An extensive study of hydroxyl salts $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OD}\right)}_3\mathrm{Cl}/\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$ utilizing muon-spin-relaxation (μSR), nuclear magnetic resonance (NMR), and Raman spectroscopy, supplemented by dielectric constant and magnetic susceptibility measurements, has revealed a unique multiferroic system with deuterium-order-type ferroelectricity and geometrically frustrated magnetism. Through the temperature-dependent dynamics of the nuclear fields probed by μSR, the deuterium atoms in $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OD}\right)}_3\mathrm{Cl}$ were found to be rapidly fluctuating in the paraelectric phase. Upon cooling, they showed a critical slowing down toward the ferroelectric transition at $T_c=T_{\varepsilon }=230\mathrm{K}$, below which eight new Raman-active phonon bands appeared as a result of symmetry breaking due to deuterium ordering. Meanwhile, the hydrogen atoms in $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$ became quasistatic at a reduced temperature near 210 K, wherein only one new Raman band appeared with a broad dielectric anomaly, suggesting incomplete ordering. Furthermore, the μSR, NMR, Raman spectroscopy, and the magnetic susceptibility all suggested an increase in magnetic couplings below $\sim 250\mathrm{K}$ in both $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OD}\right)}_3\mathrm{Cl}$ and $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Cl}$, demonstrating a multiferroic feature for $\mathrm{C}{\mathrm{o}}_2{\left(\mathrm{OD}\right)}_3\mathrm{Cl}$. This work presents a new and unique multiferroic system with an exceptional high $T_{\mathrm{c}}$. It also demonstrates that conventional μSR can be an effective tool for studying ferroelectrics.

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and GeTe in their crystalline form. The related ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ of about 0.45 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Low-energy magnetic excitations of the easy-axis antiferromagnet ${\mathrm{TbFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ are investigated by far-infrared absorption and reflection spectroscopy in high magnetic fields up to 30 T. The observed field dependence of the resonance frequencies and the magnetization are reproduced by a mean-field spin model for magnetic fields applied both along and perpendicular to the easy axis. Based on this model we determined the full set of magnetic interactions, including Fe-Fe and Fe-Tb exchange interactions, single-ion anisotropy for Tb ions and $g$ factors, which describe the ground-state spin texture and the low-energy spin excitations of ${\mathrm{TbFe}}_3{\left({\mathrm{BO}}_3\right)}_4$. Compared to earlier studies, we allow a small canting of the nearly Ising-type Tb moments to achieve a quantitative agreement with the magnetic susceptibility measurements. The additional high-energy magnetic resonance lines observed, besides the two resonances expected for a two-sublattice antiferromagnet, suggest a more complex six-sublattice magnetic ground state for ${\mathrm{TbFe}}_3{\left({\mathrm{BO}}_3\right)}_4$.

$\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ crystallizes in the tetragonal $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$ structure type and shows multiple magnetic phase transitions driven by changes in temperature and magnetic field. The nature of these transitions was investigated by a combination of magnetic and magnetoresistance measurements and both single crystal and powder neutron diffraction. The Co magnetic moments order ferromagnetically (FM) parallel to the $c$ axis at 282 K, followed by antiferromagnetic (AFM) ordering at 225 K. In the AFM structure, the Co magnetic moments align along the $c$ axis with FM $\left[\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2\right]$ layers arranged in an alternating sequence, $\uparrow \uparrow \downarrow \downarrow$, which leads to the doubling of the $c$ axis in the magnetic unit cell. Another AFM transition is observed at 27 K, due to the ordering of a half of Pr moments in the $ab$ plane. The other half of Pr moments undergoes AFM ordering along the $c$ axis at 11 K, causing simultaneous reorientation of the previously ordered Pr moments into an AFM structure with the moments being canted with respect to the $c$ axis. This AFM transition causes an abrupt decrease in electrical resistivity at 11 K. Under applied magnetic field, two metamagnetic transitions are observed in the Pr sublattice at 0.8 and 5.4 T. They correlate with two anomalies in magnetoresistance measurements at the same critical fields. A comparison of the temperature- and field-dependent magnetic properties of $\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ to the magnetic behavior of $\mathrm{PrC}{\mathrm{o}}_2{\mathrm{P}}_2$ is provided.

