We report magnetization, heat capacity, ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR), and muon-spin rotation ($\mu \mathrm{SR}$) measurements on the honeycomb $4d^5$ spin liquid candidate ${\mathrm{Li}}_2{\mathrm{RhO}}_3$. The magnetization in small magnetic fields provides evidence of the partial spin-freezing of a small fraction of ${\mathrm{Rh}}^{4+}$ moments at 6 K, whereas the Curie-Weiss behavior above 100 K suggests a pseudo-spin-1/2 paramagnet with a moment of about $2.2{\mu }_B$. The magnetic specific heat ($C_{\mathrm{m}}$) exhibits no field dependence and demonstrates the absence of long-range magnetic order down to 0.35 K. $C_{\mathrm{m}}/T$ passes through a broad maximum at about 10 K and $C_{\mathrm{m}}\propto T^2$ at low temperatures. Measurements of the spin-lattice relaxation rate ($1/T_1$) reveal a gapless slowing-down of spin fluctuations upon cooling with $1/T_1\sim T^{2.2}$. The results from NMR and $\mu \mathrm{SR}$ are consistent with a scenario in which a minority of ${\mathrm{Rh}}^{4+}$ moments are in a short-range correlated frozen state and coexist with a majority of moments in a liquid-like state that continue to fluctuate at low temperatures.

Combining electronic Raman scattering experiments with cellular dynamical mean field theory, we present evidence of the pseudogap in the superconducting state of various hole-doped cuprates. In ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ we track the superconducting pseudogap hallmark, a peak-dip feature, as a function of temperature $T$ and doping $p$, well beyond the optimal one. We show that, at all temperatures under the superconducting dome, the pseudogap disappears at the doping $p_c$, between 0.222 and 0.226, where also the normal-state pseudogap collapses at a Lifshitz transition. This demonstrates that the superconducting pseudogap boundary forms a vertical line in the $T-p$ phase diagram.

Time-space modulated media are realized by periodically modulating parameters of the media in both time and space. In this paper, we study frequency conversion induced by time-space modulated media. Two types of frequency conversion are theoretically and numerically demonstrated; their underlying mechanisms are explained by considering the unusual properties of Bloch modes. We find that the first type of conversion is induced by energy transmission between different orders of Bloch modes, which can be observed when waves inside time-space modulated media are reflected by boundaries. The second type is caused by the Bragg scattering effect occurring inside time-space modulated media; it can be observed when external waves are reflected by time-space modulated media. The frequency can be up or down converted, and the difference caused by the conversion depends on the modulation frequency, which is tunable. Therefore, these frequency conversion phenomena could be exploited to manipulate waves' frequencies for particular purposes.

Solid interfaces with exceptionally low or high thermal conductance are of intense scientific and practical interest. However, realizing such interfaces is challenging due to a lack of knowledge of the phonon transmission coefficients between specific modes on each side of the interface and how the coefficients are affected by atomic scale structure. Here, we report an ab initio based study of phonon transmission at Si/Ge interfaces using a recent extension of the atomistic Green's function method that resolves transmission coefficients by mode. These results provide a detailed framework to investigate the precise transmission and reflection processes that lead to thermal resistance and how they depend on phonon frequency as well as incident angle. We find that the transmission and reflection processes can be partly explained with familiar concepts such as conservation of transverse momentum, but we also find that numerous phonons have zero transmission coefficient despite the existence of modes that satisfy transverse momentum conservation. This work provides detailed insights into precisely which phonons transmit or reflect at interfaces, knowledge necessary to design solid interfaces with extreme values of thermal conductance for thermoelectricity and heat management applications.

A magnetostructural phase transition is investigated in single-crystal chromium nitride (CrN) thin films grown by rf plasma molecular beam epitaxy on MgO(001) substrates. While still within the vacuum environment following molecular beam epitaxy growth, in situ low-temperature scanning tunneling microscopy, and in situ variable low-temperature reflection high-energy electron diffraction are applied, revealing an atomically smooth and metallic CrN(001) surface, and an in-plane structural transition from $1\times 1$ (primitive CrN unit cell) to $\sqrt{2}\times \sqrt{2}-R{45}^{\circ }$ with a transition temperature of ($278\pm 3$) K, respectively. Ex situ temperature-dependent measurements using neutron diffraction are also performed, looking at the structural peaks and likewise revealing a first-order structural transition along the [111] out-of-plane direction, with transition temperatures of (268 $\pm$ 3) K. Turning to the magnetic peaks, neutron diffraction confirms a clear magnetic transition from paramagnetic at room temperature to antiferromagnetic at low temperatures with a sharp, first-order phase transition and a Néel temperature of ($270\pm 2$) K or ($280\pm 2$) K for two different films. In addition to the experimental measurements of structural and magnetic ordering, we also discuss results from first-principles theoretical calculations which explore various possible magnetostructural models.

