The hybridization of Yu-Shiba-Rusinov (YSR) states in dimers of magnetic impurities leads to molecularlike bonding and antibonding modes. In many-impurity systems, the interaction gives rise to YSR bands and can even result in the formation of a topologically nontrivial superconducting state, characterized by Majorana fermions at the edges of the system. To obtain a more detailed understanding of these interactions, we investigate the coupling of YSR states in short one-dimensional Fe chains on clean Nb(110). We observe a splitting of the single-atom YSR peaks into multiple states with even or odd spatial symmetry and identify a peculiar dependence of the even and odd states' energy position on the chain length.

Materials in which a Mott insulator to metal transition (Mott breakdown) can be induced using a moderate dc current (with an electric field that is much smaller than the corresponding Zener breakdown voltage) is extremely promising for electronic device applications and offer a unique platform for investigation of the underlying emergent mechanism. Here we report a Mott breakdown phenomenon in a spinel compound, namely, ${\mathrm{MgTi}}_{1.2}{\mathrm{V}}_{0.8}{\mathrm{O}}_4$, which undergoes a sharp switching of resistivity by several orders of magnitude by application of a dc current. The observed Mott breakdown effect, which is associated with a negative differential resistance as seen in the corresponding electric-voltage ($\mathit{V}$) vs electric-current ($\mathit{I}$) curve (opposite to Ohm's law dependence), occurs beyond a temperature-dependent threshold electric current (${\mathit{I}}_{\mathrm{th}}$), which corresponds to an extremely small threshold electric-field (${\mathit{E}}_{\mathrm{th}}$) (for example, ${\mathit{E}}_{\mathrm{th}}$ is found to be $\sim 40\mathrm{V}/\mathrm{cm}$ at 50 K and further decreases with an increase in temperature). The system remains in the metastable metallic state only in the presence of high-current flow upto the lowest measured temperature and switches back to the highly insulating state when the current flow is switched off. Associated experimental observations suggest against a Joule-heating driven change in state and hint towards an electronic mechanism. Using first-principles calculation and several experimental probes, we further investigate the origin of the driving mechanism. The presence of two Jahn-Teller active ions, i.e., ${\mathrm{Ti}}^{3+}$ ($3d^1$—one electron among three $t_{2\mathrm{g}}$ levels) and ${\mathrm{V}}^{3+}$ ($3d^2$—two electrons among three $t_{2\mathrm{g}}$ levels) in ${\mathrm{MgTi}}_{1.2}{\mathrm{V}}_{0.8}{\mathrm{O}}_4$ along with the obtained density of states offers a unique opportunity which seems critical for the Mott breakdown phenomenon as it supports high-current induced excitation of electrons through an activated avalanche process from ${\mathrm{Ti}}^{3+}{t}_{2g}$ levels (located at the edge of the valence band) into ${\mathrm{V}}^{3+}{t}_{2g}$ levels (located at the bottom of the conduction band) thereby removing the orbital degrees of freedom of the V and Ti electrons and leading to melting of the Jahn-Teller driven structural transition into the insulating tetragonal phase.

Measuring terahertz (THz) conductivity on an ultrafast timescale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy that allows us to measure cyclotron effective mass on a picosecond timescale. We demonstrate this technique by observing electron cooling in the technologically significant narrow-bandgap semiconductor indium antimonide. A significant reduction of electron effective mass from 0.032 to 0.017 $m_{\mathrm{e}}$ is observed in the first 200 ps after injecting hot electrons. The measured electron effective mass in InSb as a function of photoinjected electron density agrees well with conduction band nonparabolicity predictions from ab initio calculations of the quasiparticle band structure.

Diffraction is a fundamental physical phenomenon that occurs when scattering wave sources are arranged periodically. For example, when x rays are incident on the common salt crystals, diffraction occurs owing to the periodic structure of the chlorine and sodium atoms. Recent advances in nanotechnology have enabled artificial control of the scattering points even in visible wavelengths. These artificial structures are called photonic crystals and are related to many interesting phenomena. Dispersion relations between wave vector and frequency are called band structures. Similar to the band structures of electrons and holes in crystals, photonic bands for dispersion determine the direction of propagation of light waves by diffraction of the periodic structures. Herein, we introduce band engineering which enables multiple replications of bands in arbitrary wave vectors by holographic modulation. We have already realized a patterned light source named “iPMSEL” as a typical application of holographically engineered photonic band. The iPMSEL has been realized by combining photonic-crystal surface-emitting lasers and holographic engineering. It can emit arbitrary multiple beam patterns and even photographic images from a tiny semiconductor chip without any optical components. In this paper, we study the iPMSEL from the viewpoint of the photonic-band replication and operation mechanism. While this work is based on photonics, the findings can also be applied to other fields of physics concerned with diffraction. We believe that this work provides useful methods with respect to periodic structures within all scale ranges.