We study noncentrosymmetric superconductors with the tetrahedral $T_d$, tetragonal $C_{4v}$, and cubic point group $O$. The order parameter is computed self-consistently in the bulk and near a surface for several different singlet to triplet order parameter ratios. It is shown that a second phase transition below $T_c$ is possible for certain parameter values. In order to determine the surface orientation's effect on the order parameter suppression, the latter is calculated for a range of different surface orientations. For selected self-consistent order parameter profiles the surface density of states is calculated showing intricate structure of the Andreev bound states as well as spin polarization. The topology's effect on the surface states and the tunnel conductance is thoroughly investigated, and a topological phase diagram is constructed for open and closed Fermi surfaces showing a sharp transition between the two for the cubic point group $O$.

We investigate the $J_1-J_2$ Heisenberg model on the triangular lattice with an additional scalar chirality term and show that a chiral spin liquid is stabilized in a sizable region of the phase diagram. This topological phase is situated in between a coplanar ${120}^{\circ }$ Néel ordered and a noncoplanar tetrahedrally ordered phase. Furthermore we discuss the nature of the spin-disordered intermediate phase in the $J_1-J_2$ model. We compare the ground states from exact diagonalization with a Dirac spin liquid wave function and propose a scenario where this wave function describes the quantum critical point between the ${120}^{\circ }$ magnetically ordered phase and a putative ${\mathbb{Z}}_2$ spin liquid.

We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the $C2/c, Pc, Pbcn$, and $P{6}_3/m$ structures at pressures of 250, 300, and 350 GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body gap in the thermodynamic limit. By comparing our static-nucleus DMC energy gaps with available experimental results, we demonstrate the important role played by nuclear quantum effects in the electronic structure of solid hydrogen.

In the present molecular-dynamics study, external ac driving is used at frequencies outside the phonon spectrum to excite gap DBs in uniformly strained graphene nanoribbon. Harmonic displacement or harmonic force is applied to a zigzag atomic chain of graphene. In the former case, nonpropagating DBs are excited on the atoms next to the driven atoms, while in the latter case the excited DBs propagate along the nanoribbon. The energy transfer along the nanoribbon assisted by the DBs is investigated in detail, and the differences between harmonic displacement driving and harmonic force driving are discussed. It is concluded that the amplitude of external driving at out-of-phonon spectrum frequencies should not necessarily be large to obtain a noticeable energy transfer to the system. Overall, our results suggest that external harmonic driving even at relatively small driving amplitudes can be used to control excitation of DBs and consequently the energy transfer to the system.

Metal growth on graphene has many applications. Transition metals are known to favor three-dimensional (3D) cluster growth on graphene. Copper is of particular interest for cost-effective surface-supported catalysis applications and as a contact material in electronics. This paper presents an in situ real-time study of Cu growth kinetics on graphene covering all stages preceding formation of a continuous film performed by laboratory-based grazing-incidence small-angle x-ray scattering (GISAXS) technique. In particular, nucleation and 3D cluster growth, coalescence, and percolation stages were identified. The cluster nucleation saturates after reaching a density of ${10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$ at $\approx 1$ monolayer thickness. A Kratky plot and a paracrystal model with cumulative structural disorder were necessary to evaluate properly cluster growth and coalescence, respectively. The power law scaling constants $0.27\pm 0.05$ and $0.81\pm 0.02$ of the temporal evolution of Cu cluster size suggest the growth of isolated clusters and dynamic cluster coalescence keeping the cluster shape, respectively. Coalescence and percolation thresholds occur at Cu thicknesses of $2\pm 0.4$ and $8.8\pm 0.7\mathrm{nm}$, respectively. This paper demonstrates the potential of laboratory-based in situ GISAXS as a vital diagnostic tool for tailoring a large variety of Cu nanostructures on graphene based on an in situ Cu growth monitoring which is applicable in a broad range of deposition times.

