We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound $\mathrm{BaMo}{\left({\mathrm{PO}}_4\right)}_2$. The magnetic susceptibility reveals strong antiferromagnetic interactions ${\theta }_{CW}=-167\mathrm{K}$ and long-range magnetic order at $T_N=22\mathrm{K}$, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the $ac$ plane. Neutron spectroscopy reveals a large excitation gap $\mathrm{\Delta }=15\mathrm{meV}$ in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Néel temperature, and collinear order observed experimentally and is furthermore reproduced in a first-principles calculations by using a new computational scheme. We therefore propose $\mathrm{BaMo}{\left({\mathrm{PO}}_4\right)}_2$ to host $S=1$ antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

In this paper, we study the switching properties of the dynamics of magnetic moments that interact with an elastic medium. To do so, we construct a Hamiltonian framework that can take into account the dynamics in phase space of the variables that describe the magnetic moments in a consistent way. It is convenient to describe the magnetic moments as bilinears of anticommuting variables that are their own conjugates. However, we show how it is possible to avoid having to deal directly with the anticommuting variables themselves, only using them to deduce nontrivial constraints on the magnetoelastic couplings. We construct the appropriate Poisson bracket and a geometric integration scheme that is symplectic in the extended phase space and that allows us to study the switching properties of the magnetization, which are relevant for applications, for the case of a toy model for antiferromagnetic NiO, under external stresses. In the absence of magnetoelastic coupling, we recover the results reported in the literature and in our previous studies. In the presence of the magnetoelastic coupling, the characteristic oscillations of the mechanical system have repercussions on the Néel order parameter dynamics. This is particularly striking for the spin accumulation, which is more than doubled by the coupling to the strain; here as well, the mechanical oscillations are reflected on the magnetic dynamics. As a consequence of such a stress-induced strain, the switching time of the magnetization is slightly faster, and the amplitude of the magnetization is enhanced.

Previous studies on the anomalous Hall effect in coplanar noncollinear antiferromagnets are revisited and extended to magneto-optic properties, namely magneto-optic Kerr effect (MOKE) and x-ray magnetic dichroism (XMCD). Starting from group-theoretical considerations the shape of the frequency-dependent conductivity tensor for various actual and hypothetical spin configurations in cubic and hexagonal ${\mathrm{Mn}}_{3}X$ compounds is determined. Calculated MOKE and x-ray dichroism spectra are used to confirm these findings and to give estimates of the size of the effects. For ${\mathrm{Mn}}_3\mathrm{IrPt}$ and ${\mathrm{Mn}}_3\mathrm{PtRh}$ alloys the concentration dependence of the anomalous and spin Hall conductivity is studied in addition.

The temperature dependence of the upper critical field $\left(H_{c2}\right)$ in ${\mathrm{RbCr}}_3{\mathrm{As}}_3$ single crystals $(T_c\approx 7.3$ K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel $(H\parallel c,H_{c2}^{\parallel c})$ and perpendicular $(H\perp c,H_{c2}^{\perp c})$ to the Cr chains. The curves $H_{c2}^{\parallel c}\left(T\right)$ and $H_{c2}^{\perp c}\left(T\right)$ cross at $\sim 5.5$ K. As a result, the anisotropy parameter $\gamma \left(T\right)=H_{c2}^{\perp c}/H_{c2}^{\parallel c}\left(T\right)$ increases from 0.5 near $T_c$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of ${\mu }_0{H}_{c2}^{\parallel c}\left(0\right)\approx 27.2$ T and ${\mu }_0{H}_{c2}^{\perp c}\left(0\right)\approx 43.4$ T, both exceeding the BCS weak-coupling Pauli limit ${\mu }_0{H}_p=1.84T_c=13.4$ T. The results indicate that the paramagnetic pair-breaking effect is strong for $H\parallel c$ but absent for $H\perp c$, which was further confirmed by the angle dependent magnetoresistance and $H_{c2}$ measurements.

