We present a theoretical description of Bogolyubov-type excitations of exciton-polariton Bose-Einstein condensates (BECs) in semiconductor microcavities. For a typical two-dimensional (2D) BEC we focus on two limiting cases, the weak- and strong-coupling regimes, where a perturbation theory and the Thomas-Fermi approximation, respectively, are valid. We calculate integrated scattering intensity spectra for probing the collective excitations of the condensate in both considered limits. Moreover, in relation to recent experiments on optical modulation allowing localization of condensates in a trap with well-controlled shape and dimensions, we study the quasi-one-dimensional (1D) motion of the BEC in microwires and report the corresponding Bogolyubov excitation spectrum. We show that in the 1D case the characteristic polariton-polariton interaction constant is expressed as $g_1=3\lambda \mathcal{N}/\left(2L_y\right)$ ($\lambda$ is the 2D polariton-polariton interaction parameter in the cavity, $\mathcal{N}$ the number of the particles, and $L_y$ the wire cavity width). We reveal some interesting features for 2D and 1D Bogolyubov spectra for both repulsive $(\lambda >0)$ and attractive $(\lambda <0)$ interactions.

We theoretically investigate the finite size effect in quantum anomalous Hall (QAH) system. Using Mn-doped HgTe quantum well as an example, we demonstrate that the coupling between the edge states is spin dependent and is related not only to the distance between the edges but also to the doping concentration. Thus with proper tuning of the two, we can get four kinds of transport regimes: quantum spin Hall, QAH, edge conducting, and normal insulator. These transport regimes have distinguishing edge conducting properties while the bulk is insulting. Our results give a general picture of the finite size effect in a QAH system, and are important for the transport experiments in QAH nanomaterials as well as future device applications.

Coherent thermal transport in nanopatterned structures is a topic of considerable interest, but whether it occurs in certain structures remains unclear due to a poor understanding of which phonons conduct heat. Here, we perform fully three-dimensional, frequency-dependent simulations of thermal transport in nanomeshes by solving the Boltzmann transport equation with an efficient Monte Carlo method. From the spectral information in our simulations, we show that thermal transport in nanostructures that can be created with available lithographic techniques is dominated by incoherent boundary scattering at room temperature. Our result provides important insights into the conditions required for coherent thermal transport to occur in artificial structures.

The quantum dynamics of electrons in bulk states is investigated by scanning tunneling microscopy and spectroscopy on a Ag(100) surface. By measuring conductance maps above a threshold voltage, we observe standing waves at step edges and defects. We interpret these to originate from electrons in a bulk band edge at the $\Gamma$ point. From the spatially decaying waves, the wave vector and the quantum coherence parameters—coherence length, lifetime, and linewidth—are determined as a function of energy. We measure a coherence length of about 5–7 Å, which is order of magnitudes lower than typically observed for surface or image-potential states. The energy of the band edge is extracted from the dispersion relation and agrees with the peak measured in scanning tunneling spectra at 1.9 eV above the Fermi energy. Theoretical calculations confirm the nature of the state elucidating the experimental findings.

High-temperature electronic materials are in constant demand as the required operational range for various industries increases. Here we design ($A$,$A^{\prime }$)$B_2{\mathrm{O}}_6$ perovskite oxides with [111] “rock salt” $A$-site cation order and predict them to be potential high-temperature piezoelectric materials. By selecting bulk perovskites which have a tendency towards only out-of-phase $B{\mathrm{O}}_6$ rotations, we avoid possible staggered ferroelectric to paraelectric phase transitions while also retaining noncentrosymmetric crystal structures necessary for ferro- and piezoelectricity. Using density functional theory calculations, we show that (La,Pr)${\mathrm{Al}}_2{\mathrm{O}}_6$ and (Ce,Pr)${\mathrm{Al}}_2{\mathrm{O}}_6$ display spontaneous polarizations in their polar ground state structures; we also compute the dielectric and piezoelectric constants for each phase. Additionally, we predict the critical phase transition temperatures for each material from first-principles to demonstrate that the piezoelectric responses, which are comparable to traditional lead-free piezoelectrics, should persist to high temperature. These features make the rock salt $A$-site-ordered aluminates candidates for high-temperature sensors, actuators, or other electronic devices.

We have studied the underlayer thickness and temperature dependencies of the current-induced effective field in $\text{CoFeB}/\text{MgO}$ heterostructures with Ta-based underlayers. The underlayer thickness at which the effective field saturates is found to be different between the two orthogonal components of the effective field; i.e., the dampinglike term tends to saturate at a smaller underlayer thickness than the fieldlike term. For large underlayer thickness films in which the effective field saturates, we find that the measurement temperature significantly influences the size of the effective field. A striking difference is found in the temperature dependence of the two components: the dampinglike term decreases whereas the fieldlike term increases with increasing temperature. Using a simple spin diffusion-spin transfer model, we find that all of these results can be accounted for provided the real and imaginary parts of an effective spin mixing conductance are negative. These results imply that either spin transport in this system is different from conventional metallic interfaces or effects other than spin diffusion into the magnetic layer need to be taken into account in order to model the system accurately.

