The dimer Mott insulator $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ can be tuned into metallic and superconducting states on applying pressure of 1.5 kbar and more. We have performed dielectric measurements (7.5 kHz to 5 MHz) on $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ single crystals as a function of temperature (down to $T=8$ K) and pressure (up to $p=4.3$ kbar). In addition to the relaxor-like dielectric behavior seen below 50 K at $p=0$, that moves toward lower temperatures with pressure, a second peak emerges in ${\varepsilon }_1\left(T\right)$ around $T=15$ K. When approaching the insulator-metal boundary, this peak diverges rapidly reaching ${\varepsilon }_1\approx {10}^5$. Our dynamical mean-field theory calculations substantiate that the dielectric catastrophe at the Mott transition is not caused by closing the energy gap, but due to the spatial coexistence of correlated metallic and insulating regions. We discuss the percolative nature of the first-order Mott insulator-to-metal transition in all details.

$3d^2$ vanadate perovskites exhibit an unusual phase diagram where the low temperature orbital and spin orders are strongly dependent upon the rare-earth cation size. Here, we demonstrate the delicate role of strain in epitaxial ${\mathrm{LaVO}}_3$ films by performing low-energy muon spin spectroscopy and ellipsometry measurements. The electronic structure is revealed by the measurement of the multipeak structure of the charge excitations. The analysis highlights the independent roles of the $d_{xy}$ band confinement and the $d_{zx},d_{zy}$ doublet polarization for the orbital ordering to establish.

While cadmium telluride (CdTe) thin films are being used in solar cell prototyping for decades, the recent advent of two-dimensional (2D) materials challenges the fundamental limit for thickness of conventional CdTe layers. Here, we report our theoretical predictions on photocarrier dynamics in an ultimately thin (about 1 nm) CdTe slab. It corresponds to a layer that is just a single unit cell thick, when the bulk parent crystal in the zinc blende phase is cleaved along the [110] facet. Using an ab initio method based on density functional theory (DFT) and the Boltzmann equation in the relaxation time approximation (RTA), we determine the thermalization time for charge carriers excited to a certain energy for instance through laser irradiation. Our calculations include contributions arising from all phonon branches in the first Brillouin zone (BZ), thus capturing all relevant inter- and intraband carrier transitions due to electron-phonon scattering. We find that the photocarrier thermalization time is strongly reduced, by one order of magnitude for holes and by three orders of magnitude for electrons, once the CdTe crystal is thinned down from the bulk to a monolayer. Most surprisingly, the electron thermalization time becomes independent of the electron excess energy up to about 0.5 eV, when counted from the conduction band minimum (CBM). We relate this peculiar behavior to the degenerate and nearly parabolic lowest conduction band that yields a constant density of states (DOS) in the 2D limit. Our findings may be useful for designing novel CdTe-based optoelectronic devices, which employ nonequilibrium photoexcited carriers to improve the performance.

We theoretically show that materials with perovskite-type crystal structures provide a platform for spin current generation. Its mechanism is based on spin-split band structures in certain kinds of collinear antiferromagnetic states, requiring neither spin-orbit coupling nor a ferromagnetic moment. By investigating a multiband Hubbard model for transition-metal compounds by means of the Hartree-Fock approximation and the Boltzmann transport theory, we find that a pure spin current is induced by an electric field applied to a $C$-type antiferromagnetic metallic phase. The spin current generation originates from a cooperative effect of spatially anisotropic electron transfer integrals owing to the ${\mathrm{GdFeO}}_3$-type lattice distortion, which is ubiquitous in many perovskites, and the collinear spin configuration. We discuss our finding from the symmetry point of view, in comparison with other spin current generator candidates with collinear antiferromagnetism. We also propose several ways to detect the phenomenon in candidate perovskite materials.