The spin-Hall effect (SHE) is found to be strong in heavy transition metals, such as Ta and W, in their amorphous and/or high resistivity form. In this paper, we show that by employing a Cu-Ta binary alloy as a buffer layer in an amorphous $\mathrm{C}{\mathrm{u}}_{100-x}\mathrm{T}{\mathrm{a}}_x$-based magnetic heterostructure with perpendicular magnetic anisotropy, the SHE-induced dampinglike spin-orbit torque (DL-SOT) efficiency $|{\xi }_{\mathrm{DL}}|$ can be tuned linearly by adjusting the buffer layer resistivity. Current-induced SOT switching can also be achieved in these $\mathrm{C}{\mathrm{u}}_{100-x}\mathrm{T}{\mathrm{a}}_x$-based magnetic heterostructures, and we find the switching behavior better explained by a SOT-assisted domain-wall propagation picture. Through systematic studies on $\mathrm{C}{\mathrm{u}}_{100-x}\mathrm{T}{\mathrm{a}}_x$-based samples with various compositions, we determine the lower bound of spin-Hall conductivity $|{\sigma }_{\mathrm{SH}}|\approx 2.02\times {10}^4\left[\hslash /2e\right]{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$ in the Ta-rich regime. Based on the idea of resistivity tuning, we further demonstrate that $|{\xi }_{\mathrm{DL}}|$ can be enhanced from 0.087 for pure Ta to 0.152 by employing a resistive TaN buffer layer.

Slightly Co-doped ${\mathrm{MnWO}}_4$ at the lowest 5% Co concentration, for which the multiferroic cycloidal phase becomes a ground state, has been studied in magnetic fields up to 60 T by bulk magnetic and electric polarization measurements along different crystallographic directions. The field induced magnetic transitions up to 12 T and the ways they proceed were tracked also by single-crystal neutron diffraction, and the relevant field induced magnetic structures were identified and refined. The complete magnetoelectric phase diagrams for magnetic fields along distinct directions in relation to the cycloidal spin structure have been constructed for magnetic field values exceeding those necessary to induce a spin-flip transition into the paramagnetic state. Their common feature is the existence of nonpolar sinusoidal phases identified by the disappearance of the electric polarization in a field regime slightly below the spin-flip transition. At lower magnetic fields either continuous or abrupt field induced reorientations of the cycloidal magnetic structures were observed, respectively, for a field direction along the crystallographic $b$ axis or along the easy magnetic axis, and the different character of those transitions has been attributed to specific features in the magnetic anisotropy.

We report ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, with $T_{\mathrm{c}}=35$ K. ${}^{75}\mathrm{As}$ NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies ${\nu }_{\mathrm{Q}}$ of the As(1) and As(2) sites show an opposite temperature $T$ dependence. Nearly $T$ independent behavior of the Knight shifts $K$ is observed in the normal state, and a sudden decrease in $K$ in the superconducting (SC) state suggests spin-singlet Cooper pairs. ${}^{75}\mathrm{As}$ spin-lattice relaxation rates $1/T_1$ show a power-law $T$ dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector $\mathbf{q}=(\pi ,0)$ or $(0,\pi )$ in the single-iron Brillouin zone notation are revealed by $1/T_{1}T$ and $K$ measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the ${}^{75}\mathrm{As}$ quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, $1/T_1$ shows a rapid decrease below $T_{\mathrm{c}}$ without a Hebel-Slichter peak and decreases exponentially at low $T$, consistent with an $s^{\pm }$ nodeless two-gap superconductor.

