We present a systematic study of the crystal-field interactions in the $\mathrm{Li}R{\mathrm{F}}_4$ ($R=\mathrm{Gd}$, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in ${\mathrm{LiTmF}}_4$ and level splitting in ${\mathrm{LiYbF}}_4$. Furthermore, using high-resolution neutron scattering from ${\mathrm{LiHoF}}_4$, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.

A scheme suggested in the literature to determine the symmetry-imposed shape of linear response tensors is revised and extended to allow for the treatment of more complex situations. The extended scheme is applied to discuss the shape of the spin conductivity tensor for all magnetic space groups. This allows in particular investigating the character of longitudinal as well as transverse spin transport for arbitrary crystal structure and magnetic order that give rise, e.g., to the spin Hall, Nernst, and the spin-dependent Seebeck effects.

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides $\left(\mathrm{FeO}Ch\right)$ series ${\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{Fe}}_2\mathrm{O}{M}_2$ ($M=\mathrm{S}$, Se), which are among the latest Fe-based materials with the potential to show unconventional high-$T_c$ superconductivity (HTSC). A combination of incoherent Hubbard features in x-ray absorption and resonant inelastic x-ray scattering spectra, as well as resistivity data, reveal that the parent $\mathrm{FeO}Ch$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that reveal a novel Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that $\mathrm{FeO}Ch$ may constitute an ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

Hydrogen in metals has attracted much attention for a long time from both basic scientific and technological points of view. Its electronic state has been investigated in terms of a proton embedded in the electron gas mostly by the local density approximation (LDA) to the density functional theory. At high electronic densities, it is well described by a bare proton ${\mathrm{H}}^{+}$ screened by metallic electrons (charge resonance), while at low densities two electrons are localized at the proton site to form a closed-shell negative ion ${\mathrm{H}}^{-}$ protected from surrounding metallic electrons by the Pauli exclusion principle. However, no details are known about the transition from ${\mathrm{H}}^{+}$ to ${\mathrm{H}}^{-}$ in the intermediate-density region. Here, by accurately determining the ground-state electron distribution $n\left(\mathit{r}\right)$ by the use of LDA and diffusion Monte Carlo simulations with the total electron number up to 170, we obtain a complete picture of the transition, in particular, a sharp transition from short-range ${\mathrm{H}}^{+}$ screening charge resonance to long-range Kondo-type spin-singlet resonance, the emergence of which is confirmed by the presence of an anomalous Friedel oscillation characteristic to the Kondo singlet state with the Kondo temperature $T_{\mathrm{K}}$ well beyond 1000 K. This study not only reveals interesting competition between charge and spin resonances, enriching the century-old paradigm of metallic screening to a point charge, but also discovers a $\text{high-}{T}_{\mathrm{K}}$ system long sought in relation to the development of exotic superconductivity in the quantum critical regime.

Our aim is the study of magnetization phenomena of carbon powders consisting of graphene nanocones and nanodisks. Magnetization measurements were carried out using a superconducting quantum interference device in magnetic fields $-5<B<5\mathrm{T}$ for temperatures in the range $0.5\le T<300\mathrm{K}$. We have observed that magnetization $M$ depends on temperature $T$ as a power law $M\propto T^{-\alpha }$ where the scaling exponent $\alpha =0.79\pm 0.04$ was determined. We found that the magnetization consists of a diamagnetic background and a paramagnetic contribution of localized spins. Considering saturation magnetization $M_S$ in the free spin magnetization model and diamagnetic susceptibility ${\chi }_D$ for independent of temperature $T$ we found that the effective spin value $S$ increases from $S=1/2$ at temperature $T>20\mathrm{K}$ to $S=3$ at low temperatures $0.5\le T\le 20\mathrm{K}$.

The structural properties of partially hydrogenated and fluorinated graphene with different percentages of H/F atoms are investigated using molecular dynamics simulations based on reactive force field (ReaxFF) potentials. We found that the roughness of graphene varies with the percentage $\left(p\right)$ of H or F and in both cases is maximal around $p=50\%$. Similar results were obtained for partially oxidized graphene. The two-dimensional area size of partially fluorinated and hydrogenated graphene exhibits a local minimum around $p=35\%$ coverage. The lattice thermal contraction in partially functionalized graphene is found to be one order of magnitude larger than that of fully covered graphene. We also show that the armchair structure for graphene oxide (similar to the structure of fully hydrogenated and fluorinated graphene) is unstable. Our results show that the structure of partially functionalized graphene changes nontrivially with the C : H and C : F ratio as well as with temperature.

The electronic properties of the Si(111)-$7\times 7$ surface are studied using four- and two-probe conductivity measurements and tunneling spectroscopy. We demonstrate that the temperature dependence of the surface conductivity corresponds to the Efros-Shklovskii law at least in the 10–100 K temperature range. The energy gap at the Fermi level observed in tunneling spectroscopy measurements at $T\ge 5$ K vanishes by thermal fluctuations at $T\approx 30$ K, without any sign of a metal-insulator transition. We show that the low-temperature energy gap observed by the tunneling spectroscopy technique is actually a consequence of the Coulomb blockade effect.

