We have measured the electronic heat dissipation of hot electrons in highly disordered millimeter scale graphene at temperatures $T=0.3\text{–}3$ K. Disorder was introduced by hydrogenation of graphene, bringing low-temperature electron conduction below the Ioffe-Regel criterion for metallic conduction. Resistive thermometry was employed to determine the dependence of electron temperature on applied electrical power. The relation between heat flow and electron temperature was found to be well described by a power law with an exponent $\beta \sim 3.7\text{–}3.9$ and a coupling coefficient $\mathrm{\Sigma }\sim 1\mathrm{mW}/{\mathrm{m}}^2{\mathrm{K}}^{\beta }$. Our observations are similar to electronic heat dissipation of a two-dimensional electron gas in the hydrodynamic limit of electron-phonon coupling, corresponding to acoustic phonon emission into the substrate.

We demonstrate that the electromagnetic fields scattered by metallic particles hosting localized surface plasmons can be accurately reproduced by dielectric particles. We derive analytic formulas relating the permittivities of the dielectric and metallic particles that yield identical dipolar electromagnetic responses. This equivalence between dielectric and metallic particles permits the use of well-known pointlike dipolar models to predict the dipolar resonances of dielectric particles.

We present a density functional perturbation theory approach to estimate the transition temperature of the charge density wave transition of $\mathrm{TiS}{\mathrm{e}}_2$. The softening of the phonon mode at the $L$ point where in $\mathrm{TiS}{\mathrm{e}}_2$ a giant Kohn anomaly occurs, and the energy difference between the normal and distorted phase are analyzed. Both features are studied as functions of the electronic temperature, which corresponds to the Fermi-Dirac distribution smearing value in the calculation. The transition temperature is found to be 500 and 600 K by phonon and energy analysis, respectively, in reasonable agreement with the experimental value of 200 K.

The one-dimensional Holstein model and its generalizations have been studied extensively to understand the effects of electron-phonon interaction. The half-filled case is of particular interest, as it describes a transition from a metallic phase with a spin gap due to attractive backscattering to a Peierls insulator with charge-density-wave order. Our quantum Monte Carlo results support the existence of a metallic phase with dominant power-law charge correlations, as described by the Luther-Emery fixed point. We demonstrate that for Holstein and also for purely fermionic models the spin gap significantly complicates finite-size numerical studies, and explains inconsistent previous results for Luttinger parameters and phase boundaries. On the other hand, no such complications arise in spinless models. The correct low-energy theory of the spinful Holstein model is argued to be that of singlet bipolarons with a repulsive, mutual interaction. This picture naturally explains the existence of a metallic phase, but also implies that gapless Luttinger liquid theory is not applicable.

Temperature-dependent reflectivity measurements in the frequency range $85–7000{\text{cm}}^{-1}$ were performed on ${\text{BaFe}}_2{\text{As}}_2$ single crystals under pressure up to $\sim 5$ GPa. The corresponding pressure- and temperature-dependent optical conductivity was analyzed with the Drude-Lorentz model to extract the coherent and incoherent contributions. The gradual suppression of the spin-density-wave (SDW) state with increasing pressure and the appearance of the superconducting phase coexisting with the SDW phase at 3.6 GPa were observed. At 3.6 GPa, the reflectivity reaches unity below $\sim 95$ ${\text{cm}}^{-1}$ indicating the opening of the superconducting gap and shows a full gap tendency at 6 K.

We examine the real space renormalization group method of finding the excited eigenstates (RSRG-X) of the $XX$ spin-1/2 chain from entanglement perspectives. The eigenmodes of the entanglement Hamiltonian, especially the maximally entangled mode and the corresponding entanglement energies are studied and compared with the predictions of RSRG-X. Our numerical results demonstrate the accuracy of the RSRG-X method in the strong-disorder limit, and quantify its error when applied to the weak-disorder regime. Overall, our results validate the RSRG-X method qualitatively, but also show that its accuracy decreases with increasing temperature.

We report on the formation of the high-energy empty-state resonance in the electronic spectrum of the iron adatom on the Pt(111) surface. By using the combination of the first-principles methods and the finite-temperature exact diagonalization approach, we show that the resonance is the result of the valence fluctuations between atomic configurations of the impurity. Our theoretical finding is fully confirmed by the results of the scanning tunneling microscopy measurements [M. F. Crommie et al., Phys. Rev. B 48, 2851 (1993)]. In contrast to the previous theoretical results obtained by using local spin density approximation, the paramagnetic state of the impurity in the experiment is naturally reproduced within our approach. This opens a way for interpretation of STM data collected earlier for metallic surface nanosystems with iron impurities.

