We report the magnetic properties and magnetic structure of the zircon-type compound ${\mathrm{GdVO}}_4$, together with the magnetic structure of the isostructural ${\mathrm{NdVO}}_4$. At $T\simeq 2.5$ K, ${\mathrm{GdVO}}_4$ undergoes a phase transition to antiferromagnetic $G_z$, driven mainly by the exchange interactions, while the magnetic anisotropy and dipolar interactions are minor contributions. Near the liquid-helium boiling temperature, the magnetocaloric effect of ${\mathrm{GdVO}}_4$ is nearly as large as that of the structurally closely related ${\mathrm{GdPO}}_4$. It is noteworthy that ${\mathrm{GdVO}}_4$ has been recently proposed as a good passive regenerator in Gifford-McMahon cryocoolers, since adding a magnetization-demagnetization stage to the cryocooler refrigeration cycle would increase its efficiency for liquefying helium. ${\mathrm{NdVO}}_4$ is a canted $G_z$-type antiferromagnet and shows enhancement of the magnetic reflections in neutron diffraction below ca. 500 mK, due to the polarization of the Nd nuclei by the hyperfine field.

By means of powder neutron diffraction we investigate changes in the magnetic structure of the coplanar noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Ge}$ caused by an application of hydrostatic pressure up to 5 GPa. At ambient conditions the kagomé layers of Mn atoms in ${\mathrm{Mn}}_3\mathrm{Ge}$ order in a triangular ${120}^{\circ }$ spin structure. Under high pressure the spins acquire a uniform out-of-plane canting, gradually transforming the magnetic texture to a noncoplanar configuration. With increasing pressure the canted structure fully transforms into the collinear ferromagnetic one. We observed that magnetic order is accompanied by a noticeable magnetoelastic effect, namely, spontaneous magnetostriction. The latter induces an in-plane magnetostrain of the hexagonal unit cell at ambient pressure and flips to an out-of-plane strain at high pressures in accordance with the change of the magnetic structure.

The effect of double laser pulses (DPs) on the ablation process in solids is studied using a hybrid two-temperature model combining a continuum description of the conduction band electrons with a classical molecular dynamics (MD) approach for the ions. The study is concerned with double pulses with delays in the range of 0–50 ps and absorbed laser fluences of 0.5, 1.0, and $1.5{\mathrm{J}/\mathrm{m}}^2$ [i.e., 1–3 times the ablation threshold for single-pulse ablation (SP)], taking Al as a generic example of simple metals. A detailed analysis, including the assessment of thermodynamic pathways and cavitation rates, leads to a comprehensive picture of the mechanisms active during the different stages of the ablation process initiated by DPs. This study provides an explanation for several phenomena observed in DP ablation experiments. In particular, with respect to SP ablation, crater depths are reduced, which can be explained by the compensation of the rarefaction wave from the first laser pulse with the compression wave from the second pulse, or, at higher fluences and larger delays, by the fact that the target surface is shielded with matter ablated by the first laser pulse. Also, we discuss how smoother surface structures obtained using DPs may be related to features found in the simulations—viz., reduced mechanical strain and peak lattice temperatures. Finally, vaporization appears to be enhanced in DP ablation, which may improve the resolution of emission spectra.

The structural, electronic, magnetic, and vibrational properties of ${\mathrm{LaFeSiH}}_x$ for $x$ between 0 and 1 are investigated using density functional theory calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration $x$ in ${\mathrm{LaFeSiH}}_x$. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped ${\mathrm{LaFeSiH}}_x$ for $x\le 0.75$ is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe $d$ orbitals and increases electron-phonon coupling up to $50\%$, especially for modes in the 20–50 meV range that are associated with Fe atomic movement. We find competing nearest and next-nearest-neighbor exchange interactions and significant spin-phonon coupling, qualitatively similar but smaller in magnitude compared those found in LaOFeAs superconductors. Hence, it is likely that the mechanism of superconductivity for ${\mathrm{LaFeSiH}}_x$ is, like that of LaOFeAs, also unconventional. We furthermore suggest that ${\mathrm{LaFeSiH}}_x$ could be a good proton conductor due to phase stability with a wide range of hydrogen concentrations $x<1$.

We measured the ac magnetic susceptibility for the layered organic superconductor ${\mathrm{EtMe}}_3\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ under pressure with a dc magnetic field applied perpendicular to the ac field. We investigated the dc field dependence of the ac susceptibility in detail and concluded that the superconductivity in ${\mathrm{EtMe}}_3\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ is an anisotropic three-dimensional superconductivity even at low temperatures, which contrasts with the large majority of other correlated electron layered superconductors such as high-$T_{\mathrm{c}}$ cuprate and $\kappa$-(${\mathrm{ET})}_{2}X$ systems.

