's critical current is suppressed and in a few cases undergoes suppression and recovery. We explore the behavior in $H_{\parallel }$ by considering theoretically two effects: a 0-$\pi$ transition induced by tuning of the Zeeman energy and the unique effect of ripples in an atomically thin layer which create a small spatially varying perpendicular component of the field. The 2DJJs have potential utility as flexible probes for two-dimensional superconductivity in a variety of materials and introduce high $H_{\parallel }$ as a newly accessible experimental knob.

We report magnetoresistance and Hall angle measurements of the electron-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ over a wide range of dopings from $x=0.08-0.17$. Above 100 K, we find an unconventional $\sim H^{3/2}$ magnetic field dependence of the magnetoresistance observed in all samples doped within the superconducting dome. Further, the measured magnetoresistance violates Kohler's rule. Given the ubiquity of this anomalous magnetoresistance at high temperatures above the superconducting dome, we speculate that the origin of this behavior is linked to the unusual $\rho \sim T^2$ resistivity observed over the same wide parameter range at high temperatures. We also find a strong doping dependence of the Hall angle with an unconventional temperature dependence of $cot{\theta }_H\sim T^4$ ($T^{2.5}$) for samples doped below (above) the Fermi surface reconstruction doping $x_{\text{FSR}}=0.14$.

Electrides, with excess anionic electrons confined in their empty space, are promising for uses in catalysis, nonlinear optics, and spin electronics. However, the application of electrides is limited by their high chemical reactivity. In this paper, we report a group of chemically stable monolayer electrides with the presence of switchable nearly free electron (NFE) states in their electronic structures. Unlike conventional electrides, which are metals with floating electrons forming the bands crossing the Fermi level, the switchable electrides are semiconductors holding the NFE states close to the Fermi level. According to a high-throughput search, we identified 11 candidate materials with low-energy NFE states that can likely be exfoliated from the known layered materials. Under external strain, these NFE states, stemming from the surface image potential, will be pushed downward to cross the Fermi level. The critical semiconductor-metal transition can be achieved by a strain within 10% in several monolayer materials. These switchable electrides may provide an ideal platform for exploring quantum phenomena and modern electronic device applications.

We study a simple model of symmetry-enriched topological order obtained by decorating a toric code model with lower-dimensional symmetry-protected topological states. We show that the symmetry fractionalization in this model can be characterized by string order parameters, and that these signatures are robust under the effects of external fields and interactions, up to the phase transition point. This extends the recent proposal of Garre-Rubio and Iblisdir [New J. Phys. 21, 113016 (2019)] beyond the setting of fixed-point tensor network states, and solidifies string order parameters as a useful tool to characterize and detect symmetry fractionalization. In addition to this, we observe how the condensation of an anyon that fractionalizes a symmetry forces that symmetry to spontaneously break, and we give a proof of this in the framework of projected entangled pair states. This phenomenon leads to a notable change in the phase diagram of the toric code in parallel magnetic fields

The unequal electronic occupation of localized orbitals (orbital polarization), and associated lowering of symmetry and degeneracy, play an important role in the properties of transition metal oxides. Here, we examine systematically the underlying origin of orbital polarization, taking as exemplar the $3d$ manifold of ${\mathrm{Co}}^{2+}$ in a variety of spin, orbital, and structural phases in the double perovskite ${\mathrm{La}}_2{\mathrm{CoTiO}}_6$ and the (001) superlattice ${\left({\mathrm{LaCoO}}_3\right)}_1+{\left({\mathrm{LaTiO}}_3\right)}_1$ systems. Superlattices are of specific interest due to the large experimentally observed orbital polarization of their Co cations. Based on first principles calculations, we find that robust and observable orbital polarization requires symmetry reduction through the lattice structure; the role of local electronic interactions is to greatly enhance the orbital polarization.

Miscut surfaces of layered crystals can exhibit a stair-like sequence of terraces having periodic variation in their atomic structure. For hexagonal close-packed and related crystal structures with an $\alpha \beta \alpha \beta$ stacking sequence, there have been long-standing questions regarding how the differences in adatom attachment kinetics at the steps separating the terraces affect the fractional coverage of $\alpha$ versus $\beta$ termination during crystal growth. To demonstrate how surface x-ray scattering can help address these questions, we develop a general theory for the intensity distributions along crystal truncation rods (CTRs) for miscut surfaces with a combination of two terminations. We consider half-unit-cell-height steps, and variation of the coverages of the terraces above each step. Example calculations are presented for the GaN $\left(0001\right)$ surface with various reconstructions. These show which CTR positions are most sensitive to the fractional coverage of the two terminations. We compare the CTR profiles for exactly oriented surfaces to those for vicinal surfaces having a small miscut angle, and investigate the circumstances under which the CTR profile for an exactly oriented surface is equal to the sum of the intensities of the corresponding family of CTRs for a miscut surface.