We have performed temperature-dependent x-ray magnetic circular dichroism on ferromagnetic $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_3$ at the $\mathrm{Co}{L}_{2,3}$ edges to analyze the correlation of the spin and orbital angular components of the magnetization with the singular magnetostructural anomalies at ∼120 K. We found a sizable orbital angular momentum in Co atoms that follows a temperature dependence very similar to that of the spin momentum. Their sum agrees with the bulk magnetization. Taken together with recent x-ray absorption spectra showing the absence of changes in the electronic filling of Co $3d$ states across the magnetostructural transition, our results demonstrate the relevance of the spin-orbit coupling for the stabilization of two distinct ferromagnetic phases in this metallic perovskite. The Co spin-state model is discussed on the basis of charge-transfer multiplet calculations of the dichroic spectral shape.

We propose a method to read out the spin state of an electron in a quantum dot in a Voigt geometry magnetic field using cycling transitions induced by the ac Stark effect. We show that cycling transitions can be made possible by a red-detuned, circularly polarized cw laser, which modifies the spin eigenstates and polarization selection rules via the ac Stark effect. A Floquet-Liouville supermatrix approach is used to calculate the time evolution of the density matrix under the experimental conditions of a spin readout operation. With an overall detection efficiency of 2.5%, the readout is a single-shot measurement with a fidelity of 76.2%.

We report results important for the creation of a best-of-both-worlds quantum hybrid system consisting of a solid-state source of single photons and an atomic ensemble as quantum memory. We generate single photons from a GaAs quantum dot (QD) frequency matched to the Rb $D2$ transitions and then use the Rb transitions to analyze spectrally the quantum dot photons. We demonstrate lifetime-limited QD linewidths (1.42 GHz) with both resonant and nonresonant excitation. The QD resonance fluorescence in the low power regime is dominated by Rayleigh scattering, a route to match quantum dot and Rb atom linewidths and to shape the temporal wave packet of the QD photons. Noise in the solid-state environment is relatively benign: there is a blinking of the resonance fluorescence at MHz rates but negligible dephasing of the QD excitonic transition. We therefore demonstrate significant progress towards the realization of an ideal solid-state source of single photons at a key wavelength for quantum technologies.

We study the chiral separation and chiral magnetic effects in a slab of Dirac semimetal of finite thickness, placed in a constant magnetic field perpendicular to its surfaces. We utilize the Bogolyubov boundary conditions with a large Dirac mass (band gap) outside the slab. We find that, in a finite-thickness slab, the axial current density is induced by helicity-correlated standing waves and, as a consequence, is quantized. The quantization is seen in its stepped-shape dependence on the fermion chemical potential and a sawtooth-shape dependence on the thickness of the slab. In contrast to a naive expectation, there is no chiral charge accumulation anywhere in the bulk or at the boundaries of the semimetal. In the same slab geometry, we also find that a nonzero chiral chemical potential induces no electric current, as might have been expected from the chiral magnetic effect. We argue that this outcome is natural and points to the truly nonstatic nature of the latter. By taking into account a nonzero electric field of a double layer near the boundaries of the slab, we find that the low-energy modes under consideration satisfy the continuity equation for axial current density without the anomalous term.

Optically induced nuclear spin polarization in a fluorine-doped ZnSe epilayer is studied by time-resolved Kerr rotation using resonant excitation of donor-bound excitons. Excitation with helicity-modulated laser pulses results in a transverse nuclear spin polarization, which is detected as a change of the Larmor precession frequency of the donor-bound electron spins. The frequency shift in dependence on the transverse magnetic field exhibits a pronounced dispersion-like shape with resonances at the fields of nuclear magnetic resonance of the constituent zinc and selenium isotopes. It is studied as a function of external parameters, particularly of constant and radio frequency external magnetic fields. The width of the resonance and its shape indicate a strong spatial inhomogeneity of the nuclear spin polarization in the vicinity of a fluorine donor. A mechanism of optically induced nuclear spin polarization is suggested based on the concept of resonant nuclear spin cooling driven by the inhomogeneous Knight field of the donor-bound electron.

