We study a magnetization reversal by an electric current pulse in a superconductor/insulating ferromagnet/superconductor Josephson junction placed on top of a three-dimensional topological insulator. It is demonstrated that such a system is perspective for low-dissipative spintronics because of the strong spin-momentum locking in the topological insulator surface states. This property provides an ideally strong coupling between the orbital and spin degrees of freedom, thus giving a possibility of efficient reversal of the magnetic moment by current pulse with amplitude lower than the critical current. This results in strongly reduced energy dissipation. The underlying physical mechanism of the reversal is discussed. The influence of the magnetic anisotropy on the controllability of the reversal by the pulse duration is investigated. In addition, a way of a simultaneous electrical detection of the reversal is proposed.

We develop a phenomenological theory for a family of uranium-based heavy fermion superconductors $(\mathrm{URhGe}, \mathrm{UCoGe}, \mathrm{and} {\text{UTe}}_2)$. The theory unifies the understanding of both superconductivity (SC) with a weak magnetic field and reentrant superconductivity (RSC) that appears near the first-order transition line with a high magnetic field. It is shown that the magnetizations along the easy and hard axes have opposite effects on SC. The RSC is induced by the fluctuation parallel to the direction of the magnetic field. The theory makes specific predictions about the variation of triplet SC order parameters $\vec{d}$ with applied external magnetic fields and the existence of a metastable state for the appearance of the RSC.

We investigate the spin-orbit torque exerted on the magnetic moments of the transition-metal impurities Cr, Mn, Fe, and Co, embedded in the surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, in response to an electric field and a consequent electrical current flow in the surface. The multiple scattering problem of electrons off impurity atoms is solved by first-principles calculations within the full-potential relativistic Korringa-Kohn-Rostoker (KKR) Green function method, while the spin-orbit torque calculations are carried out by combining the KKR method with the semiclassical Boltzmann transport equation. We analyze the correlation of the spin-orbit torque to the spin accumulation and spin flux in the impurities and unveil the effect of resonant scattering. In addition, we relate the torque to the resistivity and Joule heat production. We predict that the $\mathrm{Mn}/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ is optimal among the studied systems.

The crystalline structure, magnetism, and magnetocaloric effect of a ${\mathrm{GdCrO}}_3$ single crystal grown with the laser-diode-heated floating-zone technique have been studied. The ${\mathrm{GdCrO}}_3$ single crystal crystallizes into an orthorhombic structure with the space group $Pmnb$ at room temperature. Upon cooling, under a magnetic field of 0.1 T, it undergoes a magnetic phase transition at $T_{\text{N-Cr}}=169.28\left(2\right)$ K with ${\mathrm{Cr}}^{3+}$ ions forming a canted antiferromagnetic (AFM) structure, accompanied by weak ferromagnetism. Subsequently, a spin reorientation takes place at $T_{\text{SR}}=5.18\left(2\right)$ K due to ${\mathrm{Gd}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ magnetic couplings. Finally, the long-range AFM order of ${\mathrm{Gd}}^{3+}$ ions establishes at $T_{\text{N-Gd}}=2.10\left(2\right)$ K. Taking into account the temperature-(in)dependent components of ${\mathrm{Cr}}^{3+}$ moments, we obtained an ideal model for describing the paramagnetic behavior of ${\mathrm{Gd}}^{3+}$ ions within 30–140 K. We observed a magnetic reversal (positive $\to$ negative $\to$ positive) at 50 Oe with a minimum centering around 162 K. In the studied temperature range of 1.8–300 K, there exists a strong competition between magnetic susceptibilities of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ ions, leading to puzzling magnetic phenomena. We have built the magnetic-field-dependent phase diagrams of $T_{\text{N-Gd}}$, $T_{\text{SR}}$, and $T_{\text{N-Cr}}$, shedding light on the nature of the intriguing magnetism. Moreover, we calculated the magnetic entropy change and obtained a maximum value at 6 K and $\mathrm{\Delta }{\mu }_{0}H=14$ T, i.e., $-\mathrm{\Delta }{S}_{\text{M}}\approx 57.5$ J/kg K. Among all $R{\mathrm{CrO}}_3$ ($R=4f^n$ rare earths, $n=$ 7–14) compounds, the single-crystal ${\mathrm{GdCrO}}_3$ compound exhibits the highest magnetic entropy change, as well as an enhanced adiabatic temperature, creating a prominent magnetocaloric effect for potential application in magnetic refrigeration.