We report an inelastic neutron scattering study on the magnetic excitations of ${\mathrm{Pr}}_2{\mathrm{NiO}}_{4+\delta }$ ($\delta \sim 0.24\pm 0.01$) at $T=10$ K. Spin stripe ordering becomes pronounced below $\sim 220$ K with an incommensurability $\epsilon \approx 0.346$, and a strong influence of interstitial oxygen is identified on establishing a long-range spin stripe ordering. Apart from the Goldstone modes emerging from the magnetic satellites (${\mathbf{q}}_m$), multiple homologous modes are observed along the spin stripe modulation separated by $\mathrm{\Delta }{\mathbf{q}}_m\approx 0.076$ in reciprocal lattice units, which is interpreted by the internal periodicity of the long-range ordered discommensurated spin stripes.

We use total scattering to study the reversible transition between the polar $1T^{\prime }$ and nonpolar $T_d$ phases of layered $\mathrm{Mo}{\mathrm{Te}}_2$ taking place at 240 K. Whereas, macroscopically, the transition appears to be first order, locally, it is not. In particular, a great deal of the stacking sequence of Te-Mo-Te layers characteristic of the polar $1T^{\prime }$ phase persists locally in the nonpolar $T_d$ phase, and vice versa, over a broad temperature range extending about 100 K both below and above the transition. The intermixing ratio for the two sequences evolves gradually across the transition temperature, consistent with a second-order transition behavior. The presence of coexisting local polar and nonpolar regions and the resulting variety of internal interfaces where the spatial inversion symmetry is broken may be behind some of the unusual electronic properties of $T_d\text{−}\mathrm{Mo}{\mathrm{Te}}_2$, including its putative type-II Weyl semimetal state.

Aperiodic metamaterials can host topological or localized wave modes, and gradually changing structures are known to induce conversion between elastic wave types. This work describes a family of one dimensional, aperiodic elastic structures that can host localized modes or achieve low-frequency attenuation by varying a single geometrical parameter, i.e., the ratio between the sizes of two interacting periodic structures. Two periodic sets of slits on the top and bottom of a slender beam scatter bending waves. The thickness profile and cross section of beams with a period ratio close to 1 varies slowly, resulting in localized wave modes. The localization happens because of spatially confined band gaps and is only triggered for certain excitation locations, unlike wave localization due to a local defect. If the periods differ more, broad band gaps appear at wave numbers equal to linear combinations of the individual Brillouin zone edges of each set of slits. We call these interaction band gaps, since they result from the interplay between two periodic sets of slits. Low-frequency wave attenuation is achieved while maintaining a high overall bending stiffness.

Motivated by the strong magnetoelectric coupling in the multiferroic geometrically frustrated triangular antiferromagnet $\mathrm{Cu}\mathrm{Cr}{\mathrm{O}}_2$ and the high sensitivity of its magnetic structure to external fields, we investigate induced magnetic phases at very low temperatures under high magnetic fields ($B$) up to 325 T applied along the [001] direction. Analytical calculations and Monte Carlo (MC) simulations based on a realistic three-dimensional classical Heisenberg model are used to reveal these magnetic phases. Interestingly, our model mimics a real distorted crystal which considers exchange interactions up to third-nearest neighbors in the $ab$ plane and an interplane interaction, in addition to hard and easy axes anisotropies along the [110] and the [001] directions, respectively. For $B\ge 70\mathrm{T}$, both our MC and analytical results are in an excellent agreement and evidence three commensurate phases, namely, the commensurate Y (CY), the up-up-down (UUD), and the V phases as the magnetic field increases. The field dependence of the characteristic angles of the CY and V phases is determined. Moreover, the saturation field is estimated at 325 T, indicating that the previously predicted values obtained by extrapolation of experimental data are too small. Below 70 T, our MC results indicate that the CY phase is no more stable and several incommensurate Y phases appear. Overall, the observed magnetic phases at nearly 0 K are in a good agreement with a recently published experimental phase diagram. It should be noted that our MC data reject the incommensurate umbrella phase at very low temperatures, which was reported in previous studies.

It is commonly assumed that a lattice of skyrmions, emerging in two-dimensional noncentrosymmetric magnets in external magnetic fields, can be represented as a sum of three magnetic helices. To test this assumption, we compare two approaches to a description of a regular skyrmion structure. We construct (i) a lattice of Belavin-Polyakov-like skyrmions within the stereographic projection method and (ii) a deformed triple helix defined with the use of elliptic functions. The estimates for the energy density and magnetic profiles show that these two ansatzes are nearly identical at zero temperature for intermediate magnetic fields. However, at higher magnetic fields, near the transition to a topologically trivial uniform phase, the stereographic projection method is preferable, particularly for the description of a disordered skyrmion liquid phase. We suggest exploring the intensities of secondary Bragg peaks to obtain additional information about the magnetic profile of individual skyrmions. We estimate these intensities to be several percent of the main Bragg peak at high magnetic fields.

