We report a high-pressure study of orthorhombic rare-earth manganites $A{\mathrm{MnO}}_3$ using Raman scattering (for $A=Pr$, Nd, Sm, Eu, Tb, and Dy) and synchrotron x-ray diffraction (XRD), for $A$ = Pr, Sm, Eu, and Dy. In all cases, a phase transition was evidenced by the disappearance of the Raman signal at a critical pressure that depends on the $A$ cation. For the compounds with $A=Pr$, Sm, and Dy, XRD confirms the presence of a corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrum can be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the $ac$ plane, and the Raman bands associated with out-of-phase ${\mathrm{MnO}}_6$ rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smaller $A$ cations considered in this study (Dy and Tb), the compression is nearly isotropic in the $ac$ plane, with only small evolutions of the tilt angles and cooperative Jahn-Teller distortion. As the radius of the $A$ cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced tilting of octahedra chains becomes conversely less pronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogy with hydrostatic pressure works quite well for manganites with the smaller $A$ cations considered in this paper but can be misleading with large $A$ cations.

The magnetic state of the noncentrosymmetric antiferromagnet ${\mathrm{CeNiC}}_2$ has been studied by magnetic susceptibility, heat capacity, muon spin relaxation $\left(\mu \mathrm{SR}\right)$, and inelastic neutron scattering (INS) measurements. ${\mathrm{CeNiC}}_2$ exhibits three magnetic phase transitions at $T_{N_1}=19.5$ K, $T_{N_2}=10$ K, and $T_{N_3}=2.5$ K. The presence of long-range magnetic order below 20 K is confirmed by the observation of oscillations in the $\mu \mathrm{SR}$ spectra between 10 and 20 K and a sharp increase in the muon depolarization rate. INS studies reveal two well-defined crystal electric field (CEF) excitations around 8 and 30 meV. INS data have been analyzed using a CEF model and the wave functions were evaluated. We also calculated the direction and magnitude of the ground state moment using CEF wave functions and compare the results with that proposed from the neutron diffraction. Our CEF model correctly predicts that the moments order along the $b$ axis (or $y$ axis) and the estimated magnetic moment is $\mathrm{0.687(5)}{\mu }_B$, which is higher than the moment observed from the neutron diffraction $(0.25{\mu }_B/\mathrm{Ce})$. We attribute the observed reduced moment due to the Kondo screening effect.

Phase-slip centers/lines and hot spots are the main mechanisms for dissipation in current-carrying superconducting thin films. The pulsed-current method has recently been shown to be an effective tool in studying the dynamics of phase-slip centers and their evolution to hot spots. We use the time-dependent Ginzburg-Landau theory in the study of the dynamics of the superconducting condensate in superconducting strips under external current and zero external magnetic field. We show that both the flux-flow state (i.e., slow-moving vortices) and the phase-slip line state (i.e., fast-moving vortices) are dynamically stable dissipative units with temperature smaller than the critical one, whereas hot spots, which are localized normal regions where the local temperature exceeds the critical value, expand in time, resulting ultimately in a complete destruction of the condensate. The response time of the system to abrupt switching on of the overcritical current decreases with increasing both the value of the current (at all temperatures) and temperature (for a given value of the applied current). Our results are in good qualitative agreement with experiments we have conducted on Nb thin strips.

A structural phase transition from cubic $Fd\overline{3}m$ to tetragonal $I4{}_1/amd$ symmetry with $c/a> 1$ is observed at $T_S=16$ K in spinel ${\mathrm{GeCo}}_2{\mathrm{O}}_4$ below the Néel temperature $T_N=21$ K. Structural and magnetic ordering appear to be decoupled with the structural distortion occurring at 16 K while magnetic order occurs at 21 K as determined by magnetic susceptibility and heat capacity measurements. An elongation of ${\mathrm{CoO}}_6$ octahedra is observed in the tetragonal phase of ${\mathrm{GeCo}}_2{\mathrm{O}}_4$. We present the complete crystallographic description of ${\mathrm{GeCo}}_2{\mathrm{O}}_4$ in the tetragonal $I4{}_1/amd$ space group and discuss the possible origin of this distortion in the context of known structural transitions in magnetic spinels. ${\mathrm{GeCo}}_2{\mathrm{O}}_4$ exhibits magnetodielectric coupling below $T_N$. The related spinels ${\mathrm{GeFe}}_2{\mathrm{O}}_4$ and ${\mathrm{GeNi}}_2{\mathrm{O}}_4$ have also been examined for comparison. Structural transitions were not detected in either compound down to $T\approx 8$ K. Magnetometry experiments reveal in ${\mathrm{GeFe}}_2{\mathrm{O}}_4$ a second antiferromagnetic transition, with $T_{N1}=7.9$ K and $T_{N2}=6.2$ K, that was previously unknown and that bears a similarity to the magnetism of ${\mathrm{GeNi}}_2{\mathrm{O}}_4$.

