The ${\mathrm{H}}_3\mathrm{S}$ compound was reproducibly synthesized by laser heating hydrogen-embedded solid sulfur samples at various pressures above 75 GPa in a diamond anvil cell. X-ray diffraction studies were conducted up to 160 GPa and the crystal structure has been identified with space group $Cccm$. The stability of this sole orthorhombic ${\mathrm{H}}_3\mathrm{S}$ phase up to 160 GPa contradicts ab initio calculations that predict the stability of a sequence of two metallic superconductive structures above 110 GPa, with $R3m$ and $Im\overline{3}m$ symmetries. This work also has strong implications for the current understanding of the $200\mathrm{K}$ superconductivity phenomenon in ${\mathrm{H}}_2\mathrm{S}$ since it seems to rule out the hypothesis of the decomposition of ${\mathrm{H}}_2\mathrm{S}$ into sulfur and superconducting ${\mathrm{H}}_3\mathrm{S}$.

${\mathrm{NaOsO}}_3$ hosts a rare manifestation of a metal-insulator transition driven by magnetic correlations, placing the magnetic exchange interactions in a central role. We use resonant inelastic x-ray scattering to directly probe these magnetic exchange interactions. A dispersive and strongly gapped (58 meV) excitation is observed, indicating appreciable spin-orbit coupling in this $5d^3$ system. The excitation is well described within a minimal model Hamiltonian with strong anisotropy and Heisenberg exchange $\left(J_1=J_2=13.9\mathrm{meV}\right)$. The observed behavior places ${\mathrm{NaOsO}}_3$ on the boundary between localized and itinerant magnetism.

Mainly ruled by oxygen octahedral rotations, perovskite oxides can exhibit zone boundary transitions (ZBTs) either with $d{T}_c/dP>0$ or $d{T}_c/dP<0$. Synchrotron structural investigations at high pressure conditions place $\mathrm{YA}{\mathrm{l}}_{0.25}\mathrm{C}{\mathrm{r}}_{0.75}{\mathrm{O}}_3$ orthorhombic perovskite at the boundary of ZBTs. The absence of changes in the octahedral tilting and a volume reduction with pressure exclusively controlled by an isotropic polyhedral compression set $\mathrm{YA}{\mathrm{l}}_{0.25}\mathrm{C}{\mathrm{r}}_{0.75}{\mathrm{O}}_3$ as the first finding of a possible asymptote at the Clapeyron relation for predicting ZBTs in perovskites. Furthermore, the discovery of a “locked-tilt perovskite” can pave the way to a new class of functional materials.

Recent experimental and theoretical efforts have focused on the effect of dissipation on quantum many-body systems in their many-body localized (MBL) phase. While in the presence of dephasing noise such systems reach a unique ergodic state, their dynamics is characterized by slow relaxation manifested in nonexponential decay of self-correlations. Here we shed light on a currently much debated issue, namely, the role of interactions for this relaxation dynamics. We focus on the experimentally relevant situation of the evolution from an initial charge density wave in the presence of strong dephasing noise. We find a crossover from a regime dominated by disorder to a regime dominated by interactions, with an accompanying change of time correlators from stretched exponential to compressed exponential form. The strongly interacting regime can be explained in terms of nucleation and growth dynamics of relaxing regions—reminiscent of the kinetics of crystallization in soft matter systems—and should be observable experimentally. This interaction-driven crossover suggests that the competition between interactions and noise gives rise to a much richer structure of the MBL phase than anticipated so far.

Magnetic properties of $\mathrm{N}{\mathrm{i}}_{46.0}\mathrm{M}{\mathrm{n}}_{36.8}\mathrm{S}{\mathrm{n}}_{11.4}\mathrm{C}{\mathrm{o}}_{5.8}/\mathrm{MgO}\left(001\right)$ epitaxial thin film, which undergo a martensitic phase transformation from cubic austenitic phase to a twinned orthorhombic martensitic phase at 270 K, were studied by the magnetic resonance at the microwave frequency of 9.45 GHz. It was found that the single resonance line observed in the austenite splits into three lines in the martensitic phase. A theoretical approach was developed to show that the additional resonance lines are caused by the weak antiferromagnetic coupling of the ferromagnetic twin components across twin boundaries. Fitting of the experimental resonance lines to model gives an effective field of antiferromagnetic coupling of about 1.5 kOe, which is two or three orders of magnitude lower than in the conventional antiferromagnetic solids because the number of magnetic ions interacting antiferromagnetically through the twin boundary is much less than the total number of magnetic ions in the twin. This feature shows a strong resemblance between the submicron twinned martensite and artificial antiferromagnetic superlattices, whereby providing a distinctive insight into magnetism of the studied magnetic shape memory material.