In this paper we investigate the strong exciton-plasmon coupling in a hybrid system consisting of an atomic thick ${\mathrm{WS}}_2$ monolayer and a gold nanogroove array. We theoretically identify the coexistence of two damping pathways: a coherent damping pathway resulting from the resonant dipole-dipole interaction and a coupling-induced incoherent damping pathway due to the spontaneous emissions of a photon by one subsystem and its subsequent reabsorption by the other. We show that the interplay between both interaction processes not only determines the optical property of the hybrid system, but also results in a pronounced modification of the radiative damping due to the formation of super- and subradiant polariton states. Importantly, we reveal that the radiative damping property of the polariton modes is determined only by the effect of coupling-induced sub- and super-radiance, which is distinctly different from that previously observed in a metal-molecular hybrid system where pure dephasing of J-aggregate excitons dominates the polariton dynamics. Our findings may pave the way towards active manipulation of polariton dynamics and offer possibilities for realizing coherent active control in novel plasmonic devices.

It has been experimentally shown that negative exchange interactions can arise in a linear three-dot system when a two-electron double quantum dot is exchange coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin tripletlike rather than singletlike ground state. Here we show using a microscopic model based on the configuration interaction (CI) method that both tripletlike and singletlike ground states are realized depending on the number of electrons. In the case of only four electrons, a full CI calculation reveals that tripletlike ground states occur for sufficiently large dots. These results hold for symmetric and asymmetric quantum dots in both Si and GaAs, showing that negative exchange interactions are robust in few-electron double quantum dots and do not require large numbers of electrons.

We investigate the occurrence of exact zero modes in one-dimensional quantum magnets of finite length that possess edge states. Building on conclusions first reached in the context of the spin-1/$2XY$ chain in a field and then for the spin-$1J_1-J_2$ Heisenberg model, we show that the development of incommensurate correlations in the bulk invariably leads to oscillations in the sign of the coupling between edge states, and hence to exact zero energy modes at the crossing points where the coupling between the edge states rigorously vanishes. This is true regardless of the origin of the frustration (e.g., next-nearest-neighbor coupling or biquadratic coupling for the spin-1 chain), of the value of the bulk spin (we report on spin-1/2, spin-1, and spin-2 examples), and of the value of the edge-state emergent spin (spin-1/2 or spin-1).

We present an analysis of neural network-based machine learning schemes for phases and phase transitions in theoretical condensed matter research, focusing on neural networks with a single hidden layer. Such shallow neural networks were previously found to be efficient in classifying phases and locating phase transitions of various basic model systems. In order to rationalize the emergence of the classification process and for identifying any underlying physical quantities, it is feasible to examine the weight matrices and the convolutional filter kernels that result from the learning process of such shallow networks. Furthermore, we demonstrate how the learning-by-confusing scheme can be used, in combination with a simple threshold-value classification method, to diagnose the learning parameters of neural networks. In particular, we study the classification process of both fully-connected and convolutional neural networks for the two-dimensional Ising model with extended domain wall configurations included in the low-temperature regime. Moreover, we consider the two-dimensional XY model and contrast the performance of the learning-by-confusing scheme and convolutional neural networks trained on bare spin configurations to the case of preprocessed samples with respect to vortex configurations. We discuss these findings in relation to similar recent investigations and possible further applications.

We determine sizes and activation energies of $k\pi$ skyrmions on a discrete lattice using the Landau-Lifshitz-Gilbert equation and the geodesic nudged elastic band method. The employed atomic material parameters are based on the skyrmionic material system Pd/Fe/Ir(111). We find that the critical magnetic fields for collapse of the $2\pi$ skyrmion and $3\pi$ skyrmion are very close to each other and considerably lower than the critical field of the $1\pi$ skyrmion. The activation energy protecting the structures does not strictly decrease with increasing $k$ as it can be larger for the $3\pi$ skyrmion than for the $2\pi$ skyrmion depending on the applied magnetic field. Furthermore, we propose a method of switching the skyrmion order $k$ by a reversion of the magnetic field direction in samples of finite size.

We consider a two leg bosonic ladder in a $U\left(1\right)$ gauge field with both interleg hopping and interleg repulsion. As a function of the flux, the interleg interaction converts the commensurate-incommensurate transition from the Meissner to a Vortex phase, into an Ising-type of transition towards a density wave phase. A disorder point is also found after which the correlation functions develop a damped sinusoid behavior signaling a melting of the vortex phase. We discuss the differences on the phase diagram for attractive and repulsive interleg interaction. In particular, we show how repulsion favors the Meissner phase at low flux and a phase with a second incommensuration in the correlation functions for intermediate flux, leading to a richer phase diagram than in the case of interleg attraction. The effect of the temperature on the chiral current is also discussed.

We report muon spin relaxation and rotation ($\mu \mathrm{SR}$) measurements on hydrothermally grown single crystals of superconducting tetragonal FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. $\mu \mathrm{SR}$ time spectra were obtained from 280 K down to 0.025 K in zero field (ZF) and applied fields up to 75 mT. In ZF, the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate ${\mathrm{\Lambda }}_{\mathrm{ZF}}$ below 13 K indicate the onset of static magnetism, which coexists with superconductivity below $T_c$. TF $\mu \mathrm{SR}$ results indicate a linear temperature dependence of the superfluid density at low temperature, consistent with nodal superconductivity. The $s+d$-wave model gives the best fit to the observed temperature and field dependencies, and yields an in-plane penetration depth value ${\lambda }_{ab}(T=0$) = 241(3) nm.