We describe a detailed study of the structural, magnetic, and magnetotransport properties of single-crystal, $n$-type, Mn-doped bismuth telluride thin films grown by molecular beam epitaxy. With increasing Mn concentration, the crystal structure changes from the tetradymite structure of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ parent crystal at low Mn concentrations towards a BiTe phase in the (${\mathrm{Bi}}_2{\mathrm{Te}}_3{)}_m({\mathrm{Bi}}_2{)}_n$ homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temperature in the range 13.8–17 K in films with $\sim$2%$-$$\sim$10% Mn concentration. Magnetization hysteresis loops reveal that the magnetic easy axis is along the $c$ axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68-nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain-wall scattering. Ordinary Hall effect measurements show that the carrier density is $n$ type, increases with Mn doping, and is high enough ($\ge 2.8\times {10}^{13}$ cm${}^{-2}$) to place the chemical potential in the conduction band. Thus the observed ferromagnetism is likely associated with both bulk and surface states. Surprisingly, the Curie temperature does not show any clear dependence on the carrier density but does increase with Mn concentration. Our results suggest that the ferromagnetism probed in these Mn-doped bismuth telluride films is not mediated by carriers in the conduction band or in an impurity band.

The stationary displacement of a magnetic vortex core in a permalloy square caused by an ultrahigh direct current has been measured utilizing scanning electron microscopy with polarization analysis. Data have been analyzed for three different generic states of the Landau structure and up to a current density of $3\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2$. This procedure allows for separating the effects caused by the Oersted field, the nonadiabatic, and the adiabatic spin-transfer torque. In addition, the spin polarization of the driving current $P=(65\pm 4)\%$ is independently determined from the spin drift velocity of $v_j=(4.79\pm 0.26)$ m/s at $j=1\times {10}^{11}\mathrm{A}/\mathrm{m}$. Ferromagnetic resonance measurements have been conducted to complete the set of parameters for our film system ($\alpha ,g,M_{\mathrm{S}}$). The full set of parameters allows for a direct comparison of the nonadiabatic spin-transfer torque with previously published results. In contrast to published values in the range of the damping parameter, a strongly increased value of $\beta =0.119\pm 0.022$ as a parameter of nonadiabaticity is found, which supports recent theoretical predictions of an additional nonlocal spin-transfer torque contribution effective at the vortex structure.

We have measured the magnetic excitation spectrum of the model square-lattice spin-1/2 antiferromagnet ${\text{Sr}}_2{\text{CuO}}_2{\text{Cl}}_2$  over a broad range of energies and momenta using high-resolution inelastic neutron scattering (INS). The magnon dispersion along the zone boundary was accurately measured to be a 43 meV between $(1/2,0)$ and $(3/4,1/4)$ indicating the importance of coupling beyond nearest neighbors in the spin Hamiltonian. We observe a strong momentum dependent damping of the zone-boundary magnons at $(1/2,0)$ revealing a high-energy continuum of magnetic excitations. A direct comparison between our INS measurements and resonant inelastic x-ray scattering (RIXS) measurements shows that the RIXS spectrum contains significant contributions from higher energy excitations not previously considered. Our observations demonstrate that this high-energy continuum of magnetic fluctuations is an ubiquitous feature of insulating monolayer cuprates, apparent in both inelastic neutron and light scattering measurements.

The concept of polar nanoregions (PNRs) in relaxor ferroelectrics has recently been discussed in several numerical and theoretical studies for the special case of barium zirconate-titanate Ba(Zr${}_x$Ti${}_{1-x})$O${}_3$ (BZT). Here we present a semiphenomenological model of relaxation in BZT and related systems in order to show that the correlation radius of a PNR is determined by the condition that the surrounding medium undergoes a local phase transformation into a correlated polar state. The model describes the growth and percolation of the PNRs on lowering the temperature or under the application of an electric field, leading to a generalized Vogel-Fulcher-type dielectric relaxation time. It is suggested that the above condition might be applied to discriminate between the relaxor state and the analogous dipolar glass state.

We present muon spin relaxation ($\mu$SR) and specific-heat versus temperature $C\left(T\right)$ measurements on polycrystalline and single-crystal samples of the pyrochlore magnet ${\text{Yb}}_2{\text{Ti}}_2{\text{O}}_7$. $C\left(T\right)$ exhibits a sharp peak at a $T_C$ of 0.21 and 0.26 K for the single-crystal and polycrystalline samples, respectively. For both samples, the magnetic entropy released between 50 mK and 30 K amounts to $Rln2$ per Yb. At temperatures below $T_C$ we observe a steep drop in the asymmetry of the zero-field $\mu$SR time spectra at short time scales, as well as a decoupling of the muon spins from the internal field in longitudinal magnetic fields of $\le 0.25$ T for both the polycrystalline and single-crystal samples. These muon data are indicative of static magnetic moments. Our results are consistent with the onset of long-range magnetic order in both forms of ${\text{Yb}}_2{\text{Ti}}_2{\text{O}}_7$.