Competing exchange interactions can produce complex magnetic states together with spin-induced electric polarizations. With competing interactions on alternating triangular and kagome layers, the swedenborgite ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$ may have one of the largest measured spin-induced polarizations of $\sim 1700$ nC/${\mathrm{cm}}^2$ below its ferrimagnetic transition temperature at 70 K. Upon rotating our sample about $\mathbf{c}=[0,0,1]$ while the magnetic field is fixed along $[1,0,0]$, the threefold splitting of the spin-wave frequencies indicates that our sample is hexagonally twinned. Magnetization measurements then suggest that roughly 20% of the sample is in a domain with the $a$ axis along $[1,0,0]$ and that 80% of the sample is in one of two other domains with the $a$ axis along either $[-1/2,\sqrt{3}/2,0]$ or $[-1/2,-\sqrt{3}/2,0]$. Powder neutron-diffraction data, magnetization measurements, and terahertz (THz) absorption spectroscopy reveal that the complex spin order in each domain can be described as a triangular array of bitetrahedral $c$-axis chains ferrimagnetically coupled to each other in the $ab$ plane. The electric-field dependence of bonds coupling those chains produces the large spin-induced polarization of ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$.

We study frustrated ferromagnetic spin-1 chains, where the ferromagnetic nearest-neighbor coupling competes with the antiferromagnetic next-nearest-neighbor coupling. We use the density-matrix renormalization group to obtain the ground states. Through the analysis of spin-spin correlations we identify the double Haldane phase as well as the ferromagnetic phase. It is shown that the ferromagnetic coupling leads to incommensurate correlations in the double Haldane phase. Such short-range correlations transform continuously into the ferromagnetic instability at the transition to the ferromagnetic phase. We also compare the results with the spin-1/2 and classical spin systems and discuss the string orders in the system.

Elastic and inelastic neutron scattering (ENS and INS) experiments were performed on a single crystal of $\left(\mathrm{N}{\mathrm{i}}_{0.4}\mathrm{M}{\mathrm{n}}_{0.6}\right)\mathrm{Ti}{\mathrm{O}}_3$ (NMTO) to study the spatial correlations and dynamics of spins in the XY-like spin-glass (SG) state. Magnetization measurements reveal signatures of SG behavior in NMTO with a freezing temperature of $T_{\mathrm{SG}}\sim 9.1\mathrm{K}$. The ENS experiments indicate that the intensity of magnetic diffuse scattering starts to increase around 12 K, which is close to $T_{\mathrm{SG}}$. Also, spin-spin correlation lengths $\left(\xi \right)$ at 1.5 K are approximately $(21\pm 1)$ and $(73\pm 2)\AA$ in the interlayer and the in-plane directions, respectively, demonstrating that magnetic correlations in NMTO exhibit quasi-two-dimensional antiferromagnetic order. In addition, critical exponent $\left(\beta \right)$ is determined to be $0.37\pm 0.02$ from the intensity of magnetic diffuse scattering confirms the XY-like SG state of NMTO. INS results show quasielastic neutron scattering (QENS) profiles below $T_{\mathrm{SG}}$. The lifetime of dynamic correlations $\tau \sim \hslash /{\mathrm{\Gamma }}_L$, obtained from the half-width at half-maximum of the Lorentzian $\left({\mathrm{\Gamma }}_L\right)$ QENS profiles, are approximately $(16\pm 1)$ and $(16\pm 2)$ ps at 10 K for two positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore, our experimental findings demonstrate that short-range-ordered antiferromagnetic clusters with short-lived spin correlations are present in the XY-like SG state of NMTO.

We explore the phase diagram of the Kitaev-Heisenberg model with nearest neighbor interactions on the honeycomb lattice using the exact diagonalization of finite systems combined with the cluster mean field approximation, and supplemented by the insights from analytic approaches: the linear spin-wave and second-order perturbation theories. This study confirms that by varying the balance between the Heisenberg and Kitaev term, frustrated exchange interactions stabilize in this model either one of four phases with magnetic long range order: Néel phase, ferromagnetic phase, and two other phases with coexisting antiferromagnetic and ferromagnetic bonds, zigzag and stripy phase, or one of two distinct spin-liquid phases. Out of these latter disordered phases, the one with ferromagnetic Kitaev interactions has a substantially broader range of stability as the neighboring competing ordered phases, ferromagnetic and stripy, have very weak quantum fluctuations. Focusing on the quantum spin-liquid phases, we study spatial spin correlations and dynamic spin structure factor of the model by the exact diagonalization technique, and discuss the evolution of gapped low-energy spin response across the quantum phase transitions between the disordered spin liquid and phases with long range magnetic order.