Large-area single-layer ${\text{WS}}_2$ is grown epitaxially on Au(111) using evaporation of W atoms in a low pressure ${\text{H}}_2\text{S}$ atmosphere. It is characterized by means of scanning tunneling microscopy, low-energy electron diffraction, and core level spectroscopy. Its electronic band structure is determined by angle-resolved photoemission spectroscopy. The valence-band maximum at $\overline{K}$ is found to be significantly higher than at $\overline{\mathrm{\Gamma }}$. The observed dispersion around $\overline{K}$ is in good agreement with density functional theory calculations for a free-standing monolayer, whereas the bands at $\overline{\mathrm{\Gamma }}$ are found to be hybridized with states originating from the Au substrate. Strong spin-orbit coupling leads to a large spin-splitting of the bands in the neighborhood of the $\overline{K}$ points, with a maximum splitting of 419(11) meV. The valence-band dispersion around $\overline{K}$ is found to be highly anisotropic with spin-branch dependent effective hole masses of $0.40\left(02\right)m_e$ and $0.57\left(09\right)m_e$ for the upper and lower split valence band, respectively. The large size of the spin splitting and the low effective mass of the valence-band maximum make single-layer ${\text{WS}}_2$ a promising alternative to the widely studied ${\text{MoS}}_2$ for applications in electronics, spintronics, and valleytronics.

We consider the properties of charged domain walls in ferroelectrics as a quantum problem. This includes determination of self-consistent attracting 1D potential for compensating charge carriers, the number and positions of discrete energy levels in this potential, dependencies on the ferroelectric characteristics, as well as the spatial structure and formation energy of the wall. Our description is based on the Hartree and Thomas-Fermi methods and Landau theory for the ferroelectric transitions. Changeover from a few to many quantum levels (with the electron binding energies $\sim 1$ eV) is controlled by a single characteristic parameter. The quantum models well describe the core of the wall, whose width is typically $\sim 10$ nm. Additionally, the walls possess pronounced long-range tails which are due to trap recharging. For the trap concentration $N_t=({10}^{17}–{10}^{18}){\mathrm{cm}}^{-3}$, the tail length $\ell$ is of the $\mu \mathrm{m}$ scale. On the distances much larger than $\ell$ the walls are electrically uncoupled from each other and the crystal faces.

We construct a family of many-body wave functions to study the many-body localization phase transition. The wave functions have a Rokhsar-Kivelson form, in which the weight for the configurations are chosen from the Gibbs weights of a classical spin glass model, known as the random energy model, multiplied by a random sign structure to represent a highly excited state. These wave functions show a phase transition into an MBL phase. In addition, we see three regimes of entanglement scaling with the subsystem size: scaling with the entanglement corresponding to an infinite temperature thermal phase, constant scaling, and a subextensive scaling between these limits. Near the phase transition point, the fluctuations of the Rényi entropies are non-Gaussian. We find that Rényi entropies with different Rényi index transition into the MBL phase at different points and have different scaling behavior, suggesting a multifractal behavior.

We investigate the low-temperature magnetic phases in ${\mathrm{TmB}}_4$, a metallic quantum magnet on the geometrically frustrated Shastry-Sutherland lattice, using coordinated experimental and theoretical studies. Our results provide an explanation for the appearance of the intriguing fractional plateau in ${\mathrm{TmB}}_4$ and accompanying magnetic hysteresis. Together with observation of the bump in the half plateau, our results support the picture that the magnetization plateau structure in ${\mathrm{TmB}}_4$ is strongly influenced by the zero-field modulated phases. We present a phenomenological model to explain the appearance of the modulated phases and a microscopic Hamiltonian that captures the complete magnetic behavior of ${\mathrm{TmB}}_4$.

