We study the low-temperature behavior of antiferromagnets in two spatial dimensions that are subjected to a magnetic field oriented perpendicular to the staggered magnetization order parameter. The evaluation of the partition function is carried to two-loop order within the systematic effective Lagrangian technique. Low-temperature series that are valid in weak magnetic and staggered fields are derived for the pressure, staggered magnetization, and magnetization. Remarkably, at $T=0$, the staggered magnetization is enhanced by the magnetic field.

In this Rapid Communication we report a phenomenon in a disordered interacting electron system. The measurements of structural, magnetic, and transport properties of $\mathrm{FeA}{\mathrm{l}}_{2-x}\mathrm{G}{\mathrm{a}}_x$ ($0\le x\le 0.5$) show that antiferromagnetic transition in these intermetallic compounds occurs concomitantly with a disorder-induced weak localization of electrons; the temperatures $T_N$ and $T_m$, at which antiferromagnetic transition and the weak localization respectively occur, closely track each other as the Ga concentration is varied. The antiferromagnetic transition is confirmed from the magnetic and specific heat measurements, and the occurrence of weak localization is confirmed from the temperature variation of resistivity and magnetoresistance measurements. With increasing Ga concentration, substitutional disorder in the system increases, and the consequent disorder-enhanced magnetic exchange interaction and disorder-induced fluctuations simultaneously drive antiferromagnetic transition and weak localization, respectively, to higher temperatures.

We report that the lowest energy transverse-optic phonon in metallic SnTe softens to near zero energy at the structural transition at $T_C=75\text{K}$ and importantly show that the energy of this mode below $T_C$ increases as the temperature decreases. Since the mode is a polar displacement this proves unambiguously that SnTe undergoes a ferroelectric displacement below $T_C$. Concentration gradients and imperfect stoichiometry in large crystals may explain why this was not seen in previous inelastic neutron scattering studies. Despite SnTe being metallic we find that the ferroelectric transition is similar to that in ferroelectric insulators, unmodified by the presence of conduction electrons: we find that (i) the damping of the polar mode is dominated by coupling to acoustic phonons rather than electron-phonon coupling, (ii) the transition is almost an ideal continuous transition, and (iii) comparison with density functional calculations identifies the importance of dipolar-dipolar screening for understanding this behavior.

The Dzyaloshinskii-Moriya interaction (DMI) is a chiral interaction that favors formation of domain walls. Recent experiments and ab initio calculations show that there are multiple ways to modify the strength of the interfacially induced DMI in thin ferromagnetic films with perpendicular magnetic anisotropy. In this paper we reveal theoretically the effects of spatially varied DMI on the magnetic state in thin films. In such heterochiral 2D structures we report several emergent phenomena, ranging from the equilibrium spin canting at the interface between regions with different DMI, over particularly strong confinement of domain walls and skyrmions within high-DMI tracks, to advanced applications such as domain tailoring nearly at will, design of magnonic waveguides, and much improved skyrmion racetrack memory.

A high-finesse optical cavity allows the establishment of long-range interactions between bosons in an optical lattice when most cold-atom experiments are restricted to short-range interactions. Supersolid phases have recently been experimentally observed in such systems. Using both exact quantum Monte Carlo simulations and the Gutzwiller approximation, we study the ground-state phase diagrams of a two-dimensional Bose-Hubbard model with infinite-range interactions which describes such experiments. In addition to superfluid and insulating Mott phases, the infinite-range checkerboard interactions introduce charge-density waves and supersolid phases. We study here the system at various particle densities, elucidate the nature of the phases and quantum phase transitions, and discuss the stability of the phases with respect to phase separation. In particular we confirm the existence and stability of a supersolid phase detected experimentally.

Effects of paramagnetic impurities on the superconducting transition of noncentrosymmetric ${\mathrm{LaNiC}}_2$ have been studied by Ce substitution for La up to 3 at.%. With the substitution the superconducting transition temperature $T_{\mathrm{c}}\sim 2.8$ K of the pure system decreases dramatically, and disappears with Ce just above 2 at.%. This intense suppression of $T_{\mathrm{c}}$ with the Ce substitution can be explained by the Abrikosov-Gor'kov theory for conventional $s$-wave superconductors with localized magnetic moments. These results, together with a quite small depression of $T_{\mathrm{c}}$ with nonmagnetic Y impurities, strongly suggest that ${\mathrm{LaNiC}}_2$ is a BCS superconductor with a full superconducting energy gap, not an unconventional one with a nodal gap.