Measurements of the ${}^{17}\mathrm{O}$ nuclear magnetic resonance (NMR) quadrupolar spectrum of apical oxygen in ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ were performed over a range of magnetic fields from 6.4–30 T in the superconducting state. Oxygen-isotope-exchanged single crystals were investigated with doping corresponding to superconducting transition temperatures from 74 K underdoped, to 78 K overdoped. The apical oxygen site was chosen since its NMR spectrum has narrow quadrupolar satellites that are well separated from any other resonance. Nonvortex contributions to the spectra can be deconvolved in the time domain to determine the local magnetic field distribution from the vortices. Numerical analysis using Brandt's Ginzburg-Landau theory was used to find structural parameters of the vortex lattice, penetration depth, and coherence length as a function of magnetic field in the vortex solid phase. From this analysis we report a vortex structural transition near 15 T from an oblique lattice with an opening angle of ${73}^{\circ }$ at low magnetic fields to a triangular lattice with ${60}^{\circ }$ stabilized at high field. The temperature for onset of vortex dynamics has been identified from spin-spin relaxation. This is independent of the magnetic field at sufficiently high magnetic field similar to that reported for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ and is correlated with mass anisotropy of the material. This behavior is accounted for theoretically only in the limit of very high anisotropy.

The generation of photocurrent in condensed matter is of main interest for photovoltaic and optoelectronic applications. Shift current, a nonlinear photoresponse, has attracted recent intensive attention as a dominant player of bulk photovoltaic effect in ferroelectric materials. In three-dimensional topological insulators ${\text{Bi}}_2{X}_3$ ($X=\text{Te}$, Se), we find that Dirac surface states with a hexagonal warping term support shift current by linearly polarized light. Moreover, we study “shift spin current” that arises in Dirac surface states by introducing time-reversal symmetry breaking perturbation. The estimate for the magnitudes of the shift charge and spin current densities are $0.13I_0$ and $0.40I_0$ (nA/m) for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ with the intensity of light $I_0$ measured in $(\text{W}/{\text{m}}^2)$, respectively, which can offer a useful method to generate these currents efficiently.

We study a Majorana island coupled to a bulk superconductor via a Josephson junction and to multiple external normal leads. In the absence of the Josephson coupling, the system displays a topological Kondo state, which had been largely studied recently. However, we find that this state is unstable even to small Josephson coupling, which instead leads at low temperature $T$ to a new fixed point. Most interesting is the case of three external leads, forming a minimal electronic realization of the long sought two-channel Kondo effect. While the $T=0$ conductance corresponds to simple resonant Andreev reflection, the leading $T$ dependence forms an experimental fingerprint for non-Fermi-liquid properties.

In this paper we theoretically study, using Floquet-Bloch theory, the influence of circularly and linearly polarized light on two-dimensional band structures with Dirac and quadratic band touching points, and flat bands, taking the nearest neighbor hopping model on the kagome lattice as an example. We find circularly polarized light can invert the ordering of this three-band model, while leaving the flat band dispersionless. We find a small gap is also opened at the quadratic band touching point by two-photon and higher order processes. By contrast, linearly polarized light splits the quadratic band touching point (into two Dirac points) by an amount that depends only on the amplitude and polarization direction of the light, independent of the frequency, and generally renders dispersion to the flat band. The splitting is perpendicular to the direction of the polarization of the light. We derive an effective low-energy theory that captures these key results. Finally, we compute the frequency dependence of the optical conductivity for this three-band model and analyze the various interband contributions of the Floquet modes. Our results suggest strategies for optically controlling band structure and interaction strength in real systems.

We consider two-dimensional interacting electrons at a monkey saddle with dispersion $\propto p_x^3-3p_x{p}_y^2$. Such a dispersion naturally arises at the multicritical Lifshitz point when three Van Hove saddles merge in an elliptical umbilic elementary catastrophe, which we show can be realized in biased bilayer graphene. A multicritical Lifshitz point of this kind can be identified by its signature Landau level behavior $E_m\propto {\left(Bm\right)}^{3/2}$ and related oscillations in thermodynamic and transport properties, such as de Haas–Van Alphen and Shubnikov–de Haas oscillations, whose period triples as the system crosses the singularity. We show, in the case of a single monkey saddle, that the noninteracting electron fixed point is unstable to interactions under the renormalization-group flow, developing either a superconducting instability or non-Fermi-liquid features. Biased bilayer graphene, where there are two non-nested monkey saddles at the $K$ and $K^{\prime }$ points, exhibits an interplay of competing many-body instabilities, namely, $s$-wave superconductivity, ferromagnetism, and spin- and charge-density waves.