Herein we show that ${\mathrm{Mo}}_4{\mathrm{Ce}}_4{\mathrm{Al}}_7{\mathrm{C}}_3$, a recently discovered nanolamellar compound displaying mixed valence, combines Kondo lattice behavior with ferromagnetism. A sizeable magnetization is carried by $3p$ states of Al as evidenced by a strong x-ray magnetic circular dichroism signal at the $K$ edge of aluminum, whereas no detectable signal was observed at the $K$ edge of carbon and $L_{2,3}$ edges of molybdenum. These results point out that the ferromagnetic behavior originates in the Ce atoms with $4f^1$ electronic configuration lying within the Al planes. The evolution with pressure of the mixed valence of Ce atoms in the Mo-C planes determined via Ce $L_3$ x-ray-absorption spectra along with the magnetoresistance measurements across the ferromagnetic transition unambiguously reveal a Kondo behavior. Angle-resolved photoemission spectroscopy and density functional theory confirm a certain degree of Ce $4f$ electron delocalization. More generally, conduction electrons are not restricted to lie in the MoC planes but are also delocalized in the Al planes.

Landau-Fermi liquid theory is conventionally believed to hold whenever the interacting single-particle density of states develops a $\delta$-like component at the Fermi surface, which is associated with quasiparticles. Here we show that a microscopic justification can be actually achieved under more general circumstances, even in the case where coherent quasiparticles are totally missing and the interacting single-particle density of states vanishes at the chemical potential as a consequence of a pole singularity in the self-energy.

With recent demonstrations of lasing, germanium-tin (GeSn) stands out as a promising candidate for the integration of a low threshold, room temperature monolithic laser source in silicon (Si) photonics. The impact of physical properties, such as energy band structure, crystal quality, and cavity loss on lasing performances (i.e., lasing threshold and maximal lasing temperature) has to be better understood, however. In this work, we calculate the theoretical band-to-band net gain for relaxed, [100] uniaxial, and (100) biaxial tensile-strained GeSn. We show that the band-to-band net gain depends not only on the interband gain introduced by the transition between valence bands and conduction bands, but also on the intervalence band absorption between the valence bands. Both approaches, GeSn with high Sn concentration and/or with high tensile strain, can yield a band-to-band net gain at room temperature. Then, the integration of another source of absorption—free carrier absorption—will be discussed. Finally, from the simulation of GeSn lasing characteristics, we show the important role the crystal quality has on the lasing threshold. Based on the results, we suggest using GeSn with low/medium Sn concentrations, under very high [100] uniaxial or (100) biaxial tensile strain, to obtain a low threshold, room temperature lasers.

Isotopic enrichment offers cutting-edge properties of materials. In semiconductors, contributions to physical properties from different isotopes can be routinely and precisely examined by different optical techniques down to a very low relative atomic content in the crystal lattice. In the case of impurity centers this can be as low as a few ppm. We report the splitting of infrared absorption lines caused by the isotopic content of boron acceptors embedded in semiconducting diamond. A blueshift as large as 0.2% of the intracenter transition energy of ${}^{10}\mathrm{B}$ relative to the ${}^{11}\mathrm{B}$ transitions has been observed in diamond with natural boron abundance. This is a large isotopic shift for hydrogenlike substitutional impurity centers in semiconductors. Also, a much smaller $(\sim 0.01\%)$ redshift of ${}^{11}\mathrm{B}$ transitions in diamond doped with natural boron has been observed relative to the ${}^{11}\mathrm{B}$ transitions in diamond doped by enriched ${}^{11}\mathrm{B}$ (up to $99\%$).

We study a quantum particle coupled to hard-core bosons and propagating on disordered ladders with $R$ legs, ranging from $R=1$ (chains) to $R\gg 1$ (planes). The particle dynamics is studied within the framework of rate equations for the boson-assisted transitions between the Anderson states. We demonstrate that for finite $R<\infty$ and sufficiently strong disorder the dynamics is nondiffusive, while two-dimensional planar systems with $R\to \infty$ remain diffusive for arbitrarily strong disorder. The transition from diffusive to subdiffusive regimes may be identified via statistical fluctuations of resistivity. Close to the transition, the corresponding distribution function in the diffusive regime has fat tails which decrease much slower than $1/\sqrt{L}$, where $L$ is the system size. Finally, we present evidence that similar non-Gaussian fluctuations arise also in standard models of many-body localization.

