The low-temperature magnetic susceptibility, heat capacity, and electrical resistivity of uranium sesquicarbide have been determined down to 2 K by combining measurements carried out on samples of U${}_2$C${}_3$, containing UC as minority phase, and on pure UC specimens. The presence of spin fluctuations with characteristic temperature $T_{SF}=7$ K is revealed by a nonanalytic contribution to the specific heat behaving (well below $T_{SF}$) as $T^3\text{ln}(T/T_{SF})$, and confirmed by a $T^2$ increase of the electrical resistivity and a $1-\kappa T^2$ decrease of the low-temperature magnetic susceptibility. The analysis of the specific-heat data above $T_{SF}$ provides a Debye temperature ${\Theta }_D$ $\approx$ 256 K and a moderate high value of the Sommerfeld coefficient, $\gamma$ $\approx$ 42 mJ mol${}_U^{-1}$ K${}^{-2}$. The zero-temperature many-body enhancement of the electron mass is found to be $m^{*}/m$ $\approx$ 2.7. Our data rule out the occurrence of magnetic ordering in U${}_2$C${}_3$. First-principles electronic structure calculations support the absence of a magnetically ordered ground state but suggest that dynamical spin fluctuations are responsible for the electron mass enhancement.

We present a quantum Monte Carlo study of the Ising model in a transverse field on a square lattice with nearest-neighbor antiferromagnetic exchange interaction $J$ and one diagonal second-neighbor interaction $J^{\prime }$, interpolating between square-lattice ($J^{\prime }=0$) and triangular-lattice ($J^{\prime }=J$) limits. At a transverse field of $B_x=J$, the disorder line first introduced by Stephenson [J. Math. Phys. 5, 1009 (1964)], where the correlations go from Neel to incommensurate, meets the zero-temperature axis at $J^{\prime }\approx 0.7J$. Strong evidence is provided that the incommensurate phase at larger $J^{\prime }$, at finite temperatures, is a floating phase with power-law decaying correlations. We sketch a general phase-diagram for such a system and discuss how our work connects with the previous quantum Monte Carlo work by Isakov and Moessner [Phys. Rev. B 68, 104409 (2003)] for the isotropic triangular lattice ($J^{\prime }=J$). For the isotropic triangular lattice, we also obtain the entropy function and constant entropy contours using a mix of quantum Monte Carlo, high-temperature series expansions and high-field expansion methods and show that phase transitions in the model in the presence of a transverse field occur at very low entropy.

Metal-insulator transitions in pyrochlore iridates (A${}_2$Ir${}_2$O${}_7$) are believed to occur due to subtle interplay of spin-orbit coupling, geometric frustration, and electron-electron interactions. In particular, the nature of magnetic ordering of iridium ions in the insulating phase is crucial for the understanding of several exotic phases recently proposed for these materials. We study the spectrum of magnetic excitations in the intermediate-coupling regime for the so-called all-in/all-out magnetic state in pyrochlore iridates with nonmagnetic $A$-site ions ($A$ $=$ Eu,Y). This state was found to be preferred in previous theoretical studies. We find that the effect of charge fluctuations on the magnetic excitations in the intermediate-coupling regime leads to a strong departure from the lowest-order spin-wave calculations based on spin models obtained in the strong-coupling limit. We discuss the characteristic features of the magnetic excitation spectrum that can lead to conclusive identification of the magnetic order in future resonant inelastic x-ray (or neutron) scattering experiments. This knowledge about the nature of magnetic order and its low-energy features may also provide useful information on the accompanying metal-insulator transition.

We measured the resistivity and the specific heat of CsFe${}_2$As${}_2$ single crystals, which were grown by using a self-flux method. The CsFe${}_2$As${}_2$ crystal shows a sharp superconducting transition at 1.8 K with a transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. A large $\Delta C_p/T_c$ accompanying a very low $T_c$, nonexponential dependence of the lowest temperature $\Delta C_p/T$ and an $H^{1/2}$ dependence of $\Delta \gamma \left(H\right)$ are observed in the superconducting-state specific heat. Combined with the fitting results by the $\alpha$ model for the superconducting-state specific heat, a pairing symmetry with nodal gaps was suggested in CsFe${}_2$As${}_2$.