Slater-Pauling (SP) rules connect the magnetic and electronic properties of the half-metallic (HM) Heusler compounds. Employing first-principles electronic structure calculations we explore the validity of the SP rules in the case of transition from HM semi-Heusler compounds to various cases of HM full-Heusler compounds. We show that the coherent-potential approximation yields half-metallicity and thus a generalized version of the SP rules can be derived. On the contrary, supercell calculations, which are expected to describe the experimental situation more accurately, show that the energy gap considerably shrinks for the intermediate compounds and in several cases the half-metallicity is completely destroyed. The origin of this behavior is attributed to the voids, which influence the symmetry of the lattice.

We report ultrasound studies of spin-lattice and single-ion effects in the spin-ice materials ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ (DTO) and ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ (HTO) across a broad field range up to 60 T, covering phase transformations, interactions with low-energy magnetic excitations, and single-ion effects. In particular, a sharp dip observed in the sound attenuation in DTO at the gas-liquid transition of the magnetic monopoles is explained based on an approach involving negative relaxation processes. Furthermore, quasiperiodic peaks in the acoustic properties of DTO due to nonequilibrium processes are found to be strongly affected by macroscopic thermal-coupling conditions: the thermal runaway observed in previous studies in DTO can be suppressed altogether by immersing the sample in liquid helium. Crystal-electric-field effects having a higher energy scale lead to a renormalization of the sound velocity and sound attenuation at very high magnetic fields. We analyze our observations using an approach based on an analysis of exchange-striction couplings and single-ion effects.

We present experimental evidence for a behavior which involves discrete breathers and vortices in a Josephson ladder. Breathers can be visualized as the creation and subsequent annihilation of vortex/antivortex pairs. An externally applied magnetic field breaks the vortex/antivortex symmetry and causes the breather to split apart. The motion of the vortex or antivortex creates multisite breathers, which are always to one side or the other of the original breather depending on the sign of the applied field. This asymmetry in the applied field is experimentally observed.

The effects of doping on the spectral properties of low-doped systems are investigated by means of the coherent potential approximation to describe the distributed disorder induced by the impurities and the phonon-phonon noncrossing approximation to characterize a wide class of electron-phonon interactions that dominate the low-energy spectral features. When disorder and electron-phonon interaction work on comparable energy scales, a strong interplay between them arises, the effect of disorder can no longer be described as a mere broadening of the spectral features, and the phonon signatures are still visible despite the presence of strong disorder. As a consequence, the disorder-induced metal-insulator transition is strongly affected by a weak or moderate electron-phonon coupling, which is found to stabilize the insulating phase.

Quantum memories are integral parts of both quantum computers and quantum communication networks. Naturally, such a memory is embedded into a hybrid quantum architecture, which has to meet the requirements of fast gates, long coherence times, and long distance communication. Erbium-doped crystals are well suited as a microwave quantum memory for superconducting circuits with additional access to the optical telecom $C$ band around 1.55 $\mu \mathrm{m}$. Here, we report on circuit QED experiments with an ${\mathrm{Er}}^{3+}$:${\mathrm{YAlO}}_3$ crystal and demonstrate strong coupling to a superconducting lumped element resonator. The low magnetic anisotropy of the host crystal allows for attaining the strong coupling regime at relatively low magnetic fields, which are compatible with superconducting circuits. In addition, ${\mathrm{Ce}}^{3+}$ impurities were detected in the crystal, which showed strong coupling as well.