Monolayer ${\mathrm{PtSe}}_2$ is a semiconducting transition-metal dichalcogenide characterized by an indirect band gap, space inversion symmetry, and high carrier mobility. Strong intrinsic spin-orbit coupling and the possibility to induce extrinsic spin-orbit fields by gating make ${\mathrm{PtSe}}_2$ attractive for fundamental spin transport studies as well as for potential spintronics applications. We perform a systematic theoretical study of the spin-orbit coupling and spin relaxation in this material. Specifically, we employ first-principles methods to obtain the basic orbital and spin-orbital properties of ${\mathrm{PtSe}}_2$, also in the presence of an external transverse electric field. We calculate the spin mixing parameters $b^2$ and the spin-orbit fields $\mathrm{\Omega }$ for the Bloch states of electrons and holes. This information allows us to predict the spin lifetimes due to the Elliott-Yafet and D'yakonov-Perel mechanisms. We find that $b^2$ is rather large, on the order of ${10}^{-2}$ and ${10}^{-1}$, while $\mathrm{\Omega }$ varies strongly with doping, being about ${10}^3\text{–}{10}^4{\mathrm{ns}}^{-1}$ for carrier density in the interval ${10}^{13}\text{–}{10}^{14}{\mathrm{cm}}^{-2}$ at the electric field of 1 V/nm. We estimate the spin lifetimes to be on the picosecond level.

A superconductor with $p_x+i{p}_y$ order has long fascinated the physics community because vortex defects in such a system host Majorana zero modes. Here, we propose a simple construction of a chiral superconductor using proximitized quantum wires and twist angle engineering as basic ingredients. A weakly coupled parallel array of these wires forms a gapless $p$-wave superconductor. Two such arrays, stacked on top of one another with a twist angle close to ${90}^{\circ }$, spontaneously break time reversal symmetry and produce a robust, fully gapped $p_x+i{p}_y$ superconductor. We map out the topological phases of this setup, demonstrate the existence of Majorana zero modes in vortices, and discuss prospects for experimental realization.

We propose a mechanism of the spin Seebeck effect attributed to excitonic condensation in a nonmagnetic insulator. We analyze a half-filled two-orbital Hubbard model with a crystalline field splitting in the strong coupling limit. In this model, the competition between the crystalline field and electron correlations brings about an excitonic insulating state, where the two orbitals are spontaneously hybridized. Using the generalized spin-wave theory and Boltzmann transport equation, we find that a spin current generated by a thermal gradient is observed in the excitonic insulating state without magnetic fields. The spin Seebeck effect originates from spin-split collective excitation modes although the ground state does not exhibit any magnetic orderings. This peculiar phenomenon is inherent in the excitonic insulating state, whose order parameter is time-reversal odd and yields a spin splitting for the collective excitation modes. We also find that the spin current is strongly enhanced and its direction is inverted in the vicinity of the phase transition to another magnetically ordered phase. We suggest that the present phenomenon is possibly observed in perovskite cobaltites with the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_3$-type lattice distortion.

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_3$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_1$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the $a^{-}{a}^{-}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_1$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\Delta } k$ point located halfway between the zone-center $\mathrm{\Gamma }$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

The magnetic ground state of $\mathrm{Co}{\mathrm{Al}}_2{\mathrm{O}}_4$ has been a controversial issue because the ratio of nearest- ($J_1$) to next-nearest-neighbor ($J_2$) exchange interactions in the diamond lattice is close to the boundary between a spiral spin liquid and a long-range ordered antiferromagnetic ground state. Recently, Ghara et al. reported a linear magnetoelectric effect in a polycrystalline $\mathrm{Co}{\mathrm{Al}}_2{\mathrm{O}}_4$ and attributed it to a long-range magnetic order. However, the magnetic ground state and the easy axis anisotropy remained unsolved. Here we propose the easy magnetic axis direction by comparing the results of magnetoelectric measurements in a high-quality single crystal with phenomenological calculations. From the value of polarization ratio $P\left|\right|\left[1\text{−}10\right]$ measured under magnetic field applied along $H\left|\right|\left[1\text{−}10\right]$ and $H\left|\right|\left[001\right]$, we determined that the magnetic easy axis is along [111]. The single ion contribution of the magnetic ions located at the noncentrosymmetric environment is responsible for the magnetoelectric effect similar to that observed in other $A$-site magnetic spinel oxides, ${\mathrm{Co}}_3{\mathrm{O}}_4$, $\mathrm{Mn}{B}_2{\mathrm{O}}_4$ ($B=\mathrm{Al},\mathrm{Ga}$).