We use spin- and angle-resolved photoemission spectroscopy (SARPES) combined with a polarization-variable laser and investigate the spin-orbit coupling effect under interband hybridization of Rashba spin-split states for the surface alloys Bi/Ag(111) and Bi/Cu(111). In addition to the conventional band mapping of photoemission for Rashba spin splitting, the different orbital and spin parts of the surface wave function are directly imaged into energy-momentum space. It is unambiguously revealed that the interband spin-orbit coupling modifies the spin and orbital character of the Rashba surface states leading to the enriched spin-orbital entanglement and the pronounced momentum dependence of the spin polarization. The hybridization thus strongly deviates the spin and orbital characters from the standard Rashba model. The complex spin texture under interband spin-orbit hybridization proposed by first-principles calculation is experimentally unraveled by SARPES with a combination of $p$- and $s$-polarized light.

We predict that a driven localized magnetic moment coupled to mobile carriers of a metal or semiconductor surface or interface generates a specific dynamics of the carrier spin density as well as a transient spin current. Numerical results illustrate the time-dependent Friedel oscillations and the associated ultrafast Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction as well as the spin current generated by the impurity spin flipping. Retardation effects of the indirect exchange interaction of the impurity spins via the conduction electrons are found and discussed. Our results point to an alternative way of controlling the local magnetization on a subpicosecond time scale via appropriate tuning of the external fields that drive the magnetic dynamics of a localized moment.

The electrical, in-plane resistance as a function of temperature $R\left(T\right)$ of bulk and mesoscopic thin graphite flakes obtained from the same batch was investigated. Samples thicker than $\sim 30$ nm show metalliclike contribution in a temperature range that increases with the sample thickness, whereas a semiconductinglike behavior was observed for thinner samples. The temperature dependence of the in-plane resistance of all measured samples and several others from literature can be very well explained between 2 and 1100 K assuming three contributions in parallel: a metalliclike conducting path at the interfaces between crystalline regions, composed of two semiconducting phases, i.e., Bernal and rhombohedral stacking. From the fits of $R\left(T\right)$ we obtain a semiconducting energy gap of $110\pm 20$ meV for the rhombohedral and $38\pm 8$ meV for the Bernal phase. The presence of these crystalline phases was confirmed by x-ray diffraction measurements. We review similar experimental data from literature of the last 33 years and two more theoretical models used to fit $R\left(T\right)$.

We present a theoretical study of the anisotropy of the spin relaxation and decoherence in typical quantum wells with an arbitrary magnetic field. In such systems, the orientation of the magnetic field relative to the main crystallographic directions is crucial, owing to the lack of spin-rotation symmetry. For typical high mobility samples, relaxation anisotropies owing to the interplay of Rashba and Dresselhaus spin-orbit coupling are calculated. We also include the effect of the cubic-in-momentum terms. Although commonly ignored in literature, the latter were experimentally evidenced by the observation of strong anisotropy in spin decoherence measurements by different experimental groups and has long remained unexplained. This work suggests a method to determine the relative strength of spin-orbit coupling terms by angular resolution of decoherence in electron spin resonance experiments.

We apply the generalized Boltzmann theory to describe thermoelectric transport properties of monolayer phosphorene in the presence of short- and long-range charged impurity interactions. First, we propose a low-energy Hamiltonian to explore the accurate electronic band structure of phosphorene in comparison with those results obtained by density-functional simulations. We explain the effect of the coupling between the conduction and valence bands on the thermoelectric properties. We show that the electric conductivity of phosphorene is highly anisotropic, while the Seebeck coefficient and figure of merit, without being influenced via either the presence or absence of the coupling term, are nearly isotropic. Furthermore, we demonstrate that the conductivity for the $n$ type of doping is more influenced by the coupling term than that of the $p$ type. Along with thermopower sign change, profound thermoelectric effects can be achieved.