We analyze a one-dimensional ring structure composed of many two-level systems, in the limit where only one excitation is present. The two-level systems are coupled to a common environment, where the excitation can be lost, which induces super- and subradiant behavior, an example of cooperative quantum coherent effect. We consider time-independent random fluctuations of the excitation energies. This static disorder, also called inhomogeneous broadening in literature, induces Anderson localization and is able to quench superradiance. We identify two different regimes: (i) weak opening, in which superradiance is quenched at the same critical disorder at which the states of the closed system localize; (ii) strong opening, with a critical disorder strength proportional to both the system size and the degree of opening, displaying robustness of cooperativity to disorder. Relevance to photosynthetic complexes is discussed.

We construct a minimal four-band model for the two-dimensional (2D) topological insulators and quantum anomalous Hall insulators based on the $p_x$- and $p_y$-orbital bands in the honeycomb lattice. The multiorbital structure allows the atomic spin-orbit coupling which lifts the degeneracy between two sets of on-site Kramers doublets $j_z=\pm \frac{3}{2}$ and $j_z=\pm \frac{1}{2}$. Because of the orbital angular momentum structure of Bloch-wave states at $\Gamma$ and $K\left(K^{\prime }\right)$ points, topological gaps are equal to the atomic spin-orbit coupling strengths, which are much larger than those based on the mechanism of the $s-p$ band inversion. In the weak and intermediate regime of spin-orbit coupling strength, topological gaps are the global gap. The energy spectra and eigen wave functions are solved analytically based on Clifford algebra. The competition among spin-orbit coupling $\lambda$, sublattice asymmetry $m$, and the Néel exchange field $n$ results in band crossings at $\Gamma$ and $K\left(K^{\prime }\right)$ points, which leads to various topological band structure transitions. The quantum anomalous Hall state is reached under the condition that three gap parameters $\lambda$, $m$, and $n$ satisfy the triangle inequality. Flat bands also naturally arise which allow a local construction of eigenstates. The above mechanism is related to several classes of solid state semiconducting materials.

We present a theoretical description of excitonic spectra in GaN-based quantum wells in a wide range of magnetic fields taking into account built-in strain and electric fields, and valence band mixing due to the quantum well confinement. Our calculations performed for the GaN/AlGaN quantum wells reveal a nonlinear behavior of the excitonic Zeeman splitting on magnetic field. We have determined that the low magnetic field $g$-factor dependence on the quantum well width shows a steplike variation due to the reordering of the light- and heavy-hole valence subbands. Sharp change of the $g$ factor is also predicted for InGaN/GaN quantum wells grown on a virtual metamorphic InGaN substrate.

Using the nonequilibrium Green's function (NEGF) approach, we develop a microscopic ac transport theory in the presence of electron-phonon interaction. Taking into account the self-consistent Coulomb interaction, the displacement current is included. This ensures that our theory satisfies the current-conserving and gauge-invariant conditions. Importantly, the inclusion of self-consistent Coulomb interaction naturally connects the NEGF formalism to the density functional theory (DFT). This allows us to calculate the self-consistent Hamiltonian using DFT within the NEGF framework, which paves the way for first principles ac transport calculation of nanoelectronic devices in the presence of electron-phonon interaction. It is known that the inelastic electron tunneling spectroscopy (IETS) is a powerful tool in studying the inelastic dc quantum transport in molecular devices. The basic idea of IETS is to obtain the information of vibrational spectrum of molecular devices by measuring the second derivative of the dc current with respect to the bias voltage. In the ac transport, we find that the phonon spectrum and electron-phonon coupling strength can be obtained from the second derivative of the admittance with respect to the frequency which is the working principle of the inelastic electron admittance spectroscopy (IEAS). Hence we propose to use IEAS to probe the effect of the phonon in ac transport. As an example, dynamic conductance of a quantum dot is discussed in detail and the concept of IEAS is demonstrated.

We investigate the kagome lattice Hubbard model at half filling using the variational Monte Carlo method by testing the $U\left(1\right)$-Dirac spin-liquid, uniform, and valence bond crystal states. Even for the large-$U$ case, the $U\left(1\right)$ Dirac state, which is the optimal state in the Heisenberg model, cannot be recovered. While the finite-$U$ Hubbard model allows for the introduction of vacancies in a different manner compared to the $t\text{−}J$ model, the physics appears to have many similarities. In particular, a valence bond crystal is formed in the intermediate-$U$ regime. We observe an impact of the formation of this valence bond crystal on a possible Mott transition in this model and discuss the properties of the wave function.