We analytically study the three-dimensional (3D) quantum Hall effect (QHE) in a thin film of a Weyl semimetal from the perspective of bulk states. We derive the Chern numbers for $B\ne 0$ from the Chern numbers for $B=0$ through a topological analysis and obtain a phase diagram of Chern numbers in Weyl semimetals. We demonstrate how the relative alignment of the Weyl nodes and the thickness of the film influence the quantum Hall plateaus and predict a peculiar phase diagram for the QHE in a thin film of a Weyl semimetal. Our work reveals the nature of the 3D QHE in Weyl semimetals from the bulk states.

Transition metal dichalcogenides (TMDs) offer a new platform for theoretical study of two-dimensional materials and their applications, beyond graphene. Previous studies indicate that monolayers of TMDs become ferromagnetic when put in proximity to conventional ferromagnetic, leading to remarkable spin and valley transport properties when combined with the intrinsic spin-orbit coupling (SOC) inherent in these materials. In this work, we show that a magnetic tunnel junction setup consisting of an asymmetric ferromagnetic/ferromagnetic/normal ${\mathrm{WSe}}_2$ junction gives rise to a fully spin- and valley-polarized current for both parallel and antiparallel magnetization configurations in the presence of an off-resonance light. Due to the strong SOC of ${\mathrm{WSe}}_2$ together with off-resonance light, both the spin and valley polarizations are tunable and switchable. We also find that the tunneling magnetoresistance (TMR) could be tuned to 1 by off-resonance light. The relatively much stronger SOC in the conduction and valence bands of ${\mathrm{WSe}}_2$ compared to other compounds of the dichalcogenides family in collaboration with an off-resonance light leads to significant negative TMR at weak enough exchange fields. In appropriate parameter regimes, the TMR oscillations from negative to positive values can be tuned by the off-resonance light. Our results suggest that magnetic tunnel junctions involving ${\mathrm{WSe}}_2$ have very promising potential for applications in magnetic memory and spin and valleytronics devices.

Recently, intensive studies have revealed fascinating physics, such as charge density wave and superconducting states, in the newly synthesized kagome-lattice materials ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\text{K}, \mathrm{Rb}, \mathrm{Cs})$. Despite the rapid progress, fundamental aspects such as the magnetic properties and electronic correlations in these materials have not been clearly understood yet. Here, based on density functional theory plus single-site dynamical mean-field theory calculations, we investigate the correlated electronic structure and magnetic properties of the ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ family materials in the normal state. We show that these materials are good metals with weak local correlations. The obtained Pauli-like paramagnetism and the absence of local moments are consistent with a recent experiment. We reveal that the band crossings around the Fermi level form three groups of nodal lines protected by the spacetime inversion symmetry, each carrying a quantized $\pi$ Berry phase. Our result suggests that the local correlation strength in these materials appears to be too weak to generate unconventional superconductivity, and nonlocal electronic correlation might be crucial in this kagome system.

A seminal gedankenexperiment by Laughlin describes the charge transport in quantum Hall systems via the pumping of flux. Here, we propose an optical scheme which probes and manipulates quantum Hall systems in a similar way: When light containing orbital angular momentum interacts with electronic Landau levels, it acts as a flux pump which radially moves the electrons through the sample. We investigate this effect for a graphene system with Corbino geometry and calculate the radial current in the absence of any electric potential bias. Remarkably, the current is robust against the disorder which is consistent with the lattice symmetry, and in the weak excitation limit, the current shows a power-law scaling with intensity characterized by the novel exponent 2/3.