We investigate how dynamic correlations of hard-core bosonic excitation at finite temperature are affected by additional interactions besides the hard-core repulsion which prevents them from occupying the same site. We focus especially on dimerized spin systems, where these additional interactions between the elementary excitations, triplons, lead to the formation of bound states, relevant for the correct description of scattering processes. In order to include these effects quantitatively, we extend the previously developed Brückner approach to include also nearest-neighbor (NN) and next-nearest neighbor (NNN) interactions correctly in a low-temperature expansion. This leads to the extension of the scalar Bethe-Salpeter equation to a matrix-valued equation. As an example, we consider the Heisenberg spin ladder to illustrate the significance of the additional interactions on the spectral functions at finite temperature, which are proportional to inelastic neutron scattering rates.

We study the effect of band anisotropy with discrete rotational symmetry $C_N$ (where $N\ge 2)$ in the quantum Hall regime of two-dimensional electron systems. We focus on the composite Fermi liquid (CFL) at half-filling of the lowest Landau level. We find that the magnitude of anisotropy transferred to the composite fermions decreases very rapidly with $N$. We demonstrate this by performing density matrix normalization group calculations on the CFL, and comparing the anisotropy of the composite fermion Fermi contour with that of the (noninteracting) electron Fermi contour at zero magnetic field. We also show that the effective interaction between the electrons after projecting into a single Landau level is much less anisotropic than the band, a fact which does not depend on filling and thus has implications for other quantum Hall states as well. Our results confirm experimental observations on anisotropic bands with warped Fermi contours, where the only detectable effect on the composite Fermi contour is an elliptical distortion $\left(N=2\right)$.

We report the occurrence of reentrant metallic behavior in the Weyl semimetal NbP. When the applied magnetic field $H$ is above a critical value $H_{\mathrm{c}}$, a reentrance appears as a peak in the temperature-dependent resistivity ${\rho }_{xx}\left(T\right)$ at $T=T_{\mathrm{p}}$, similar to that observed in graphite where it was attributed to local superconductivity. The $T_{\mathrm{p}}\left(H\right)$ relationship follows a power-law dependence $T_p\sim {(H-H_c)}^{1/\nu }$ where ν can be derived from the temperature dependence of the zero-field resistivity ${\rho }_0\left(T\right)\sim T^{\nu }$. From concurrent measurements of the transverse ${\rho }_{xx}\left(T\right)$ and Hall ${\rho }_{xy}\left(T\right)$ magnetoresistivities, we reveal a clear correlation between the rapidly increasing ${\rho }_{xy}\left(T\right)$ and the occurrence of a peak in the ${\rho }_{xx}\left(T\right)$ curve. Quantitative analysis indicates that the reentrant metallic behavior arises from the competition of the magnetoconductivity ${\sigma }_{xx}\left(T\right)$ with an additional component $\mathrm{\Delta }{\sigma }_{xx}\left(T\right)={\kappa }_H{\sigma }_{xx}\left(T\right)$ where ${\kappa }_H={[{\rho }_{xy}\left(T\right)/{\rho }_{xx}\left(T\right)]}^2$ is the Hall factor.  We find that the Hall factor (${\kappa }_H\approx 0.4$) at peak temperature $T_{\mathrm{p}}$ is nearly field independent, leading to the observed $T_p\left(H\right)$ relationship. Furthermore, the reentrant metallic behavior in ${\rho }_{xx}\left(T\right)$ also is reflected in the behavior of ${\rho }_{xx}\left(H\right)$ that ranges from nonsaturating at $T>70\mathrm{K}$ to saturation at liquid-helium temperatures. The latter can be explained with the magnetic field dependence of the Hall factor ${\kappa }_H\left(H\right)$. Our paper demonstrates that a semiclassical theory can account for the “anomalies” in the magnetotransport phenomena of NbP without invoking an exotic mechanism.