Based on a two-orbital honeycomb lattice model and by use of a random phase approximation analysis we investigate the pairing symmetry of Ni-based transition metal trichalcogenides. We find that an $I$-wave $\left(A_{2g}\right)$ state and a chiral $d$-wave $\left(E_g\right)$ state are dominant and nearly degenerate for typical electron and hole dopings. Both pairing states exhibit nontrivial topological properties, which manifest themselves by the chiral edge states for the $d+id$-wave state and dispersionless Andreev bound state at zero energy for the $I$-wave state. We thus show that Ni-based transition metal trichalcogenides provide a promising platform to study exotic topological phenomena emerging from electronic correlation effects.

Spin-orbit coupling (SOC) plays an important role at superconductor/ferromagnet interfaces. By using the generalized London and Ginzburg-Landau theories, we demonstrate that SOC induces spontaneous vortices in zero applied magnetic field in an $s$-wave superconductor placed below a ferromagnetic metal with intrinsic or interfacial SOC. Even for small SOC strength, the interaction between SOC and vortices is attractive, pinning vortices along the edges of the ferromagnet with the SOC reinforcing the superconducting state. This Rapid Communication also highlights experimental schemes to investigate these phenomena, which provide a platform for Abrikosov vortex memory.

The low formation energy of atomic vacancies in black phosphorus allows it to serve as an ideal prototypical system for exploring the dynamics of interlayer interstitial-vacancy (I-V) pairs (i.e., Frenkel defects) which account for Wigner energy release. Based on a few-layer model of black phosphorus, we conduct discrete geometry analysis and investigate the structural dynamics of intimate interlayer Frenkel pairs from first-principles calculations. We reveal a highly metastable I-V pair state driven by anisotropic dipolelike strain fields which can build strong connections between neighboring layers. In the two-dimensional limit (monolayer), the intimate I-V pair exhibits a relatively low formation energy of 1.54 eV and is energetically favored over its isolated constituents by up to 1.68 eV. The barrier for annihilation of the Frenkel pair is 1.46 eV in the bilayer, which is remarkably higher than that of similar defects in graphite. The findings reported in this work suggest that there exist rich bridging pathways in black phosphorus, leading to stable dimensional reduction and structural condensation on exposure to moderate electron excitation or thermal annealing. This study paves the way for creating novel dimensional-hybrid polymorphs of phosphorus via the introduction of such metastable interlayer I-V pair defects.

Energy bands of electrons in a square lattice potential threaded by a uniform magnetic field exhibit a fractal structure known as the Hofstadter butterfly. Here we study a Fermi gas in a 2D optical lattice within a linear cavity with a tilt along the cavity axis. The hopping along the cavity axis is only induced by resonant Raman scattering of transverse pump light into a standing-wave-cavity mode. Choosing a suitable pump geometry allows us to realize the Hofstadter-Harper model with a cavity-induced dynamical synthetic magnetic field, which appears at the onset of the superradiant phase transition. The dynamical nature of this cavity-induced synthetic magnetic field arises from the delicate interplay between collective superradiant scattering and the underlying fractal band structure. Using a sixth-order expansion of the free energy as a function of the order parameter and by numerical simulations, we show that at low magnetic fluxes the superradiant ordering phase transition is first order, while it becomes second order for higher flux. The dynamic nature of the magnetic field induces a nontrivial deformation of the Hofstadter butterfly in the superradiant phase. At strong pump far above the self-ordering threshold, we recover the Hofstadter butterfly one would obtain in a static magnetic field.

We consider the insulating spin-orbit coupled Bose atoms confined within one-dimensional optical lattices and explore their ground-state magnetic phase transitions. Under strong interactions, the charge degrees of atoms are frozen and the system can be described by an anisotropic XXZ Heisenberg chain with Dzyaloshinskii-Moriya interaction and transverse field. We apply the matrix product state method to obtain the ground states, and analyze spiral correlation functions, spin-spin correlation functions, and first energy gap. Under weakly transverse fields, the ground state is a ferromagnetic phase if the interspin $s$-wave interaction strength is stronger than that of the intraspin ones, otherwise, it is a paramagnetic phase, an antiferromagnetic phase, or a gapless spiral phase with algebraic decaying spin correlations. When the transverse field is strengthened, the gapless spiral phase is broken and a new spiral phase with long-range spin-spin correlations emerges.