The activation-relaxation technique is used to explore the distribution of escape times from metastable minima of a model metallic glass, estimated using the harmonic transition state theory. We investigate in particular the distribution of attempt frequencies and find that these prefactors are distributed over a range of almost ten orders of magnitude, in stark contrast with the common assumption that they are given by a typical vibrational frequency. Moreover, the typical attempt frequencies appear to decrease with increasing barrier energy, a behavior inverse to the so-called Meyer-Neldel rule observed in many processes with apparent Arrhenius behavior. When combined with a multiple-hop analysis that takes into account the multiplicity of possible transitions, a direct Meyer-Neldel behavior is recovered, albeit with a reduced characteristic prefactor.

We study features in the optically detected magnetic resonance (ODMR) signals associated with negatively charged nitrogen-vacancy (NV${}^{-}$) centers coupled to other paramagnetic impurities in diamond. Our results are important for understanding ODMR line shapes and for optimization of devices based on NV${}^{-}$ centers. We determine the origins of several side features to the unperturbed NV${}^{-}$ magnetic resonance by studying their magnetic field and microwave power dependences. Side resonances separated by around 130 MHz are due to hyperfine coupling between NV${}^{-}$ centers and nearest-neighbor ${}^{13}$C nuclear spins. Side resonances separated by approximately $\{40,260,300\}$ MHz are found to originate from simultaneous spin flipping of NV${}^{-}$ centers and single substitutional nitrogen atoms. All results are in agreement with the presented theoretical calculations.

The lattice dynamics of solid ${}^4$He has been explored using pulsed NMR methods to study the motion of ${}^3$He impurities in the temperature range (0.05–0.20 K) where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid ${}^4$He lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the ${}^3$He motion that results from quantum tunneling in the ${}^4$He matrix. The measurements were made for ${}^3$He concentrations $16<x_3<2000$ ppm. For ${}^3$He concentrations $x_3=16$ and 24 ppm, large changes are observed for both the spin-lattice relaxation time $T_1$ and the spin-spin relaxation time $T_2$ at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher ${}^3$He concentrations.

Electric control of magnetism is a vision which drives intense research on magnetic semiconductors and multiferroics. Recently, ultrathin metallic films were also reported to show magnetoelectric effects at room temperature. Here we demonstrate much stronger effects by exploiting reduction/oxidation reactions in a naturally grown oxide layer exchange coupled to an underlying ferromagnet. For the exemplarily studied FePt/iron oxide composite in an electrolyte, a large and reversible change of magnetization and anisotropy is obtained. The principle can be transferred to various metal/oxide combinations. It represents an approach towards multifunctionality.

We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in MgO-based tunnel junctions. We use ab initio alloy theory to consider different Fe${}_x$Co${}_{1-x}$ alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experimental results. In addition, changing the barrier thickness from six to ten monolayers of MgO leads to a nontrivial change of the temperature dependence. Our results emphasize that the tunneling magneto-Seebeck effect depends very crucially on and is very sensitive to material parameters and show that further experimental and theoretical investigations are necessary.

Magnetic susceptibility measurements and a microscopic magnetic model of the mineral clinoclase Cu${}_3$(AsO${}_4$)(OH)${}_3$ are reported. This spin-1/2 material can be well described as a combination of two nonequivalent spin dimers with the sizable magnetic couplings of $J\simeq 700$ K and $J_{\text{D2}}\simeq 300$ K but only small interdimer couplings. Based on density functional theory calculations, we pinpoint the location of dimers in the rather complex crystal structure. Counterintuitively, the largest exchange coupling operates between the structural Cu${}_2$O${}_6$ dimers. Additionally, we investigate magnetostructural correlations in Cu${}_2$O${}_6$ structural dimers by considering the influence of the hydrogen position on the magnetic coupling. To evaluate accurate exchange couplings, we establish the hydrogen positions that were not known so far and analyze the pattern of hydrogen bonding.