The electron spin resonance spectrum of a quasi-1D $S=1/2$ antiferromagnet ${\mathrm{K}}_2{\mathrm{CuSO}}_4{\mathrm{Br}}_2$ was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Neèl temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in $S=1/2$ antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

Plastic flow of solid ${}^3\mathrm{He}$ through a porous membrane frozen in the crystal is observed in the temperature range from 0.1 to 0.5 K. The flow of helium through the pores of the membrane occurs under the mechanical stress caused by the electrostatic force. It was found that the temperature dependence of the flow velocity $V$ has two characteristic regions: at temperatures above $T\sim 0.2$ K, $V$ decreases exponentially with lowering temperature, which corresponds to the thermally activated process, and at temperatures below 0.2 K, $V$ is independent on $T$, which indicates the quantum mechanism of mass transfer. For the high-temperature region the values of thermal activation volume and energy of activation are estimated. The results are analyzed in the framework of vacancy and dislocation models of plastic flow.

We investigate the quantum many-body instabilities of the extended Hubbard model for spinless fermions on the honeycomb lattice with repulsive nearest-neighbor and second-nearest-neighbor density-density interactions. Recent exact diagonalization and infinite density matrix renormalization group results suggest that a putative topological Mott insulator phase driven by the second-nearest-neighbor repulsion is suppressed, while other numerically exact approaches support the topological Mott insulator scenario. In the present work, we employ the functional renormalization group (fRG) for correlated fermionic systems. Our fRG results hint at a strong suppression of the scattering processes stabilizing the topological Mott insulator. From analyzing the effects of fermionic fluctuations, we obtain a phase diagram, which is the result of the competition of various charge ordering instabilities.

We study an “observable-based” notion of equilibration and its application to realistic systems like spin qubits in quantum dots. On the basis of the so-called distinguishability, we analytically derive general equilibration bounds, which we relate to the standard deviation of the fluctuations of the corresponding observable. Subsequently, we apply these ideas to the central spin model describing the spin physics in quantum dots. We probe our bounds by analyzing the spin dynamics induced by the hyperfine interaction between the electron spin and the nuclear spins using exact diagonalization. Interestingly, even small numbers of nuclear spins as found in carbon or silicon based quantum dots are sufficient to significantly equilibrate the electron spin.

The properties of the heavy-fermion superconductor ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$ are investigated by thermodynamic and transport measurements at ambient and under hydrostatic pressure. At ambient pressure the compound exhibits two successive magnetic transitions at $T_1\simeq 2.2$ K and $T_{\mathrm{N}}\simeq 2$ K into antiferromagnetically ordered states and enters into a heavy-fermion superconducting phase below $T_{\mathrm{c}}\simeq 0.32$ K. The coexistence of long-range magnetic order and superconductivity is discussed in the context of the existence of the two crystallographically inequivalent Ce sites in the unit cell of ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$. The experimental data allow us to construct the pressure-temperature phase diagram.

We measure the second harmonic (SH) response of ${\mathrm{MoS}}_2$ in a broad fundamental photon energy range between 0.85 and 1.7 eV, and present a continuous SH spectrum capturing signatures of both the fundamental $A$ and $B$ excitons in addition to the intense $C$ resonance. Moreover, we interpret our results in terms of an exciton simulation of the SH properties of multilayered ${\mathrm{MoS}}_2$ samples, represented by a study of trilayer ${\mathrm{MoS}}_2$. The good agreement between theory and experiments allows us to establish a connection between the measured spectrum and the underlying electronic structure, in the process elucidating the nonlinear excitation of electron-hole pairs.

We study the Holstein-Hubbard model at half filling to explore the ordered phases such as the charge density wave and antiferromagnet. The Coulomb interaction is rewritten in terms of auxiliary fields. By treating the auxiliary fields and phonons as classical, we obtain real-space features of the system and transition between the phases from weak to strong coupling. When both interactions are weak, mutual competition between them leads to a metallic phase in an otherwise insulator-dominated phase diagram. Spatial correlations induced by thermal fluctuations lead to pseudogap features at intermediate range of coupling.

We report a combined experimental and theoretical study of the optical response of epitaxial silicene on silver. The silicene/Ag(111) ultraviolet-visible absorption spectra, which turn out to be strongly nonadditive, are analyzed in the framework of ab initio calculations. Electronic transitions involving silver states are found to provide huge contributions to the optical absorption of silicene, compatible with a strong Si-Ag hybridization. The results are independent of the specific silicene configuration and are also worked out for thin amorphous silicon. This points to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of transient-reflectance spectroscopy. The latter shows a metalliclike relaxation time, hence demonstrating the effects of the strong hybridization arising in silicene/Ag(111) systems on charge carriers dynamical properties.

We study charge transport of a topological superconductor connected to different electromagnetic environments using a low-energy description where only the Majorana bound states in the superconductor are included. Extending earlier findings that found a crossover between perfect Andreev reflection with conductance $2e^2/h$ and a regime with blocked transport when the resistance of the environment is larger than $2e^2/h$, we consider Majorana bound states coupled to metallic dots. In particular, we study two topological superconducting leads connected by a metallic quantum dot in both the weak-tunneling and strong-tunneling regimes. For weak tunneling, we project onto the most relevant charge states. For strong tunneling, we start from the Andreev fixed point and integrate out charge fluctuations, which gives an effective low-energy model for the nonperturbative gate-voltage-modulated cotunneling current. In both regimes and in contrast to cotunneling with normal leads, the conductance is temperature independent because of the resonant Andreev reflections, which are included to all orders.