Synchronization and chaos caused by alternating current and microwave field in a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized reference layer is comprehensively studied theoretically. A forced synchronization by the alternating current is observed in numerical simulation over wide ranges of its amplitude and frequency. An analytical theory clarifies that the nonlinear frequency shift, as well as the spin-transfer torque asymmetry, plays a key role in determining locking range and phase difference between the oscillator and current. Chaos caused by the alternating current is identified for a region of large alternating current by evaluating the Lyapunov exponent. Similar results are also obtained for microwave field, although the parameter regions causing chaos are narrower than those by the alternating current.

We consider a superconductor with surface suppression of the BCS pairing constant $\lambda \left(x\right)$. We analytically find the gap in the surface density of states (DOS), behavior of the DOS $\nu \left(E\right)$ above the gap, a “vertical” peculiarity of the DOS around an energy equal to the bulk order parameter ${\mathrm{\Delta }}_0$, and a perturbative correction to the DOS at higher energies. The surface gap in the DOS is parametrically different from the surface value of the order parameter due to a difference between the spatial scale $r_c$ at which $\lambda \left(x\right)$ is suppressed and the coherence length. The vertical peculiarity implies an infinite-derivative inflection point of the DOS curve at $E={\mathrm{\Delta }}_0$ with square-root behavior as $E$ deviates from ${\mathrm{\Delta }}_0$. The coefficients of this dependence are different at $E<{\mathrm{\Delta }}_0$ and $E>{\mathrm{\Delta }}_0$, so the peculiarity is asymmetric.

We report the results of high-pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of the Kondo insulator (KI) ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$ up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted topological KI state.

We propose an effective model called the “charge model,” for the half-filled one-dimensional Hubbard and extended Hubbard models. In this model, spin-charge separation, which has been justified from an infinite on-site repulsion ($U$) in the strict sense, is compatible with charge fluctuations. Our analyses based on the many-body Wannier functions succeeded in determining the optical conductivity spectra in large systems. The obtained spectra reproduce the spectra for the original models well even in the intermediate $U$ region of $U=5$–$10T$, with $T$ being the nearest-neighbor electron hopping energy. These results indicate that the spin-charge separation works fairly well in this intermediate $U$ region against the usual expectation and that the charge model is an effective model that applies to actual quasi-one-dimensional materials classified as strongly correlated electron systems.

$\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ has an exotic physical property, i.e., the $C_2$ spin-density-wave phase suddenly changes to the $C_4$ magnetic phase, and superconductivity is suppressed within a narrow composition range around $x=0.25$. We have investigated the doping dependence of the electronic structure of $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ using angle-resolved photoemission spectroscopy (ARPES), and we found an anomaly in the superconducting (SC) gap structure only for $x=0.25$ in both the hole and electron Fermi surfaces. We also propose that the mechanism of a newly observed double-resonance peak by inelastic neutron scattering is understandable because of the SC gap obtained by ARPES. Our discovery reveals the important relationship between the $C_4$-magnetic phase and superconductivity, and it provides an opportunity to survey the underlying relationship between the SC gap and the resonance mode in the hole-doped iron-based superconductor.

We present a coordinated study of the paramagnetic-to-antiferromagnetic, rhombohedral-to-monoclinic, and metal-to-insulator transitions in thin-film specimens of the classic Mott insulator ${\mathrm{V}}_2{\mathrm{O}}_3$ using low-energy muon spin relaxation, x-ray diffraction, and nanoscale-resolved near-field infrared spectroscopic techniques. The measurements provide a detailed characterization of the thermal evolution of the magnetic, structural, and electronic phase transitions occurring in a wide temperature range, including quantitative measurements of the high- and low-temperature phase fractions for each transition. The results reveal a stable coexistence of the high- and low-temperature phases over a broad temperature range throughout the transition. Careful comparison of temperature dependence of the different measurements, calibrated by the resistance of the sample, demonstrates that the electronic, magnetic, and structural degrees of freedom remain tightly coupled to each other during the transition process. We also find evidence for antiferromagnetic fluctuations in the vicinity of the phase transition, highlighting the important role of the magnetic degree of freedom in the metal-insulator transition.