Analytic solutions for the longitudinal and transverse components of the magnetization of a single-domain ferromagnetic nanoparticle with the simplest possible form of uniaxial magnetocrystalline anisotropy and Zeeman energy are obtained in terms of the appropriate Jacobi elliptic functions and elliptic integrals via the undamped limit of the inertial Landau-Lifshitz-Gilbert (ILLG) equation. Moreover, the nutation frequency is also determined in terms of the inverse period of a Jacobi elliptic function. All the results originate by starting from the analogy of the ILLG equation to a symmetric top with an electric dipole lying along its axis of symmetry as used to model inertial, i.e., high-frequency, effects in the Debye theory of dielectric relaxation of assemblies of noninteracting polar molecules, an analogy which allows one to simultaneously obtain closed-form solutions for typical THz or ultrahigh-frequency magnetization observables as well as the GHz ones associated with the usual ferromagnetic resonance due to precessional motion.

Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (${\mathrm{YH}}_{3-2x}{\mathrm{O}}_x$), show a photochromic effect where the transparency of the films decreases reversibly upon exposure to UV light. However, the exact mechanism behind this effect is unknown. In this paper, we describe the behavior of ${\mathrm{YH}}_{3-2x}{\mathrm{O}}_x$ thin films, with different ${\mathrm{O}}^{2-}:{\mathrm{H}}^{-}$ ratios, under dark and illuminated conditions using in situ muon spin relaxation (${\mu }^{+}\mathrm{SR}$), and compare that to an oxygen-free reference compound, yttrium dihydride (${\mathrm{YH}}_{2-\delta }$). The muon acts as a local magnetic probe in our compounds, giving information related to electronic, structural, and photochromic properties. Although ${\mathrm{YH}}_{2-\delta }$ is the parent compound to ${\mathrm{YH}}_{3-2x}{\mathrm{O}}_x$, the muon behavior in these two materials is different—the muon electrostatically interacts primarily with ${\mathrm{H}}^{-}$ (dihydride) or ${\mathrm{O}}^{2-}$ (oxyhydride)—leading to the use of different theoretical models. For ${\mathrm{YH}}_{2-\delta }$, we observed the formation of an entangled H-$\mu$ complex and the onset of ${\mathrm{Mu}}^{+}$ diffusion and ${\mathrm{H}}^{-}$ rearrangement above 150 K ($E_{A,\mathrm{\Gamma }}=67\pm 13\mathrm{meV}$). For the oxyhydrides, we adopted a transition state model, where ${\mathrm{Mu}}^0$ formation and gradual ${\mathrm{Mu}}^{+}$ recovery take place, accompanied by the formation of a ${\mathrm{Mu}}^{+}\text{−}{\mathrm{O}}^{2-}$ complex and a polaron at the Y cation. The activation energy ($E_{A,\mathrm{dia}}$) associated with ${\mathrm{Mu}}^{+}$ recovery is dependent on lattice relaxation and is lower for thin films of higher H content ($E_{A,\mathrm{dia}}=29–45\mathrm{meV}$). In situ illumination further reduces this energy barrier for all measured oxyhydrides, suggesting that the photochromic effect involves a reversible structural rearrangement during photodarkening.

The low temperature acoustic properties of bulk metallic glasses measured over a broad range of frequencies rigorously test the predictions of the standard tunneling model. The strength of these experiments and their analyses is mainly based on the interaction of the tunneling states with conduction electrons or quasiparticles in the superconducting state. A series of experiments at kHz and GHz frequencies on the same sample material essentially confirms previous measurements and their discrepancies with theoretical predictions. These discrepancies can be lifted by considering more correctly the linewidths of the dominating two-level atomic-tunneling systems. In fact, dephasing caused or mediated by interaction with conduction electrons may lead to particularly large linewidths and destroy the tunneling sytems' two-level character in the normal conducting state.

We study the longitudinal spin susceptibility inside a magnetically ordered phase, which exhibits a superconducting instability leading to a coexistence of the two ordered phases. Inside the magnetic phase, the superconducting gap acquires a linear term in a magnetic field applied along the direction of the magnetic moment. We find that such a linear term generates a jump of the longitudinal spin susceptibility when the superconducting instability occurs via a continuous phase transition. This anomaly at the superconducting instability is a thermodynamic signature of the microscopic coexistence of superconductivity and magnetism and can be a general feature associated with the breaking of spin rotational symmetry inside the magnetic phase.