Photonic structures with topologically nontrivial bands are usually designed by arranging simple meta-atoms, ideally, single-mode ones, in a carefully designed photonic lattice with symmetry that guarantees the emergence of topological states. Here we investigate an alternative option that does not require complex lattice geometry but instead relies on the tuning of the parameters of the individual meta-atoms to achieve the degeneracy of the modes with different symmetry. As an illustrative example, we consider a one-dimensional array of equidistant identical periodic nanophotonic waveguides supporting degenerate modes with strongly asymmetric near-field profiles giving rise to the coupling modulation. Exploiting this feature, we demonstrate that the proposed system supports topological edge modes and can be viewed as a generalization of the paradigmatic Su-Schrieffer-Heeger model, reducing to it for the suitable parameter choice. Our results thus provide an avenue to engineer topological states via mode interference which further expands the plethora of topological structures available in photonics being especially promising for nonlinear topological systems.

Unidirectional in-plane structural anisotropy in rhenium-based dichalcogenides introduces a new class of 2D materials, exhibiting anisotropic optical properties. In this work, we perform temperature-dependent, polarization-resolved photoluminescence and reflectance measurements on several-layer ${\mathrm{ReS}}_2$. We discover two additional excitonic resonances (${\mathrm{X}}_3$ and ${\mathrm{X}}_4$), which can be attributed to the splitting of spin degenerate states. Strong in-plane oscillator strength of exciton species ${\mathrm{X}}_1$ and ${\mathrm{X}}_2$ is accompanied by weaker counterparts ${\mathrm{X}}_3$ and ${\mathrm{X}}_4$ with similar polarization orientations. The in-plane anisotropic dielectric function has been obtained for ${\mathrm{ReS}}_2$, which is essential for engineering light matter coupling for polarization-sensitive optoelectronic devices. Furthermore, our temperature-dependent study revealed the existence of low-lying momentum-forbidden dark states causing an anomalous PL intensity variation at 30 K, which has been elucidated using a rate equation model involving phonon scattering from these states. Our findings of the additional excitonic features and the momentum-dark states can shed light on the true nature of the electronic band structure of ${\mathrm{ReS}}_2$.

Localization due to the presence of disorder has proven crucial for our current understanding of relaxation in isolated quantum systems. The many-body localized phase constitutes a robust alternative to the thermalization of complex interacting systems, but recently the importance of disorder has been brought into question. A number of disorder-free localization mechanisms have been put forward connected to local symmetries of lattice gauge theories. Here, starting from translationally invariant $(1+1)$-dimensional quantum electrodynamics, we modify the dynamics of the gauge field which allows us to construct a lattice model with a $U$(1) local gauge symmetry revealing a mechanism of disorder-free localization. We consider two different discretizations of the continuum model resulting in a free-fermion soluble model in one case and an interacting model in the other. We diagnose the localization of our translationally invariant model in the far-from-equilibrium dynamics following a global quantum quench.

We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity ${\rho }_{xx}$($T,H$), Hall resistivity ${\rho }_{xy}$($T,H$), and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for $\mathrm{Mo}{\mathrm{O}}_2$ with a monoclinic crystal structure. The band structure calculations show that $\mathrm{Mo}{\mathrm{O}}_2$ is a nodal-line semimetal when spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching $5.03\times {10}^4\%$ at 2 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of ${\rho }_{xx}$($T$), the characteristics common for many topologically nontrivial as well as trivial semimetals, emerge also in $\mathrm{Mo}{\mathrm{O}}_2$. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for magnetic field along the current direction and the nonzero Berry phase in de Haas–van Alphen measurements indicate that pairs of Weyl points appear in $\mathrm{Mo}{\mathrm{O}}_2$, which may be due to the crystal symmetry breaking. These results highlight $\mathrm{Mo}{\mathrm{O}}_2$ as a platform for studying the topological properties of oxides.