The thermal noise in amorphous oxides is the limiting factor for gravitational wave detectors and other high-precision optical devices. Through the fluctuation-dissipation theorem, the thermal noise is directly connected to the internal friction ($Q^{-1}$). Computational calculations of $Q^{-1}$ that use a two-level system (TLS) model have previously been performed for several coating materials, facilitating the search for coatings with lower thermal noise. However, they are based on a historical approximation made within the TLS model that treats the TLS distribution as uncorrelated, which has limited the predictive power of the model. In this paper, we demonstrate that this approximation limits the physical description of amorphous oxides using the TLS model and a fully correlated distribution must be used to calculate high-temperature behavior. Not only does using a correlated distribution improve the theoretical standing of the TLS model, calculations of $Q^{-1}$ using a fully correlated distribution reproduce and uncover the physical mechanisms of a second peak observed in measurements of ion-beam sputtered amorphous silica. We also explore the details of the thermal activation of TLSs and analyze the atomic transitions that contribute to $Q^{-1}$ in amorphous silica.

We theoretically and experimentally report subwavelength resonant panels for low-frequency quasiperfect sound absorption including transmission by using the accumulation of cavity resonances due to the slow sound phenomenon. The subwavelength panel is composed of periodic horizontal slits loaded by identical Helmholtz resonators (HRs). Due to the presence of the HRs, the propagation inside each slit is strongly dispersive, with near-zero phase velocity close to the resonance of the HRs. In this slow sound regime, the frequencies of the cavity modes inside the slit are down-shifted and the slit behaves as a subwavelength resonator. Moreover, due to strong dispersion, the cavity resonances accumulate at the limit of the band gap below the resonance frequency of the HRs. Near this accumulation frequency, simultaneously symmetric and antisymmetric quasicritical coupling can be achieved. In this way, using only monopolar resonators quasiperfect absorption can be obtained in a material including transmission.

We use first-principles calculations combined with the Boltzmann transport equation and semiclassical analysis to investigate the thermal conductivity $\kappa$, electrical conductivity $\sigma$, and thermopower $S$ of crystalline LiH and NaH. Remarkably, the calculated $S$ is extraordinarily high while the lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ is fairly low, which, as a result, leads to a much high thermoelectric power factor $\sigma S^2$ and good thermoelectric properties, with the figure of merit $zT$ even larger than 1.5 in the $p$-type doped NaH. Further analyses reveal that (i) the large band gap and the flat band around the Fermi level cause the high $S$ and (ii) strong anharmonic phonon scatterings and relevant phonon group velocities result in the low ${\kappa }_{\mathrm{L}}$ in these light materials. Our results support that crystalline LiH and NaH may be potential materials for thermoelectric applications.

We present the zero-temperature phase diagram of a Heisenberg antiferromagnet on a frustrated triangular lattice with nearest-neighbor $\left(J_1\right)$ and next-nearest-neighbor $\left(J_2\right)$ interactions, in a magnetic field. We show that the classical model has an accidental degeneracy for all $J_2/J_1$ and all fields, but the degeneracy is lifted by quantum fluctuations. We show that at large spin $S$, for $J_2/J_1<1/8$, quantum fluctuations select the same sequence of three sublattice co-planar states in a field as for $J_2=0$, and for $1/8<J_2/J_1<1$, they select the canted stripe state for all nonzero fields. The transition between the two states is first order in all fields, with the hysteresis width set by quantum fluctuations. We study the model with arbitrary $S$, including $S=1/2$, near the saturation field by exploring the fact that near saturation the density of bosons is small for all $S$. We show that for $S>1$, the transition remains first order, with a finite hysteresis width, but for $S=1/2$ and, possibly, $S=1$, there appears a new intermediate phase without a quasiclassical long-range order.

We investigate the effect of strong disorder on a system with strong electronic repulsion. In the absence of disorder, the system has a d-wave superconducting ground state with strong non-BCS features due to its proximity to a Mott insulator. We find that while strong correlations make superconductivity in this system immune to weak disorder, superconductivity is destroyed efficiently when disorder strength is comparable to the effective bandwidth. The suppression of charge motion in regions of strong potential fluctuation leads to the formation of Mott insulating patches, which anchor a larger nonsuperconducting region around them. The system thus breaks into islands of Mott insulating and superconducting regions, with Anderson insulating regions occurring along the boundary of these regions. Thus, electronic correlation and disorder, when both are strong, aid each other in destroying superconductivity, in contrast to their competition at weak disorder. Our results shed light on why zinc impurities are efficient in destroying superconductivity in cuprates, even though it is robust to weaker impurities.