We have investigated the electronic structure of the high oxidation state material ${\mathrm{YCrO}}_4$ within the framework of the Zaanen-Sawatzky-Allen phase diagram. While ${\mathrm{Cr}}^{4+}$-based compounds such as ${\mathrm{SrCrO}}_3/{\mathrm{CaCrO}}_3$ and ${\mathrm{CrO}}_2$ can be classified as small-gap or metallic negative-charge-transfer systems, we find using photoelectron spectroscopy that ${\mathrm{YCrO}}_4$ is a robust insulator despite the fact that its Cr ions have an even higher formal valence state of 5+. We reveal using band-structure calculations that the tetrahedral coordination of the ${\mathrm{Cr}}^{5+}$ ions in ${\mathrm{YCrO}}_4$ plays a decisive role, namely to diminish the bonding of the Cr $3d$ states with the top of the O $2p$ valence band. This finding not only explains why the charge-transfer energy remains effectively positive and the material stable, but also opens up a new route to create doped carriers with symmetries different from those of other transition-metal ions.

We study the critical spin-polarization energy $\left({\alpha }_{\mathrm{C}}\right)$ above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spin polarized. We find a significant decrease of ${\alpha }_{\mathrm{C}}$ as we increase the well width. In systems with comparable electron layer thickness, ${\alpha }_{\mathrm{C}}$ for fractional states near Landau level filling $\nu =3/2$ is about twice larger than those near $\nu =1/2$, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness capture certain qualitative features of the experimental results.

We develop a theory for the proximity effect in superconductor–semiconductor–normal-metal tunneling structures, which have recently been extensively studied experimentally, leading to the observation of transport signatures consistent with the predicted zero-energy Majorana bound states. We show that our model for the semiconductor nanowire having multiple occupied subbands with different transmission probabilities through the barrier reproduces the observed “soft-gap” behavior associated with substantial subgap tunneling conductance. We study the manifestations of the soft-gap phenomenon both in the tunneling conductance and in local density of states measurements and discuss the correlations between these two quantities. We emphasize that the proximity effect associated with the hybridization between low-lying states in the multiband semiconductor and the normal-metal states in the lead is an intrinsic effect leading to the soft-gap problem. In addition to the intrinsic contribution, there may be extrinsic effects, such as, for example, interface disorder, exacerbating the soft-gap problem. Our work establishes the generic possibility of an ubiquitous presence of an intrinsic soft gap in the superconductor–semiconductor–normal-metal tunneling transport conductance induced by the inverse proximity effect of the normal metal.

The excitation of localized or delocalized surface plasmon-polaritons in nanostructured or extended graphene has attracted a steadily increasing attention due to their promising applications in sensors, switches, and filters. These single resonances may couple and intriguing spectral signatures can be achieved by exploiting the entailing hybridization. Whereas thus far only the coupling between localized or delocalized surface plasmon-polaritons has been studied in graphene nanostructures, we consider here the interaction between a localized and a delocalized surface plasmon-polariton. This interaction can be achieved by two different schemes that reside on either evanescent near-field coupling or far-field interference. All observable phenomena are corroborated by analytical considerations, providing insight into the physics and paving the way for compact and tunable optical components at infrared and terahertz frequencies.

In this paper we consider the system composed of two magnetic planes attached to zigzag terminations of the graphene nanostructure being an ultrashort fragment of the armchair nanoribbon. We investigate theoretically an indirect coupling between these magnetic planes mediated by charge carriers as a function of an external in-plane electric field. The calculations are based on a tight-binding model supplemented with a Hubbard term to account for Coulombic interactions. For selected sizes of the graphene nanostructure, particularly high sensitivity of the coupling to the electric field is found. This leads to the possibility of control over coupling magnitude and continuous switching of its sign between an antiferromagnetic and a ferromagnetic one. Such a phenomenon is demonstrated in the numerical results and its origin is analyzed. The robustness of this effect against armchair edge deformation and variation of exchange energy between magnetic planes and spins of charge carriers is discussed in a detailed way.

We study electron transport in granular film placed above the ferroelectric substrate. We show that the conductivity of granular film depends strongly on the ferroelectric state due to screening effects that modify the Coulomb blockade in granular film. In particular, the electric current in granular film is controlled by the direction of ferroelectric polarization. We show that the ferroelectric/granular film system has a large electroresistance effect. This effect can be utilized in memory and electric-field sensor applications.