In this work, we study the complex entanglement between spin interactions, electron correlation, and Janh-Teller structural instabilities in the ${5d}^1{J}_{\mathrm{eff}}=\frac{3}{2}$ spin-orbit coupled double perovskite ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$ using first principles approaches. By combining noncollinear magnetic calculations with multipolar pseudospin Hamiltonian analysis and many-body techniques, we elucidate the origin of the observed quadrupolar canted antifferomagnetic. We show that the noncollinear magnetic order originates from Jahn-Teller distortions due to the cooperation of Heisenberg exchange, quadrupolar spin-spin terms, and both dipolar and multipolar Dzyaloshinskii-Moriya interactions. We find a strong competition between ferromagnetic and antiferromagnetic canted and collinear quadrupolar magnetic phases: the transition from one magnetic order to another can be controlled by the strength of the electronic correlation ($U$) and by the degree of Jahn-Teller distortions.

${\left(\mathrm{MnNiSi}\right)}_{0.56}{\left(\mathrm{FeNiGe}\right)}_{0.44}$ belongs to a new family of alloys, similar to MnAs, showing a magnetostructural first-order transition near room temperature with large latent heat and magnetocaloric effect (MCE). From isothermal magnetization, remarkable values of the entropy change have been reported, such as $|\mathrm{\Delta }{S}_T|=11.5$ J/kg K at 290 K for the small field change of 1 T, or $|\mathrm{\Delta }{S}_T|=70.1$ J/kg K for 5 T on a slightly different composition. Strangely, this last value almost doubles that obtained from the Clausius-Clapeyron equation. Therefore, a very detailed set of calorimetric determinations have been made including heat capacity by adiabatic and differential scanning calorimetry (DSC), and direct measurement of the heat absorbed on isothermal demagnetization for several magnetothermal histories of the sample. We found high values of $|\mathrm{\Delta }{S}_T|=45.7$ J/kg K at 289.4 K for a field change of 7 T, for a sample previously cooled to a low temperature and then heated under magnetic field to the target temperature. This value is high but very far from previously reported data, which happened to be nonphysical but obtained from a very frequently used, but incorrect, experimental protocol to determine $\mathrm{\Delta }{S}_T$ from magnetization, via the Maxwell relation. The spurious contribution has been analyzed and computed, explaining the nonphysical reported values. The strong thermal and magnetic hysteresis of 8.4 K and 8.3 T, respectively, make this alloy useless for magnetic refrigeration, but the results encourage searching for other derivatives with lower hysteresis which could have even higher $|\mathrm{\Delta }{S}_T|$.

Tuning the topological and magnetic properties of materials by applying an electric field is widely used in spintronics. In this work, we find a topological phase transition from topologically trivial to nontrivial states at an external electric field of about 0.1 V/Å in a ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ monolayer that is a topologically trivial ferromagnetic semiconductor. It is shown that when electric field increases from 0 to 0.15 V/Å, the magnetic anisotropy energy (MAE) increases from about 0.1 to 6.3 meV, and the Curie temperature $T_C$ increases from 13 to about 61 K. The increased MAE mainly comes from the enhanced spin-orbit coupling due to the applied electric field. The enhanced $T_C$ can be understood from the enhanced $p\text{−}d$ hybridization and decreased energy difference between $p$ orbitals of Te atoms and $d$ orbitals of Mn atoms. Moreover, we propose two Janus materials, ${\mathrm{MnBi}}_2{\mathrm{Se}}_2{\mathrm{Te}}_2$ and ${\mathrm{MnBi}}_2{\mathrm{S}}_2{\mathrm{Te}}_2$ monolayers with different internal electric polarizations, which can realize the quantum anomalous Hall effect (QAHE) with Chern numbers $C=1$ and $C=2$, respectively. Our study not only exposes the electric field induced exotic properties of the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ monolayer but also proposes materials to realize QAHE in ferromagnetic Janus semiconductors with electric polarization.

We study thermal transport in a two-dimensional system with coexisting $s$- or $d$-wave superconducting (SC) and spin density wave (SDW) orders. We analyze the nature of coexistence phase in a tight-binding square lattice with $\mathbf{Q}=(\pi ,\pi )$ SDW ordering. The electronic thermal conductivity is computed within the framework of the Boltzmann kinetic theory, using Born approximation for the impurity scattering collision integral. We describe the influence of the Fermi surface (FS) topology, the competition between the SC and SDW order parameters, and the presence or absence of zero energy excitations in the coexistence phase, on the low temperature behavior of thermal conductivity of the various pairing states. We present qualitative analytical and fully numerical results that show that the heat transport signatures of various SC states emerging from collinear SDW order are quite distinct and depend on the symmetry properties of the SC order parameter under translation by the SDW nesting vector $\mathbf{Q}$. A combination of $(\pi ,\pi )$-SDW and the $d_{x^2-y^2}$ pairing state results in fully gapped excitations, whereas $(\pi ,\pi )$-SDW coexisting with either $d_{xy}$ or $s$-wave pairing states may always have gapless excitations. There appear special stable Dirac nodal points that are not gapped by the SC order in the coexistence phase, resulting in finite residual heat conductivity.