Hopping of carriers between localized states dominates charge transport in amorphous organic and inorganic semiconductors. We suggest a comprehensive description of this transport regime based on the percolation approach that allows one to determine not only very pronounced exponential dependencies of the hopping conductivity on material parameters, but also the more weakly dependent pre-exponential factors. The problem of the variable-range hopping (VRH) via sites with exponential energy distribution is mapped onto a universal geometrical problem of percolation via spheres with distributed sizes. An exact solution of the latter problem provides accurate results for the VRH in systems with exponential density-of-states (DOS). Our analytical results are confirmed by straightforward computer simulations and compared to former results present in the literature. We also discuss the case of nearest-neighbor hopping on a lattice, where the pre-exponential factors are provided by the percolation approach for any shape of the DOS.

The combined effect of Rashba and Dresselhaus spin-orbit interactions on the physics of a helical Luttinger liquid coupled to a Kondo impurity is studied. A Rashba coupling can potentially destroy the Kondo singlet formation in certain parameter regimes [Phys. Rev. B 86, 161103(R) (2012)]. This effect is here shown to vanish for sufficiently strong Dresselhaus interaction. The transport properties of the system are investigated by calculating electrical conductance, current and current fluctuations, and thermal conductance.

We apply quantum optimal control theory to establish a local voltage-control scheme that operates in conjunction with the numerically exact solution of the time-dependent Schrödinger equation. The scheme is demonstrated for high-fidelity coherent control of electronic charge in semiconductor double quantum dots. We find tailored gate voltages in the viable gigahertz regime that drive the system to a desired charge configuration with $>99\%$ yield. The results could be immediately verified in experiments and would play an important role in applications towards solid-state quantum computing.

We studied Sr${}_2$IrO${}_4$ and Sr${}_3$Ir${}_2$O${}_7$ using angle-resolved photoemission spectroscopy, making direct experimental determinations of intra- and intercell coupling parameters as well as Mott correlations and gap sizes. The results are generally consistent with LDA+$U$+spin-orbit coupling calculations, though the calculations missed the momentum positions of the dominant electronic states and neglected the importance of intercell coupling on the size of the Mott gap. The calculations also ignore the correlation-induced spectral peak widths, which are critical for making a connection to activation energies determined from transport experiments. The data indicate a dimensionality-controlled Mott transition in these 5$d$ transition-metal oxides.

First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS${}_2$/Ti${}_2$C and MoS${}_2$/Ti${}_2$C$Y_2$ ($Y$ $=$ F and OH) semiconductor/metal contacts. The structure of the interfaces is determined. Strong chemical bonds formed at the MoS${}_2$/Ti${}_2$C interface result in additional states next to the Fermi level, which extend over the three atomic layers of MoS${}_2$ and induce a metallic character. The interaction in MoS${}_2$/Ti${}_2$C$Y_2$, on the other hand, is weak and not sensitive to the specific geometry, and the semiconducting nature thus is preserved. The energy level alignment implies weak and strong $n$-type doping of MoS${}_2$ in MoS${}_2$/Ti${}_2$CF${}_2$ and MoS${}_2$/Ti${}_2$C(OH)${}_2$, respectively. The corresponding $n$-type Schottky barrier heights are 0.85 and 0.26 eV. We show that the MoS${}_2$/Ti${}_2$CF${}_2$ interface is close to the Schottky limit. At the MoS${}_2$/Ti${}_2$C(OH)${}_2$ interface, we find that a strong dipole due to charge rearrangement induces the Schottky barrier. The present interfaces are well suited for application in all-two-dimensional devices.

Manifestations of chiral excitons on a magnetically gapped surfaces of a topological insulator thin film in Kerr and Faraday effects are analyzed. Excitonic contribution to a surface optical conductivity tensor is calculated. Chiral excitons contrary to conventional ones resonantly contribute to Hall conductivity due to the lack of symmetry between the states with opposite angular momentum. They can lead to the considerable enhancement of Faraday angle and ellipticity of transmitted electromagnetic wave. Chiral excitons cause a decrease of Kerr angle and prominent signatures in ellipticity of reflected electromagnetic wave. Conditions for experimental observation of the described effects are discussed.