Dipole-dipole (DD) interactions are known to play an important role in conditioning ferroelectricity in ${AB\text{O}}_3$ perovskite crystals. Their possible role in conditioning nonferroelectric (inhomogeneous) cation ordering is presently unexplored. I analyze the energy of DD interactions in perovskite structure for different inhomogeneous spatial patterns of electric dipoles localized at the lattice sites. I show that DD interactions can favor inhomogeneous ordering when other terms of the Hamiltonian allow large total polarizability of $A$ sites, comparably large total polarizability of oxygen sites in $A$-O planes, and relatively weak total polarizability of oxygen sites in the $B$-O direction. These conditions are naturally expected in ${\mathrm{PbZrO}}_3$ crystals, for which the reason behind antiferroelectric and incommensurate inhomogeneous ordering is currently being debated.

The recent realization of epitaxial ${\mathrm{SrCoO}}_3$ thin films has triggered a renewed interest in their electronic, magnetic, and ionic properties. Here we uncover several unusual magnetotransport properties of this compound, suggesting that it hosts persistent spin fluctuation down to low temperatures. We achieve the metallic ${\mathrm{SrCoO}}_3$ with record-low resistivity from insulating ${\mathrm{SrCoO}}_{2.5}$ by the ionic liquid gating. We find a linear relationship between the anomalous Hall resistivity and the longitudinal resistivity, which cannot be accounted for by the conventional mechanisms. We theoretically propose that the impurity induced chiral spin fluctuation gives rise to such a dependence. The existence of spin fluctuation manifests itself as negatively enhanced magnetoresistance of ${\mathrm{SrCoO}}_3$ when the temperature approaches zero. Our study brings further insight into the unique spin state of ${\mathrm{SrCoO}}_3$ and unveils a skew scattering mechanism for the anomalous Hall effect.

We report a single-crystal study on the magnetism of the rare-earth compound ${\mathrm{PrTiNbO}}_6$ that experimentally realizes the zigzag pseudospin-1/2 quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between nonmagnetic ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Nb}}^{5+}$ results in the non-Kramers ground-state quasidoublet of ${\mathrm{Pr}}^{3+}$ with the effective pseudospin-1/2 Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, while the system approaches its ground state with almost zero residual spin entropy. At low temperatures, a sizable gap of about 1 K is observed in zero field. We ascribe this gap to off-diagonal anisotropy terms in the pseudospin Hamiltonian, and argue that rare-earth oxides open an interesting venue for studying magnetism of quantum spin chains.

We have conducted high-pressure x-ray diffraction studies on the $\mathrm{MnC}{\mathrm{r}}_2{\mathrm{O}}_4$ and $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ spinels at room temperature. Both compounds undergo pressure-induced structural transitions into diverse tetragonal modifications. Based on these experimental observations and our ab initio calculations, we show that the Cr-oxide spinels with magnetic $A^{2+}$ cations $(A^{2+}=\mathrm{Mn}$, Fe, Co, Ni) follow a similar trend as their chalcogenide counterparts with nonmagnetic $A^{2+}$ ions, i.e., the transition pressure is proportionally related with the magnitude of the Cr-Cr magnetic exchange interactions. Therefore, we reach the conclusion that the Cr-Cr magnetic exchange interactions alone suffice to account for the high-pressure behavior of these systems. Our results clearly depict the close relationship between the structural and magnetic degrees of freedom in Cr-bearing spinels.

We study the spin-1 Heisenberg model on the square lattice with the antiferromagnetic nearest-neighbor $J_1$ and the next-nearest-neighbor $J_2$ couplings by using the infinite projected entangled-pair state (iPEPS) ansatz and the density matrix renormalization group (DMRG) calculation. The iPEPS simulation, which studies the model directly in the thermodynamic limit, finds a crossing of the ground state from the Néel magnetic state to the stripe magnetic state at $J_2/J_1\simeq 0.549$, showing a direct phase transition. In the finite-size DMRG calculation on the cylinder geometry up to the cylinder width $L_y=10$, we find that around the same critical point the Néel and the stripe orders are strongly suppressed, which implies the absence of an intermediate phase. Both calculations identify that the stripe order comes with a first-order transition at $J_2/J_1\simeq 0.549$. Our results indicate that unlike the spin-1/2 $J_1\text{−}{J}_2$ square model, quantum fluctuations in the spin-1 model may not be strong enough to stabilize an intermediate nonmagnetic phase.

Whereas considerable evidence exists for the conversion of singlet Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous magnetic fields, recent theoretical proposals have suggested an alternative way to exert control over triplet generation: intrinsic spin-orbit coupling in a homogeneous ferromagnet coupled to a superconductor. Here, we proximity couple Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit-coupled ferromagnet owing to structural inversion asymmetry. Unconventional modulation of the superconducting critical temperature as a function of in-plane and out-of-plane applied magnetic fields suggests the presence of triplets that can be controlled by the magnetic orientation of a single homogeneous ferromagnet. Our studies demonstrate an active role of spin-orbit coupling in controlling the triplets, an important step towards the realization of novel superconducting spintronic devices.