Group-III post-transition-metal monochalcogenides like gallium sulfide (GaS) are layered semiconductors with weakly interacting adjacent layers, which allow them to be reduced to the two-dimensional nanometric thickness level by different exfoliation approaches, similar to graphene. Here, we investigate the intensity polarization dependence of the Raman modes for a different number of GaS layers and use symmetry analysis and density-functional perturbation theory to provide further information on these structures. The Raman polarization-dependent behaviors of the bulk relative modes ${\mathrm{A}}_{1g}$ and ${\mathrm{E}}_{2g}$ were found to be independent of the number of layers, being proportional to ${cos}^2\left(\theta \right)$ for ${\mathrm{A}}_{1g}$ modes and constant for ${\mathrm{E}}_{2g}$ modes. The computational calculations for two and three layers show Raman active modes emerging at Raman shifts near the bulk Raman modes, with ${\mathrm{A}}_{1g}$ (${\mathrm{A}}_1^{\prime }$) and ${\mathrm{E}}_g$ (${\mathrm{E}}^{\prime }$) symmetries for an even (odd) number of layers, some of them being observed as “shoulders” in the experimental Raman spectra. These phonon modes present Raman tensors with components similar to those observed in bulk, thus explaining the same polar dependencies for different GaS thicknesses. The Raman intensity calculations were made by implementing the specific experimental geometry used here, thus resulting in good qualitative agreement. These results are fundamental for the understanding of the structural and vibrational changes when GaS is reduced to the few-layer limit, layer-number differentiation, and for further symmetry-lowering studies by strain manipulation or substrate interaction, which are routine issues in both fundamental research and device fabrication.

We reveal a one-dimensional (1D) charge density wave (CDW) at the lateral interface of black/blue phosphorene heterostructure (BBPH). This emerging CDW is induced by Fermi surface nesting and characterized by dimerization of unsaturated P atoms at the boundary. Accordingly, lattice distortion and band-gap opening simultaneously occur in the BBPH with prominently lowered total energy from metallic structure (MS) to the CDW phase. Due to the quantum confinement effect, the band gap increases as the heterostructure width reduces. Under an accessible electron doping level of $\sim {10}^{14}\mathrm{c}{\mathrm{m}}^{-2}$, the relative stability between the CDW phase and the MS can be flipped because of their different electron occupation distributions and enhanced electrostatic repulsion at the boundary. In addition, this junction boundary provides a good platform to study the 1D Su-Schrieffer-Heeger (SSH) model. We predict that an SSH-type electron soliton would emerge when there is a dimerization fault, which carries a topologically protected unpaired electron, and can migrate along the boundary with a tunable energy barrier.

A quantum pump in a buckled graphene ribbon with armchair edges is discussed numerically. By solving the Su-Schrieffer-Heeger model and performing the computer simulation of quantum transport we find that a kink adiabatically moving along the metallic ribbon results in highly efficient pumping, with a charge per kink transition close to the maximal value determined by the Fermi velocity in graphene. Remarkably, insulating nanoribbon show the quantized value of a charge per kink $\left(2e\right)$ in a relatively wide range of the system parameters, providing a candidate for the quantum standard ampere. We attribute it to the presence of a localized electronic state, moving together with a kink, whose energy lies within the ribbon energy gap.

The sulfur-hydrogen system is the first one in which superconductivity at temperatures over 200 K has been reported, albeit at high pressure. The particular phases causing the measured $T_c$ and their structures are not yet firmly identified. Here, synchrotron single-crystal x-ray diffraction studies of S-H samples were performed up to 150 GPa and revealed two previously unobserved and unpredicted sulfur-hydrogen phases—${\mathrm{H}}_{6\pm x}{\mathrm{S}}_5$ with $x\sim 0.4$, and ${\mathrm{H}}_{2.85\pm y}{\mathrm{S}}_2$ with $y\sim 0.35$. The crystallographic data obtained in this work, both for the new phases and for the previously identified ${\mathrm{H}}_3\mathrm{S}$ polymorphs, provide an unambiguous experimental proof of the chemical richness of the S-H system and the structural diversity of compounds forming at high pressures and high temperatures. Our results have profound implications for the interpretation of the resistance, superconductivity, and other physical properties measurements on the complex S-H system.

Exploiting the Lanczos method in momentum space, we determine accurately the quasiparticle properties of disordered two-dimensional Weyl fermions with anisotropic dispersion, which is linear in one direction and quadratic along the other. For comparison, we also present a perturbative analytical analysis based on the Born approximation and renormalization group methods. We reveal that low-energy quasiparticle properties are substantially corrected by multiple impurity scattering processes and manifested by the power-law function of self-energy. Near the nodal point, quasiparticle residue is considerably reduced and vanishes as $Z_E\propto E^r$ with a disorder-dependent exponent $r$. To highlight the importance of such unconventional quasiparticle residue behavior, we compute the classical diffusive conductivity via the Kubo formalism. We show that the sharp change of $Z_E$ in the vicinity of the nodal point gives rise to the strong temperature dependence of classical conductivity, which can be directly tested by transport measurements in the future.