Canonical and microcanonical Monte Carlo simulations are carried out to study the electrocaloric effect (ECE) in ferroelectrics and relaxor ferroelectrics (RFEs) by direct computation of field-induced temperature variations at the ferroelectric-to-paraelectric phase transition and the nonergodic-to-ergodic state transformation. A lattice-based Hamiltonian is introduced, which includes a thermal energy, a Landau-type term, a dipole-dipole interaction energy, a gradient term representing the domain-wall energy, and an electrostatic energy contribution describing the coupling to external and random fields. The model is first parametrized and studied for the case of ${\mathrm{BaTiO}}_3$. Then, the ECE in RFEs is investigated, with particular focus on the influence of random fields and domain-wall energies. If the strength or the density of the random fields increases, the ECE peak shifts to a lower temperature but the temperature variation is reduced. On the contrary, if the domain-wall energy increases, the peak shifts to a higher temperature and the ECE becomes stronger. In RFEs, the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results imply that the presence of random fields reduces the entropy variation in an ECE cycle by pinning local polarization.

We study theoretically the effects of strong pinning centers on a charge density wave in the limit that the charge density wave coherence length is shorter than the average interimpurity distance. An analysis based on a Ginzburg-Landau model shows that long-range forces arising from the elastic response of the charge density wave induce a kind of collective pinning which suppresses impurity-induced phase fluctuations, leading to a long-range ordered ground state. The correlations induced by impurities are characterized by a length scale parametrically longer than the average interimpurity distance. Long-wavelength fluctuations are found to be gapped, implying the stability of the ground state. We also present Monte Carlo simulations that confirm the basic features of the analytical results.

The multiple scattering-based theory of Gaspari and Gyorffy for the electron-ion matrix element in close packed metals is applied to $Im\overline{3}m{\mathrm{H}}_3\mathrm{S}$, which has been predicted by Duan et al. and Bernstein et al. to be the stable phase at this stoichiometry around 190 GPa, thus is the leading candidate to be the phase observed to superconduct at 190 K by Drozdov, Eremets, and Troyan. The nearly perfect separation of vibrational modes into those of S and of H character provides a simplification that enables identification of contributions of the two atoms separately. The picture that arises is basically that of superconducting atomic H stabilized by strong covalent mixing with S $p$ and $d$ character. The reported isotope shift is much larger than the theoretical one, suggesting there is large anharmonicity in the H vibrations. Given the relative unimportance of sulfur, hydrides of lighter atoms at similarly high pressures may also lead to high temperature superconductivity.

We consider the dynamics of a quantum phase-slip junction (QPSJ), a dual Josephson junction, connected to a microwave source with frequency ${\omega }_{\mathrm{mw}}$. With respect to an ordinary Josephson junction, a QPSJ can sustain dual Shapiro steps, consisting of well-defined current plateaus at multiple integers of $e{\omega }_{\mathrm{mw}}/\pi$ in the current-voltage $\left(I\text{−}V\right)$ characteristic. The experimental observation of these plateaus has been elusive up to now. We argue that thermal as well as quantum fluctuations can smear the $I\text{−}V$ characteristic considerably. In order to understand these effects, we study a current-biased QPSJ under microwave irradiation and connected to an inductive and resistive environment. We find that the effect of the fluctuations is governed by the resistance of the environment and by the ratio of the phase-slip energy and the inductive energy. Our results are of interest for experiments aiming at the observation of dual Shapiro steps in QPSJ devices for the definition of a new quantum current standard.

A generalized trial wave function termed as the “multi-$D_1$ ansatz” has been developed to study the ground state of the spin-boson model with simultaneous diagonal and off-diagonal coupling in the sub-Ohmic regime. Ground-state properties including energy and spin polarization are investigated, and the results are consistent with those from exact diagonalization and density matrix renormalization group approaches for the cases involving two oscillators and two baths described by a continuous spectral density function. Breakdown of the rotational and parity symmetries along the continuous quantum phase transition separating the localized phase from the critical phase has been uncovered. Moreover, the phase boundary is determined accurately with the corresponding rotational- and parity-symmetry parameters. A critical value of the spectral exponent $s^{*}=0.49\left(1\right)$ is predicted in the weak coupling limit, which is in agreement with the mean-field prediction of $1/2$, but much smaller than the earlier literature estimate of $0.75\left(1\right)$.