Coherent excitation of magnons is conventionally achieved through Raman scattering processes, in which the difference-frequency components of the driving field are resonant with the magnon energy. Here, we describe mechanisms by which the sum-frequency components of the driving field can be used to coherently excite magnons through two-particle absorption processes. We use the Landau-Lifshitz-Gilbert formalism to compare the spin-precession amplitudes that different types of impulsive stimulated and ionic Raman scattering processes and their sum-frequency counterparts induce in an antiferromagnetic model system. We show that sum-frequency mechanisms enabled by linearly polarized driving fields yield excitation efficiencies comparable or larger than established Raman techniques, while elliptical polarizations produce only weak and circular polarizations no sum-frequency components at all. The mechanisms presented here complete the map for dynamical spin control by means of Raman-type processes.

We propose a phase prediction method for pattern formation in a two-dimensional kinetic Ising model with dipole-dipole interactions under the time-dependent Ginzburg-Landau dynamics. Considering the effects of the material thickness by assuming uniformness along the magnetization axis, the model corresponds to thin magnetic materials with long-range repulsive interactions. We formulate a theoretical basis to understand the effects of the material parameters on the formation of the magnetic domain patterns in terms of the equilibrium equations governing the balance between the linear and nonlinear forces in the equilibrium state. Further, we develop a method for predicting the phase in the equilibrium state achieved after the dynamical evolution of a system with given initial parameters. The analytically hard third-order term is approximated using the restricted phase-space approximation [Anzaki et al., Ann. Phys. (NY) 353, 107 (2015)] for the ${\varphi }^4$ models. Although the proposed method does not have perfect concordance with the actual numerical results, it has no arbitrary parameters and functions to tune the prediction. In other words, it is a method with no a priori assumptions of the domain patterns.

We report on the low-dimensional magnetic behavior of the series of compounds ${\mathrm{NiNb}}_{2-x}{\mathrm{V}}_x{\mathrm{O}}_6$ ($0\le x\le 2$), with a columbite-type crystal structure stabilized at high pressure and temperature. Based on susceptibility, magnetization, and specific-heat measurements, the system is characterized as presenting quasi-one-dimensional magnetism, with ${\mathrm{Ni}}^{2+}$ magnetic moments that can be modeled as Ising spins, placed along zigzag chains in the crystal structure. The low-temperature phase is found to consist of an antiferromagnetic arrangement of ferromagnetic chains, and a metamagnetic transition to uniform ferromagnetic order is observed under magnetic fields slightly above ${\mu }_{0}H=1\mathrm{T}$. We discuss the effects of substituting vanadium for niobium, maintaining the same crystal structure along the whole series of samples. In particular, the long-range magnetic order, most clearly seen for $x=0$, tends to be suppressed as the vanadium content is increased. The exchange interactions are quantified, revealing that the ferromagnetic intrachain interactions vary from about $7\mathrm{K}$ for ${\mathrm{NiNb}}_2{\mathrm{O}}_6$ to $2\mathrm{K}$ for ${\mathrm{NiV}}_2{\mathrm{O}}_6$, remaining one order of magnitude larger than the mean antiferromagnetic interchain coupling.

Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound, and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is on the order of a few meV and can be fine tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multilayer graphene are superconducting. The superconducting gap size is sensitive to the Fermi energy of the hole band. Remarkably, the FeSe monolayer provides the opportunity to study the physics in the crossover regime where the Fermi energy and superconducting gap are comparable to each other.

We consider the origin of the anomalous Hall effect in a general model of a clean two-band chiral superconductor. Within the Kubo formalism we derive an analytic expression for the high-frequency ac Hall conductivity valid close to the critical temperature. This expression involves two distinct gauge-invariant time-reversal-odd bilinear (TROB) functions involving the pairing potential and its Hermitian conjugate. We argue that the existence of at least one of these TROBs generically implies a nonzero ac Hall conductivity. The TROBs allow us to clarify the roles of intra- and interband pairing, and provide a straightforward criterion for a superconducting state to exhibit the anomalous Hall effect. We briefly exemplify our results with model calculations of two different chiral $p$-wave pairing states in strontium ruthenate and a chiral $d$-wave pairing state on the honeycomb lattice.

A system of hard spheres exhibits physics that is controlled only by their density. This comes about because the interaction energy is either infinite or zero, so all allowed configurations have exactly the same energy. The low-density phase is liquid, while the high-density phase is crystalline, an example of “order by disorder” as it is driven purely by entropic considerations. Here we study a family of hard spin models, which we call hard-core spin models, where we replace the translational degrees of freedom of hard spheres with the orientational degrees of freedom of lattice spins. Their hard-core interaction serves analogously to divide configurations of the many spin system into allowed and disallowed sectors. We present detailed results on the square lattice in $d=2$ for a set of models with ${\mathbb{Z}}_n$ symmetry, which generalize Potts models, and their $\text{U}\left(1\right)$ limits, for ferromagnetic and antiferromagnetic senses of the interaction, which we refer to as exclusion and inclusion models. As the exclusion and inclusion angles are varied, we find a Kosterlitz-Thouless phase transition between a disordered phase and an ordered phase with quasi-long-ranged order, which is the form order by disorder takes in these systems. These results follow from a set of height representations, an ergodic cluster algorithm, and transfer matrix calculations.

The recent experimental confirmation of a transformation between meron and skyrmion topological spin textures in the chiral magnet ${\mathrm{Co}}_8{\mathrm{Zn}}_9{\mathrm{Mn}}_3$ [S.-Z. Lin et al., Phys. Rev. B 91, 224407 (2015); X. Z. Yu et al., Nature 564, 95 (2018)] confirms that the skyrmion crystals discovered in 2009 [S. Mühlbauer et al., Science 323, 915 (2009)] are just the tip of the iceberg. Crystals of topological textures, including skyrmions, merons, vortices, and monopoles, can be stabilized by combining simple physical ingredients, such as lattice symmetry, frustration, and spin anisotropy. The current challenge is to find the combinations of these ingredients that produce specific topological spin textures. Here we report a simple mechanism for the stabilization of meron, skyrmion, and vortex crystals in centrosymmetric tetragonal magnets. In particular, the meron/skyrmion crystals can form even in absence of magnetic field. The application of magnetic field leads to a rich variety of topological spin textures that survive in the long wavelength limit of the theory. When conduction electrons are coupled to the spins, these topological spin textures twist the electronic wave functions to induce Chern insulators and Weyl semimetals for specific band filling fractions.

Recently, magnetic multilayer systems have received again a great attention owing to their suitability for the generation of magnetic chiral spin structure. In this study, we experimentally investigated the interfacial Dzyaloshisnkii-Moriaya interaction (DMI) and the spin-Hall angle (SHA) of magnetic Co/Gd/Pt multilayers with respect to the repetition number $N$ of the multilayers. The DMI and SHA are important variables that govern the stability and mobility of a homochiral spin structure, respectively. The experimental results show that the ratio between the DMI and the dipole energy from the domain wall gradually decreases as $N$ increases, which is expected to be hard to achieve homochiral spin structure for larger $N$ and the values of SHA remain constant irrespective of $N$. The observed SHA invariance indicates that the Pt layers in repetition have a negligible effect on the SHA.