We report the crystal structure and highly anisotropic magnetic, transport, and thermal properties of an exceptionally good single crystal of $\mathrm{U}{\left({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08}\right)}_2{\mathrm{Si}}_2$, prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine ${\mathrm{URu}}_2{\mathrm{Si}}_2$; however, the superconductivity, hidden order, and remnant weak antiferromagnetic orders are absent. Instead, the ground state of the doped system can be classified as a spin liquid that preserves the heavy-fermion character. $\mathrm{U}{\left({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08}\right)}_2{\mathrm{Si}}_2$ exhibits a short-range magnetic order distinguished by reflections of a Lorentzian profile at ${\mathrm{q}}_{\mathit{III}}=\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ positions that disappear above $\sim 15$ K. The short-range order seems to be a precursor of a long-range magnetic order that occurs with higher Rh concentration. We indicate that these short-range fluctuations involve, at least partially, inelastic scattering processes.

The $GW$ approximation is a well-known method to obtain the quasiparticle and spectral properties of systems ranging from molecules to solids. In practice, $GW$ calculations are often employed with many different approximations and truncations. In this work, we describe the implementation of a fully self-consistent $GW$ approach based on the solution of the Dyson equation using a plane wave basis set. Algorithmic, numerical, and technical details of the self-consistent $GW$ approach are presented. The fully self-consistent $GW$ calculations are performed for GaAs, ZnO, and CdS including semicores in the pseudopotentials. No further approximations and truncations apart from the truncation on the plane wave basis set are made in our implementation of the $GW$ calculation. After adopting a special potential technique, a $\sim 100\mathrm{Ry}$ energy cutoff can be used without the loss of accuracy. We found that the self-consistent $GW$ (sc-$GW)$ significantly overestimates the bulk band gaps, and this overestimation is likely due to the underestimation of the macroscopic dielectric constants. On the other hand, the sc-$GW$ accurately predicts the $d$-state positions, most likely because the $d$-state screening does not sensitively depend on the macroscopic dielectric constant. Our work indicates the need to include the high-order vertex term in order for the many-body perturbation theory to accurately predict the semiconductor band gaps. It also sheds some light on why, in some cases, the $G_0{W}_0$ bulk calculation is more accurate than the fully self-consistent $GW$ calculation, because the initial density-functional theory has a better dielectric constant compared to experiments.

We study the effect of interactions on the Hofstadter butterfly of the honeycomb lattice. We show that the interactions induce charge ordering that breaks the translational and rotational symmetries of the system. These phase transitions are prolific and occur at many values of the flux and particle density. The breaking of the translational symmetry introduces a new length scale in the problem and this affects the energy-band diagram resulting in the disintegration of the fractal structure in the energy flux plot, the Hofstadter butterfly. This disintegration increases with increase in the interaction strength. Many of these phase transitions are accompanied with change in the Hall conductivity. Consequently, the disintegration of the Hofstadter butterfly is manifested in the Landau fan diagram also.

KMgBi single crystals have been grown successfully by using the Bi flux method. KMgBi shows a semiconducting behavior with a metal-semiconductor transition in high temperature regions. Moreover, both electron- and hole-type carriers exist in KMgBi with a strong temperature dependence of carrier concentrations and mobilities. These results suggest that KMgBi is a narrow-band semiconductor with multiband features in the bulk rather than a semimetal as predicted theoretically. On the other hand, KMgBi exhibits a resistivity plateau in low temperature regions, similar to other topological insulators. It implies that there might be a nontrivial topological surface state in KMgBi.

${\mathrm{ZrTe}}_5$ is a newly discovered topological material. Shortly after a single layer ${\mathrm{ZrTe}}_5$ had been predicted to be a two-dimensional topological insulator, a handful of experiments have been carried out on bulk ${\mathrm{ZrTe}}_5$ crystals, which however suggest that its bulk form may be a three-dimensional topological Dirac semimetal. We report a transport study on ultrathin ${\mathrm{ZrTe}}_5$ flakes down to 10 nm. A significant modulation of the characteristic resistivity maximum in the temperature dependence by thickness has been observed. Remarkably, the metallic behavior, occurring only below about 150 K in bulk, persists to over 320 K for flakes less than 20 nm thick. Furthermore, the resistivity maximum can be greatly tuned by ionic gating. Combined with the Hall resistance, we identify contributions from a semiconducting and a semimetallic band. The enhancement of the metallic state in thin flakes are a consequence of shifting of the energy bands. Our results suggest that the band structure sensitively depends on the film thickness, which may explain the divergent experimental observations on bulk materials.