The dependence of the critical current $I_{\mathrm{C}}$ on magnetic field has been systematically studied in thin-film superconducting Nb, NbN, and TaN bridges with widths smaller than the Pearl length. We have shown that application of a magnetic field of a proper orientation to the bridges with bends results in a suppression of the current crowding effect. This leads to an increase up to about 30% of the critical current in such bridges with respect to $I_{\mathrm{C}}$ measured at zero magnetic fields. We have demonstrated a good qualitative and quantitative agreement between experimental results and theoretical predictions obtained in frame of the London and Ginzburg-Landau models for a dominating edge pinning mechanism of vortices. Also we observed oscillations of $I_{\mathrm{C}}$ at magnetic fields, which increase with a decrease of the width of superconducting stripe. These oscillations are qualitatively similar to the Fraunhofer-like dependence of a critical current on the magnetic field in wide Josephson junctions. In the frame of the Ginzburg-Landau model, a penetration and pining of vortex chain is considered as a mechanism responsible for this nonmonotonic behavior of the critical current.

We analytically and numerically compute three equilibrium Fermi-liquid coefficients of the fully screened $N$-channel Kondo model, namely $c_B$, $c_T$, and $c_{\varepsilon }$, characterizing the magnetic field and temperature dependence of the resisitivity, and the curvature of the equilibrium Kondo resonance, respectively. We present a compact, unified derivation of the $N$ dependence of these coefficients, combining elements from various previous treatments of this model. We numerically compute these coefficients using the numerical renormalization group, with non-Abelian symmetries implemented explicitly, finding agreement with Fermi-liquid predictions on the order of 5% or better.

The thermoelectric transport properties of elemental tellurium are investigated by density functional theory combined with the Boltzmann transport equation in the rigid band approximation. We find that the thermoelectric transport properties parallel and perpendicular to the helical chains are highly asymmetric (almost symmetric) for $p$- ($n$-) type doped tellurium due to the anisotropic (isotropic) hole (electron) pockets of the Fermi surface. The electronic band structure shows that the lone-pair derived uppermost heavy-hole and extremely light-hole lower valence bands offer the opportunity to obtain both a high Seebeck coefficient and electrical conductivity along the chains through Sb or Bi doping. Furthermore, the stairlike density of states yields a large asymmetry for the transport distribution function relative to the Fermi energy which leads to large thermopower. The calculations reveal that tellurium has the potential to be a good $p$-type thermoelectric material with an optimum figure of merit $zT$ of 0.31 (0.56) at room temperature (500 K) at a hole concentration around $1\times {10}^{19}$ cm${}^{-3}$. Exploiting the rich chemistry of lone pairs in chiral solids may have important implications for the discovery of high-$zT$ polychalcogenide-based thermoelectric materials.

We have studied the vibrational modes and Raman spectra of P-doped Si nanocrystals using pseudopotential density functional theory and the Placzek approximation. We find that Si nanocrystal vibrations are largely unaffected by the introduction of P dopants. However, the Raman spectra of doped nanocrystals are enhanced relative to those of pristine nanocrystals, and demonstrate a strong dependence on dopant position. Thus, Raman has the potential of being developed as a tool for probing the location of the dopant within the nanocrystal. Our analysis shows that vibrational modes involving atoms in the vicinity of the dopant give the largest contributions to the Raman spectra.

The thermal expansion of a hole, in a planar system, follows the same trend as the thermal expansion of the whole system, i.e., the hole expands (contracts) if the material expands (contracts) under thermal excitation. At nanoscale, this phenomenon has not been studied so far. Here, using tools of classical molecular dynamics simulations, we show that graphene nanomeshes (GNMs) behave oppositely: While the whole structure contracts (expands), the nanoholes expand (contract) under thermal excitation. We propose and test a simple mechanism to describe this unexpected behavior in terms of out-of-plane vibrations of the atoms close to and far from the edges of the holes. This mechanism allows us to see that, contrary to usual planar systems, this behavior comes from nonuniform thermal expansion along the structure. Although the thermal expansion of holes in GNMs is contrary to the classical prediction, we verify that the thermal expansion of the whole GNM structure is the same as that of pristine graphene.

Angle-resolved photoemission spectroscopy, low-energy electron diffraction, and density functional theory calculations were employed to investigate the electronic and structural properties of a graphene layer grown on the regularly stepped $\mathrm{Pt}\left(997\right)$ surface. The measurements show the predominance of a ($2\times 2$) multidomain superstructure of graphene, lying flat on the Pt terraces and covering the step edges in a carpetlike mode. The linearly dispersing $\pi$ band of graphene appears to be weakly affected by the substrate at the $K$ point, while an appreciable hybridization is found at the $\Gamma$ point. Replicas of the Dirac cone are seen perpendicularly to the steps, with double real-space periodicity compared to the Pt terrace width.