We study the Majorana surface states of higher-spin topological superconductors (TSCs) that could be realized in ultracold atomic systems or doped semimetals with spin-orbit coupling. As a paradigmatic example, we consider a model with $p$-wave pairing of spin-3/2 fermions that generalizes ${}^3\mathrm{He}-B$. This model has coexisting linear and cubic dispersing Majorana surface bands. We show that these are unstable to interactions, which can generate a spontaneous surface thermal quantum Hall effect (TQHE). By contrast, nonmagnetic quenched disorder induces a surface conformal field theory (CFT) that is stable against weak interactions: topological protection is enhanced by disorder. Gapless surface states of higher-spin TSCs could therefore be robustly realized in solid-state systems, where disorder is inevitable. The surface CFT is characterized by universal signatures that depend only on the bulk topological winding number, and they include power-law scaling of the density of states, a universal multifractal spectrum of local density of states fluctuations, and a quantized ratio of the longitudinal thermal conductivity ${\kappa }_{xx}$ divided by temperature $T$. By contrast, ${\kappa }_{xx}/T$ for the clean surface without TQHE order would diverge as $T\to 0$. Since disorder stabilizes the conducting Majorana surface fluid and quantizes thermal transport, our results suggest a close analogy between bulk TSCs and the integer quantum Hall effect.

We investigate multilayers of a normal insulator and a Weyl semimetal using two models: an effective model and a lattice model. As a result, we find that the behavior of the multilayers is qualitatively different depending on the stacking direction relative to the displacement vector between the Weyl nodes. When the stacking direction is perpendicular to the displacement vector between the Weyl nodes, the system shows either the normal insulator or the Weyl semimetal phases depending on the thicknesses of the two layers. In contrast, when the stacking direction is parallel to that, the phase diagram is rich, containing the normal insulator phase, the Weyl semimetal phase, and the quantum anomalous Hall phases with various values of the Chern number. As a superlattice period increases, the Chern number in the quantum anomalous Hall phases increases. Thus, one can design a quantum anomalous Hall system with various Chern numbers in a multilayer of a Weyl semimetal and a normal insulator. Applications to Weyl semimetal materials are discussed.

The fermion sign problem is often viewed as a sheer inconvenience that plagues numerical studies of strongly interacting electron systems. Only recently has it been suggested that fermion signs are fundamental for the universal behavior of critical metallic systems and crucially enhance their degree of quantum entanglement. In this work we explore potential connections between emergent scale invariance of fermion sign structures and scaling properties of bipartite entanglement entropies. Our analysis is based on a wave-function Ansatz that incorporates collective, long-range backflow correlations into fermionic Slater determinants. Such wave functions mimic the collapse of a Fermi liquid at a quantum critical point. Their nodal surfaces, a representation of the fermion sign structure in many-particle configurations space, show fractal behavior up to a length scale $\xi$ that diverges at a critical backflow strength. We show that the Hausdorff dimension of the fractal nodal surface depends on $\xi$, the number of fermions and the exponent of the backflow. For the same wave functions we numerically calculate the second Rényi entanglement entropy $S_2$. Our results show a crossover from volume scaling, $S_2\sim {\ell }^{\theta }$ $(\theta =2$ in $d=2$ dimensions), to the characteristic Fermi-liquid behavior $S_2\sim \ell ln\ell$ on scales larger than $\xi$. We find that volume scaling of the entanglement entropy is a robust feature of critical backflow fermions, independent of the backflow exponent and hence the fractal dimension of the scale invariant sign structure.

We study the evolution of the Kondo effect in heavy fermion compounds, $\mathrm{Yb}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{Zn}}_{20}$ ($0\le x\le 1$), by means of temperature-dependent electric resistivity and specific heat. The ground state of ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ can be well described by a Kondo model with degeneracy $N$ = 8 and a $T_K\sim 30$ K. The ground state of ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ is close to a Kondo state with degeneracy $N$ = 2 and a much lower $T_K\sim$ 2 K, even though the total crystalline electric field (CEF) splittings are similar for ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ and ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$. Upon Co substitution, the coherence temperature of ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For $0.4\lesssim x\lesssim$ 0.9, the ground state remains roughly the same, which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence in resistivity data down to 500 mK within this substitution range. The coherence reappears at around $x\gtrsim$ 0.9 and the coherence temperature increases with higher Co concentration levels.

We implemented a combination of DMFT and $GW$ in its fully self-consistent way, one shot $GW$ approximation, and quasiparticle self-consistent scheme, and studied how well these combined methods perform on a ${\mathrm{H}}_2$ molecule as compared to more established methods such as LDA+DMFT. We found that most flavors of $GW+\mathrm{DMFT}$ break down in a strongly correlated regime due to causality violation. Among $GW+\mathrm{DMFT}$ methods, only the self-consistent quasiparticle $GW+\mathrm{DMFT}$ with static double-counting, and a method with causal double-counting, correctly recover the atomic limit at large H-atom separation. While some flavors of $GW+\mathrm{DMFT}$ improve the single-electron spectra as compared to LDA+DMFT, the total energy is best predicted by LDA+DMFT, for which the exact double-counting is known, and is static.

We demonstrate the use of laser-induced phase gratings to control the emission characteristics of self-assembled semiconductor quantum dots. The microscopic Coulomb interaction between the photoinduced charge densities in a dot, referred to as the local field effect, affects the macroscopic optical properties of a dot ensemble even with inhomogeneous broadening, and forms a phase grating by spatially modulating the exciton resonant frequency. In the low excitation regime, the diffracted light intensity (observed using photon echoes) gradually rose with time delay—a result very different from the conventional instantaneous response to pulse excitation. With increasing excitation intensity, the response of the diffracted signal became more immediate and exhibited a biexponential decay. The change in the temporal profile can be systematically explained by analyzing the dynamics of the phase grating. Our findings suggest an optical switching mechanism using this intrinsic property of semiconductor quantum dots.