We report a detailed theoretical study of a coherent macroscopic quantum-mechanical phenomenon—quantum beats of a single magnetic fluxon trapped in a two-cell SQUID of high kinetic inductance. We calculate numerically and analytically the low-lying energy levels of the fluxon and explore their dependence on externally applied magnetic fields. The quantum dynamics of the fluxon shows quantum beats originating from its coherent quantum tunneling between the SQUID cells. We analyze the experimental setup based on a three-cell SQUID, allowing for time-resolved measurements of quantum beats of the fluxon.

Structural and magnetic transitions in a double perovskite hosting $5d^1$ Re ions are discussed in the context of recently published high-resolution x-ray diffraction patterns [D. Hirai et al., Phys. Rev. Res. 2, 022063(R) (2020)]. Our symmetry-inspired analysis of a reported structural transition below room temperature, from cubic to tetragonal symmetry, reveals order of a lone $E_g$-type charge quadrupole. A magnetic motif at lower temperature is shown to be composed of two order parameters, with propagation vectors $\mathbf{k}=\left(0,0,1\right)$ and $\mathbf{k}=\left(0,0,0\right)$, which imply a magnetic space group $Pn{n}^{\prime }{m}^{\prime }$ . A concomitant lowering of Re site symmetry adds a $T_{2g}$ type to permitted quadrupoles. Findings from our studies, for structural and magnetic properties of ${\mathrm{Ba}}_2{\mathrm{MgReO}}_6$, surface in predicted amplitudes for x-ray diffraction at rhenium $L_2$ and $L_3$ absorption edges, and magnetic neutron Bragg diffraction. Specifically, entanglement of anapole and spatial degrees of freedom creates a quadrupole in the neutron scattering amplitude. It would be excluded in an unexpected scenario whereby the rhenium atomic state is adequately described by a single value of the total angular momentum. Quadrupoles in x-ray and neutron diffraction amplitudes possess different angular symmetries, since the former is purely electronic and the latter purely magnetic. A chiral signature visible in resonant x-ray diffraction will be one consequence of predicted chargelike quadrupole and magnetic dipole orders. A model Re wave function consistent with all current knowledge is a guide to electronic and magnetic multipoles engaged in x-ray and neutron diffraction investigations.

For fractional quantum Hall states on a finite disk, we study the thermoelectric transport properties under the influence of the presence of an edge and its reconstruction. In a recent study on a torus [Phys. Rev. B 101, 241101(R) (2020)], Sheng and Fu found a universal non-Fermi liquid power-law scaling of the thermoelectric conductivity ${\alpha }_{xy}\propto T^{\eta }$ for the gapless composite Fermi liquid state. The exponent $\eta \sim 0.5$ appears an independence of the filling factors and the details of the interactions. In the presence of an edge, we find the properties of the edge spectrum dominants the low-temperature behaviors and a nonuniversal scaling behavior in low temperature is observed. To consider individually the effect of the edge states, the real space entanglement spectrum of the model wave function, which contains only the spectrum of edge excitation is employed. Its spectrum energies could be tuned by varying the radius of the subsystem. In the non-Abelian Moore-Read state, the Majorana neutral edge mode is found to have more significant effects than that of the charge mode in the low temperature.

We study a one-dimensional lattice model of generalized statistics in which particles have next-nearest-neighbor hopping between sites which depends on the occupation number at the intermediate site and a statistical parameter $\varphi$. The model breaks parity and time-reversal symmetries and has four-fermion interactions if $\varphi \ne 0$. We first analyze the model using mean field theory and find that there are four Fermi points whose locations depend on $\varphi$ and the filling $\eta$. We then study the modes near the Fermi points using the technique of bosonization. Based on the quadratic terms in the bosonized Hamiltonian, we find that the low-energy modes form two decoupled Tomonaga-Luttinger liquids with different values of the Luttinger parameters which depend on $\varphi$ and $\eta$; further, the right- and left-moving modes of each system have different velocities. A study of the scaling dimensions of the cosine terms in the Hamiltonian indicates that the terms appearing in one of the Tomonaga-Luttinger liquids will flow under the renormalization group and the system may reach a nontrivial fixed point in the long distance limit. We examine the scaling dimensions of various charge density and superconducting order parameters to find which of them is the most relevant for different values of $\varphi$ and $\eta$. Finally, we look at two-particle bound states that appear in this system and discuss their possible relevance to the properties of the system in the thermodynamic limit. Our work shows that the low-energy properties of this model of generalized statistics have a rich structure as a function of $\varphi$ and $\eta$.