Using two approaches to strongly correlated systems, the extremely correlated Fermi liquid theory and the dynamical mean field theory, we compute the transverse transport coefficients, namely, the Hall constants $R_H$ and Hall angles ${\theta }_H$, and the longitudinal and transverse optical response of the $U=\infty$ Hubbard model in the limit of infinite dimensions. We focus on two successive low-temperature regimes, the Gutzwiller-correlated Fermi liquid (GCFL) and the Gutzwiller-correlated strange metal (GCSM). We find that the Hall angle $cot{\theta }_H$ is proportional to $T^2$ in the GCFL regime, while upon warming into the GCSM regime it first passes through a downward bend and then continues as $T^2$. Equivalently, $R_H$ is weakly temperature dependent in the GCFL regime, but becomes strongly temperature dependent in the GCSM regime. Drude peaks are found for both the longitudinal optical conductivity ${\sigma }_{xx}\left(\omega \right)$ and the optical Hall angles $tan{\theta }_H\left(\omega \right)$ below certain characteristic energy scales. By comparing the relaxation rates extracted from fitting to the Drude formula, we find that in the GCFL regime there is a single relaxation rate controlling both longitudinal and transverse transport, while in the GCSM regime two different relaxation rates emerge. We trace the origin of this behavior to the dynamical particle-hole asymmetry of the Dyson self-energy, arguably a generic feature of doped Mott insulators.

The intrinsic ferromagnetic semiconductor GdN offers a unique opportunity to separate the anomalous and ordinary contributions to the Hall effect, and to investigate the strength of the anomalous Hall effect (AHE) as a function of carrier concentration and relaxation time. The data show an AHE that is inversely proportional to the carrier concentration $n$ in a single spin channel. There is no dependence at all on the relaxation time $\tau$, rather than the usual ${\tau }^1$ or ${\tau }^2$ dependencies predicted by conventional mechanisms. However, the $n$ and $\tau$ dependencies are identical to those of the ordinary Hall effect, which suggests a semiclassical wave-packet description of an intrinsic AHE contribution that ultimately provides a quantitative agreement with the data.

We consider a condensate of exciton polaritons in a diluted magnetic semiconductor microcavity. Such a system may exhibit magnetic self-trapping in the case of sufficiently strong coupling between polaritons and magnetic ions embedded in the semiconductor. We investigate the effect of the nonequilibrium nature of exciton polaritons on the physics of the resulting self-trapped magnetic polarons. We find that multiple polarons can exist at the same time, and we derive a critical condition for self-trapping that is different from the one predicted previously in the equilibrium case. Using the Bogoliubov–de Gennes approximation, we calculate the excitation spectrum and provide a physical explanation in terms of the effective magnetic attraction between polaritons, mediated by the ion subsystem.

Density functional theory calculations for the electronic and phononic band structures of Pb/Si(111) thin films with a thickness of 4 and 5 monolayers (ML) are performed. We employ a Si(111)$(\sqrt{3}\times \sqrt{3})$ unit cell to model the Pb films including the Si substrate, and identify quantum well (QW) states for film thicknesses between 3 and 6 ML. The calculations show that the quantum-confined state closest to the Si band gap acquires the character of a quantum well resonance with a major part of its wave function extending into the Si(111) substrate. This finding explains the unusually low dispersion of this state and its lacking sensitivity to phonons in the 5 ML Pb film. Moreover, several unoccupied QW states are identified in the calculations and are assigned to previously observed features in structurally simpler freestanding Pb films. The calculated phonon band structures of the Pb/Si(111)$(\sqrt{3}\times \sqrt{3})$ films display stiff surface phonon modes in the 2.3–2.5 THz range. The electron-phonon coupling strength in the quantum-confined states is addressed by means of deformation-potential theory using the calculated atomic displacements of $\mathrm{\Gamma }$-point phonons. It is found that both the acoustic shear deformation potential as well as the optical deformation potentials of unoccupied QW states are sizable. Comparing the results for 4 and 5 ML Pb films, we conclude that the optical deformation potentials are generally larger for the 4 ML film. The occupied QW resonance in the 5 ML Pb film shows weak electron-phonon coupling, in qualitative agreement with the small experimentally observed lifetime broadening of this state. Our results form the basis for addressing the role of electron-phonon scattering for the lifetime of unoccupied QWs acting as intermediate states in two-photon photoemission from Pb/Si(111) films.

By means of a multiband effective mass Hamiltonian, a theoretical characterization of the effect of the geometrical features of the confinement profile—in particular, a longitudinal groove—on the subband dispersion and spin-related properties of a rectangular HgTe/CdTe nanowire is presented. Through an external electric field applied perpendicular to the wire, the interplay of the induced Rashba spin splitting and these geometrical features is investigated. It is found that by exploiting this interplay, a rich complexity of the subband structure arises, permitting the generation and modulation of spin-polarized currents without magnetic fields.