The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. Here, we present a comparative electronic structure study of $\mathrm{ZrSi}X$ (where $X=\text{S}$, Se, Te) using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Our ARPES studies show that the overall electronic structure of $\mathrm{ZrSi}X$ materials comprises the diamond-shaped Fermi pocket, the nearly elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone center ($\mathrm{\Gamma }$) point, the $M$ point, and the $X$ point of the Brillouin zone, respectively. We also observe a small Fermi surface pocket along the $M-\mathrm{\Gamma }-M$ direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe. Furthermore, our theoretical studies show a transition from nodal-line to nodeless gapped phase by tuning the chalcogenide from S to Te in these material systems. Our findings provide direct evidence for the tunability of the TNL phase in $\mathrm{ZrSi}X$ material systems by adjusting the spin-orbit coupling strength via the $X$ anion.

The magnetic structure of the heavy fermion antiferromagnet ${\mathrm{CePt}}_2{\mathrm{In}}_7$ is determined using neutron diffraction. We find a magnetic wave vector ${\mathbf{q}}_M=(1/2,1/2,1/2)$, which is temperature independent up to $T_N=5.5$ K. A staggered moment of $0.45\left(1\right){\mu }_B$ at 2 K resides on the Ce ion. The nearest-neighbor moments in the tetragonal basal plane are aligned antiferromagnetically. The moments rotate by ${90}^{\circ }$ from one ${\mathrm{CeIn}}_3$ plane to another along the $c$ axis. A much weaker satellite peak with an incommensurate magnetic wave vector ${\mathbf{q}}_M=(1/2,1/2,0.47)$ seems to develop at low temperature. However, the experimental data available so far are not sufficient to draw a definitive conclusion about the possible coexistence of commensurate and incommensurate magnetic structures in this material.

We propose a set of three compatible photonic structures emulating quantum topologically insulating phases corresponding to Hall, spin-Hall, and valley-Hall effects. It is shown that an interface between any two of these photonic topological insulators supports scattering-free edge states. Spin and valley degrees of freedom characterizing such topologically protected edge waves determine their unique pathways through complex photonic circuits comprised of multiple heterogeneous interfaces.

The band structures of single-layer ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ present a coupling between spin and valley degrees of freedom that suppresses spin-flip scattering and spin dephasing. Here we show that out-of-plane deformations, such as corrugations or ripples, enhance spin-flip scattering in armchair ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ nanoribbons. Spin transport in the presence of surface roughness is systematically investigated, employing the nonequilibrium Green's function method along with the tight-binding approximation. Both transmission and conductance have been calculated as a function of surface roughness. Our results indicate that the spin-flip rate, usually neglected in flat pristine samples, increases significantly with the surface roughness amplitude. These results are important for the design and fabrication of transition metal dichalcogenides based spintronic devices.

Transition-metal dichalcogenide (TMD) monolayers $MXY(M=\mathrm{Mo},\mathrm{W};X\ne Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ are two-dimensional polar semiconductors. Setting the WSeTe monolayer as an example and using density functional theory calculations, we investigate the manipulation of Rashba spin-orbit coupling (SOC) in the MXY monolayer. It is found that the intrinsic out-of-plane electric field due to the mirror symmetry breaking induces the large Rashba spin splitting around the $\mathrm{\Gamma }$ point, which, however, can be easily tuned by applying the in-plane biaxial strain. Through a relatively small strain (from $-2\%$ to 2%), a large tunability (from around $-50\%$ to 50%) of Rashba SOC can be obtained due to the modified orbital overlap, which can in turn modulate the intrinsic electric field. The orbital selective external potential method further confirms the significance of the orbital overlap between $\mathrm{W}\text{−}{d}_{z^2}$ and $\mathrm{Se}\text{−}{p}_z$ in Rashba SOC. In addition, we also explore the influence of the external electric field on Rashba SOC in the WSeTe monolayer, which is less effective than strain. By calculating the electric-field-induced Rashba SOC in all six $M{X}_2$ monolayers, the rule of the electric-field influence on Rashba SOC in TMD monolayers is demonstrated. The large Rashba spin splitting, together with the valley spin splitting in MXY monolayers, may make a special contribution to semiconductor spintronics and valleytronics.

The formation of monolayer and multilayer ice with a square lattice structure has recently been reported on the basis of transmission electron microscopy experiments, renewing interest in confined two-dimensional ice. Here we report a systematic density functional theory study of double-layer ice in nanoconfinement. A phase diagram as a function of confinement width and lateral pressure is presented. Included in the phase diagram are honeycomb hexagonal, square-tube, hexagonal-close-packed, and buckled-rhombic structures. However, contrary to experimental observations, square structures do not feature: our most stable double-layer square structure is predicted to be metastable. This study provides general insight into the phase transitions of double-layer confined ice and a fresh theoretical perspective on the stability of square ice in graphene nanocapillary experiments.