The complex nature of magnetic ordering in the spinel $\mathrm{C}{\mathrm{o}}_2\mathrm{Ti}{\mathrm{O}}_4$ is investigated by analyzing the temperature and magnetic field dependence of its magnetization ($M$), specific heat ($C_{\mathrm{p}}$), and ac magnetic susceptibilities ${\chi }^{\prime }$ and ${\chi }^{″}$. X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be $\mathrm{C}{\mathrm{o}}_2\mathrm{Ti}{\mathrm{O}}_4=\left[\mathrm{C}{\mathrm{o}}^{2+}\right]\left[\mathrm{C}{\mathrm{o}}^{3+}\mathrm{T}{\mathrm{i}}^{3+}\right]{\mathrm{O}}_4$. From analysis of the temperature dependence of the dc paramagnetic susceptibility, the magnetic moments $\mu \left(A\right)=3.87{\mu }_{\mathrm{B}}$ and $\mu \left(B\right)=5.19{\mu }_{\mathrm{B}}$ on the $A$ and $B$ sites are determined with $\mu \left(B\right)$ in turn yielding $\mu \left(\mathrm{T}{\mathrm{i}}^{3+}\right)=1.73{\mu }_{\mathrm{B}}$ and $\mu \left(\mathrm{C}{\mathrm{o}}^{3+}\right)=4.89{\mu }_{\mathrm{B}}$. Analysis of the dc and ac susceptibilities combined with the weak anomalies observed in the $C_{\mathrm{p}}$ vs $T$ data shows the existence of a quasi-long-range ferrimagnetic state below $T_{\mathrm{N}}\sim 47.8\mathrm{K}$ and a compensation temperature $T_{\mathrm{comp}}\sim 32\mathrm{K}$, the latter characterized by sign reversal of magnetization with its magnitude depending on the applied magnetic field and the cooling protocol. Analysis of the temperature dependence of $M$ (field cooled) and $M$ (zero field cooled) data and the hysteresis loop parameters is interpreted in terms of large spin clusters. These results in $\mathrm{C}{\mathrm{o}}_2\mathrm{Ti}{\mathrm{O}}_4$, significantly different from those reported recently in isostructural $\mathrm{C}{\mathrm{o}}_2\mathrm{Sn}{\mathrm{O}}_4=\left[\mathrm{C}{\mathrm{o}}^{2+}\right]\left[\mathrm{C}{\mathrm{o}}^{2+}\mathrm{S}{\mathrm{n}}^{4+}\right]{\mathrm{O}}_4$, warrant further investigations of its magnetic structure using neutron diffraction.

We study multilayer triangular-lattice Ising antiferromagnets with interlayer interactions that are weak and frustrated in an abc stacking. By analyzing a coupled height model description of these systems, we show that they exhibit a classical spin liquid regime at low temperature, in which both intralayer and interlayer correlations are strong but there is no long-range order. Diffuse scattering in this regime is concentrated on a helix in reciprocal space, as observed for charge ordering in the materials ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ and ${\mathrm{YbFe}}_2{\mathrm{O}}_4$.

Recent radio frequency scanning tunneling spectroscopy (rf-STS) experiments have demonstrated nuclear and electron spin excitations up to $\pm 12\hslash$ in a single molecular spin quantum dot (qudot). Despite the profound experimental evidence, the observed independence of the well-established dipole selection rules is not described by existing theory of magnetic resonance—pointing to a new excitation mechanism. Here we solve the puzzle of the underlying mechanism by discussing the relevant mechanistic steps. At the heart of the mechanism, periodic transient charging and electric polarization due to the rf-modulated tunneling process cause a periodic asymmetric deformation of the adsorbed qudot, enabling efficient spin transitions via spin-phonon-like coupling. The mechanism has general relevance for a broad variety of different spin qudots exhibiting internal mechanical degrees of freedom (organic molecules, doped semiconductor qudots, nanocrystals, etc.).

There is great interest in lead-free $\left(\mathrm{B}{\mathrm{a}}_{0.85}\mathrm{C}{\mathrm{a}}_{0.15}\right)\left(\mathrm{T}{\mathrm{i}}_{0.90}\mathrm{Z}{\mathrm{r}}_{0.10}\right){\mathrm{O}}_3$ (15/10BCTZ) because of its exceptionally large piezoelectric response [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this paper, we have analyzed the nature of: (i) crystallographic phase coexistence at room temperature, (ii) temperature- and field-induced phase transformation to throw light on the atomistic mechanisms associated with the large piezoelectric response of this system. A detailed temperature-dependent dielectric and lattice thermal expansion study proved that the system exhibits a weak dielectric relaxation, characteristic of a relaxor ferroelectric material on the verge of exhibiting a normal ferroelectric-paraelectric transformation. Careful structural analysis revealed that a ferroelectric state at room temperature is composed of three phase coexistences, tetragonal $\left(P4mm\right)+\text{orthorhombic}\left(\mathit{Amm}2\right)+\text{rhombohedral}\left(R3m\right)$. We also demonstrate that the giant piezoresponse is associated with a significant fraction of the tetragonal phase transforming to rhombohedral. It is argued that the polar nanoregions associated with relaxor ferroelectricity amplify the piezoresponse by providing an additional degree of intrinsic structural inhomogeneity to the system.