In order to establish the doping dependence of the critical current properties in the iron-based superconductors, the in-plane critical current density $J_{\mathrm{c}}$ of $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$-based superconductors $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (K-Ba122), $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x{)}_2\mathrm{A}{\mathrm{s}}_2$ (Co-Ba122), and $\mathrm{BaF}{\mathrm{e}}_2{\left(\mathrm{A}{\mathrm{s}}_{1-x}{\mathrm{P}}_x\right)}_2$ (P-Ba122) in a wide range of doping concentration $x$ was investigated by means of magnetization hysteresis loop (MHL) measurements on single-crystal samples. Depending on the dopant elements and their concentration, $J_{\mathrm{c}}$ exhibits a variety of magnetic-field $H$ and temperature $T$ dependences. (1) In the case of K-Ba122, the MHL of the underdoped samples $(x\le 0.33)$ exhibits a second magnetization peak (SMP), which sustains high $J_{\mathrm{c}}$ at high $H$ and high $T$, exceeding ${10}^5\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ at $T=25\mathrm{K}$ and ${\mu }_{0}H=6\mathrm{T}$ for $x=0.30$. On the other hand, the SMP is missing in the optimally $(x\sim 0.36–0.40)$ and overdoped $(x\sim 0.50)$ samples and consequently $J_{\mathrm{c}}$ rapidly decreases by more than one order of magnitude, although the change in $T_{\mathrm{c}}$ is within a few K. (2) For Co-Ba122, the SMP is always present over the entire superconducting (SC) dome from the underdoped $(x\sim 0.05)$ to the overdoped $(x\sim 0.12)$ region. However, the magnitude of $J_{\mathrm{c}}$ significantly changes with $x$, exhibiting a sharp maximum at $x\sim 0.057$, which is a slightly underdoped composition for Co-Ba122. (3) For P-Ba122, the highest $J_{\mathrm{c}}$ is attained at $x=0.30$, corresponding to the highest $T_{\mathrm{c}}$ composition. For the overdoped samples, the MHL is characterized by a SMP located close to the irreversibility field $H_{\mathrm{irr}}$. Common to the three doping variations, $J_{\mathrm{c}}$ becomes highest at the underdoping side of the SC dome near the phase boundary between the SC phase and the antiferromagnetic-orthorhombic (AFO) phase. Also, the peak appears in a narrow range of doping, distinct from the $T_{\mathrm{c}}$ dome with a broad maximum. These similarities in the three cases indicate that the observed doping dependence of $J_{\mathrm{c}}$ is intrinsic to the $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$-based superconductors. The scaling analysis of the normalized pinning force density $f_{\mathrm{p}}$ as a function of the reduced magnetic field $h=H/H_{\mathrm{irr}}$ shows that the peak position in the pinning force $h_{\max }$ depends on $x$, indicating a change in pinning with $x$. On the other hand, $\mathrm{high}\text{−}{J}_{\mathrm{c}}$ samples always attain similar $h_{\max }$ values of 0.40–0.45 for all the dopants, which may suggest that a common pinning source causes the highest $J_{\mathrm{c}}$. A quantitative analysis of the $T$-dependent $J_{\mathrm{c}}$ indicates that the two pinning mechanisms, namely, the spatial variations in $T_{\mathrm{c}}$ (referred to as $\delta T_{\mathrm{c}}$ pinning) and the fluctuations in the mean free path $(\delta l$ pinning), are enhanced for the underdoped samples, which results in the enhancement of $J_{\mathrm{c}}$. Possible origins for the different pinning mechanism are discussed in connection with the $x$ dependence of $T_{\mathrm{c}}$, the residual resistivity, AFO domain boundaries, and a possible quantum critical point.