We present a $4^{\prime }/m^{\prime }$-respecting crisscross AFM model in two dimensions (2D) and three dimensions (3D), both belonging to the $Z_2$ classification and exhibiting interesting magnetic high-order topological insulating (HOTI) phases. The topologically nontrivial phase in the 2D model is characterized by the fractional charge localized around the corners and the quantized charge quadrupole moment. Moreover, our 2D model also exhibits the quantized magnetic quadrupole moment, which is a unique feature compared with previous studies. The 3D system stacked from layers of the 2D model possesses the HOTI phase holding chiral one-dimensional (1D) metallic states on the hinge, which corresponds to the Wannier center flow between the valence and conduction bands. The novel transport properties such as the half-quantum spin-flop pumping phenomena on the side surfaces of the HOTI phase are also discussed.

We address the increase of electron drift velocity that arises in semiconductor superlattices (SLs) subjected to constant electric and magnetic fields. It occurs if the magnetic field possesses nonzero components both along and perpendicular to the SL axis and the Bloch oscillations along the SL axis become resonant with cyclotron rotation in the transverse plane. It is a phenomenon of considerable interest, so that it is important to understand the underlying mechanism. In an earlier paper [Phys. Rev. Lett. 114, 166802 (2015)], we showed that, contrary to a general belief that drift enhancement occurs through chaotic diffusion along a stochastic web (SW) within semiclassical collisionless dynamics, the phenomenon actually arises through a nonchaotic mechanism. In fact, any chaos that occurs tends to reduce the drift. We now provide fuller details, elucidating the mechanism in physical terms, and extending the investigation. In particular, we (i) demonstrate that pronounced drift enhancement can still occur even in the complete absence of an SW; (ii) show that, where an SW does exist and its characteristic slow dynamics comes into play, it suppresses the drift enhancement even before strong chaos is manifested; (iii) generalize our theory for nonsmall temperature, showing that heating does not affect the enhancement mechanism and accounting for some earlier numerical observations; (iv) demonstrate that certain analytic results reported previously are incorrect; (v) provide an extended critical review of the subject and closely related issues; and (vi) discuss some challenging problems for the future.

Numerically investigating mode-conversion effects of phonons on Anderson localization in random superlattices, we make it clear the changes in localization properties when the LA and TA phonons which independently show Anderson localization for the normal incidence interact as the incident angles increase. The mode conversions which occur for oblique propagation at small incident angles induce collaboration among the phonons, giving rise to extension of the localization length or delocalization, depending on the system size. On the other hand, at large incident angles, the transmission decay becomes larger than for the normal incidence. In contrast to the small incident angles, the transmission decay and transmission fluctuations become independent of the incident phonon modes and show the frequency dependences that are not predictable from those at the small incident angles, evidencing that the coupled modes of phonons are formed by the mode conversions.

We propose a quantum heat engine based on an Aharonov-Bohm interferometer in a two-terminal geometry and investigate its thermoelectric performances in the linear response regime. Sizeable thermopower (up to $\sim 0.3\text{mV}$/K) as well as $ZT$ values largely exceeding unity can be achieved by simply adjusting parameters of the setup and temperature bias across the interferometer, leading to thermal efficiency at maximum power approaching $30\%$ of the Carnot limit. This is close to the optimal efficiency at maximum power achievable for a two-terminal heat engine. Changing the magnetic flux, the asymmetry of the structure, a side-gate bias voltage through a capacitively coupled electrode, and the transmission of the T junctions connecting the AB ring to the contacts allows us to finely tune the operation of the quantum heat engine. The exploration of the parameters' space demonstrates that the high performances of the Aharonov-Bohm two-terminal device as a quantum heat engine are stable over a wide range of temperatures and length imbalances, promising for experimental realization.