Although often ignored in first-principles studies of material behavior, electronic free energy can have a profound effect in systems with a high-temperature threshold for kinetics and a high Fermi-level density of states (DOS). ${\mathrm{Nb}}_3\mathrm{Sn}$ and many other members of the technologically important A15 class of superconductors meet these criteria. This is no coincidence: both electronic free energy and superconducting transition temperature $T_{\text{c}}$ are closely linked to the electronic density of states at the Fermi level. Antisite defects are known to have an adverse effect on $T_{\text{c}}$ in these materials because they disrupt the high Fermi-level density of states. We observe that this also locally reduces electronic free energy, giving rise to large temperature-dependent terms in antisite defect formation and interaction free energies. This work explores the effect of electronic free energy on antisite defect behavior in the case of ${\mathrm{Nb}}_3\mathrm{Sn}$. Using ab initio techniques, we perform a comprehensive study of antisite defects in ${\mathrm{Nb}}_3\mathrm{Sn}$, and find that their effect on the Fermi-level DOS plays a key role determining their thermodynamic behavior, their interactions, and their effect on superconductivity. Based on our findings, we calculate the A15 region of the Nb-Sn phase diagram and show that the phase boundaries depend critically the electronic free energy of antisite defects. In particular, we show that extended defects such as grain boundaries alter the local phase diagram by suppressing electronic free-energy effects, explaining experimental measurements of grain boundary antisite defect segregation. Finally, we quantify the effect of antisite defects on superconductivity with the first ab initio study of $T_{\text{c}}$ in ${\mathrm{Nb}}_3\mathrm{Sn}$ as a function of composition, focusing on tin-rich compositions observed in segregation regions around grain boundaries. As tin-rich compositions are not observed in bulk, their properties cannot be directly measured experimentally; our calculations therefore enable quantitative Ginzburg-Landau simulations of grain boundary superconductivity in ${\mathrm{Nb}}_3\mathrm{Sn}$. We discuss the implications of these results for developing new growth processes to improve the properties of ${\mathrm{Nb}}_3\mathrm{Sn}$ thin films.

The edge states in the fractional quantum Hall systems at filling factor $\nu =1/3$ are studied by the density matrix renormalization group method. It is shown that the density oscillation induced by the local boundary condition at the edge is characterized by the wave number of the minimum magnetoroton excitation, and this structure is partially reconstructed with the change in the confinement potential shape. In particular, the $\nu =1$ counterpropagating edge channel appears with the change in the chemical potential, which is consistent with recent experiments on heat transport. The stability of the bulk states against the change in the number of electrons confirms that the bulk part of the fractional quantum Hall state is incompressible, while the edge state is compressible.

High-resolution angle-resolved photoemission spectroscopy and ultrafast optical pump-probe spectroscopy were used to study semimetallic $1T-{\mathrm{TiTe}}_2$ quasiparticle dispersion and dynamics. A kink and a flat band, having the same energy scale and temperature-dependent behaviors along the $\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ direction, were detected. Both manifested at low temperatures but blurred as temperature increased. The kink was formed by an electron-phonon coupling. And the localized flat band might be closely related to an electron-phonon coupling. Ultrafast optical spectroscopy identified multiple distinct time scales in the 10–300 K range. Quantitative analysis of the fastest decay process evidenced a significant lifetime temperature dependence at high temperatures, while this starts to change slowly below $\sim 100$ K where an anomalous Hall coefficient occurred. At low temperature, a coherent $A_{1}g$ phonon mode with a frequency of $\sim 4.36$ THz was extracted. Frequency temperature dependence suggests that phonon hardening occurs as temperature falls and anharmonic effects can explain it. Frequency fluence dependence indicates that the phonons soften as fluence increases.