Understanding the electronic properties of dopants near an interface is a critical challenge for nanoscale devices. We have determined the effect of dielectric mismatch and quantum confinement on the ionization energy of individual acceptors beneath a hydrogen passivated silicon (100) surface. While dielectric mismatch between the vacuum and the silicon at the interface results in an image charge which enhances the binding energy of subsurface acceptors, quantum confinement is shown to reduce the binding energy. Using scanning tunneling spectroscopy we measure resonant transport through the localized states of individual acceptors. Thermal broadening of the conductance peaks provides a direct measure for the absolute energy scale. Our data unambiguously demonstrates that these two independent effects compete with the result that the ionization energy is less than 5 meV lower than the bulk value for acceptors less than a Bohr radius from the interface.

We explore incommensurate magnetic structures and phase diagrams of multiferroics through accurate micromagnetic analysis, taking into account the spin flexoelectric interaction (the so-called Lifshitz invariant). We consider ${\text{BiFeO}}_3$ single crystals and epitaxial films grown on (111) substrates. The main control parameters of these systems are the magnetic field, magnetic anisotropy, and, in the case of thin films, epitaxial strain. We construct phase diagrams representing stability regions for the homogeneous magnetic states and incommensurate structures for two geometries of the field (parallel to the principal crystal axis $\mathit{H}\parallel {\mathit{C}}_3$ and perpendicular to this direction $\mathit{H}\perp {\mathit{C}}_3$). It is shown that the direction of applied magnetic field substantially affects the magnetic phases, the properties of incommensurate structures, and the character of phase transitions. A conical type of cycloidal ordering is revealed to exist during the transition from the incommensurate cycloidal structure to the homogeneous magnetic state. Constructed phase diagrams provide the combination of control parameters required to suppress the cycloidal ordering. Our results show that the critical magnetic field for suppression of the cycloid is lower in thin films than in single crystals, and can also be lowered by appropriate selection of applied magnetic field orientation. These results provide a stronger understanding of the complex magnetic ordering in ${\text{BiFeO}}_3$ and can be useful for strain engineering of new multiferroic materials on demand.

We investigate the spin-dependent electronic density of states near and above the Fermi level in bismuth iron garnet (BIG), Bi${}_3$Fe${}_5$O${}_{12}$, by magnetic circular dichroism and magneto-optical Faraday spectroscopy. BIG is a recently synthesized material, as its preparation requires special nonequilibrium conditions. Its scientific and applicative interest resides in huge specific Faraday rotation of the incident light, useful for magneto-optic applications. We show experimentally the presence of spin gaps in the conduction band as recently predicted theoretically by Oikawa et al. [T. Oikawa, S. Suzuki, and K. Nakao, J. Phys. Soc. Jpn. 74, 401 (2005)]. In the range of photon energies, where full spin polarization is expected, completely asymmetric Faraday hysteresis loops were observed, similar to those observed in half-metals such as (Pr,La)${}_{0.7}$Ca${}_{0.3}$MnO${}_3$ and Fe${}_3$O${}_4$. These results were modeled using even and odd (with respect to magnetization) contributions into hysteresis loops. The odd contribution appears only in the energy ranges where the density of states is fully spin polarized and vanishes at the Curie temperature. These results open a new perspective for the use of bismuth iron garnet in optic spintronics at room temperature and above.

We studied the spin-state responses to light impurity substitution in low-spin perovskite LaCoO${}_3$ (Co${}^{3+}$: $d^6$) through magnetization, x-ray fluorescence, and electrical resistivity measurements of single-crystal LaCo${}_{0.99}{M}_{0.01}$O${}_3$ ($M$ $=$ Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe substitution but was observed for Ni substitution. Strong magnetic anisotropy was also found in the Ni-substituted sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr${}^{3+}$, Mn${}^{(4-\delta )+}$, Fe${}^{(3+{\delta }^{\prime })+}$, and Ni${}^{3+}$. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping [Podlesnyak et al., Phys. Rev. Lett. 101, 247603 (2008)], the emergence of highly anisotropic spin-state molecular ferromagnets induced by low-spin Ni${}^{3+}$ with Jahn-Teller activity is suggested. We also discuss applicability of the present results to other materials with Fe ($d^6$).