Femtosecond (fs) coherent control of collective order parameters is important for nonequilibrium phase dynamics in correlated materials. Here, we propose such control of ferromagnetic order based on using nonadiabatic optical manipulation of electron-hole ($e-h$) photoexcitations to create fs carrier-spin pulses with controllable direction and time profile. These spin pulses are generated due to the time-reversal symmetry breaking arising from nonperturbative spin-orbit and magnetic exchange couplings of coherent photocarriers. By tuning the nonthermal populations of exchange-split, spin-orbit-coupled semiconductor band states, we can excite fs spin-orbit torques that control complex magnetization pathways between multiple magnetic memory states. We calculate the laser-induced fs magnetic anisotropy in the time domain by using density matrix equations of motion rather than the quasiequilibrium free energy. By comparing to pump-probe experiments, we identify a “sudden” out-of-plane magnetization canting displaying fs magnetic hysteresis, which agrees with switchings measured by the static Hall magnetoresistivity. This fs transverse spin-canting switches direction with magnetic state and laser frequency, which distinguishes it from the longitudinal nonlinear optical and demagnetization effects. We propose that sequences of clockwise or counterclockwise fs spin-orbit torques, photoexcited by shaping two-color laser-pulse sequences analogous to multidimensional nuclear magnetic resonance (NMR) spectroscopy, can be used to timely suppress or enhance magnetic ringing and switching rotation in magnetic memories.

We present a study of photoexcited magnetization dynamics in ferromagnetic (Ga,Mn)As films observed by time-resolved magneto-optical measurements. The magnetization precession triggered by linearly polarized optical pulses in the absence of an external field shows a strong dependence on photon frequency when the photoexcitation energy approaches the band edge of (Ga,Mn)As. This can be understood in terms of magnetic anisotropy modulation by both laser heating of the sample and by hole-induced nonthermal paths. Our findings provide a means for identifying the transition of laser-triggered magnetization dynamics from thermal to nonthermal mechanisms, a result that is of importance for ultrafast optical spin manipulation in ferromagnetic materials via nonthermal paths.

We present a study of graphene/substrate interactions on ultrahigh-vacuum-grown graphene islands with minimal surface contamination using in situ low-temperature scanning tunneling microscopy. We compare the physical and electronic structure of the sample surface with atomic spatial resolution on graphene islands versus regions of bare Cu(111) substrate. We find that the Rydberg-like series of image potential states is shifted toward lower energy over the graphene islands relative to Cu(111), indicating a decrease in the local work function, and the resonances have a much smaller linewidth, indicating reduced coupling to the bulk. In addition, we show the dispersion of the occupied Cu(111) Shockley surface state is influenced by the graphene layer, and both the band edge and effective mass are shifted relative to bare Cu(111).

Here we study the emergence of different symmetry-protected topological (SPT) phases in a spin-2 quantum chain. We consider a Heisenberg-like model with bilinear, biquadratic, bicubic, and biquartic nearest-neighbor interactions, as well as uniaxial anisotropy. We show that this model contains four different effective spin-1 SPT phases, corresponding to different representations of the $({\mathbb{Z}}_2\times {\mathbb{Z}}_2)+T$ symmetry group, where ${\mathbb{Z}}_2$ is some $\pi$ rotation in the spin internal space and $T$ is time reversal. One of these phases is equivalent to the usual spin-1 Haldane phase, while the other three are different but also typical of spin-1 systems. The model also exhibits an $SO\left(5\right)$-Haldane phase. Moreover, we also find that the transitions between the different effective spin-1 SPT phases are continuous and can be described by a $c=2$ conformal field theory. At such transitions, indirect evidence suggests a possible effective field theory of four massless Majorana fermions. The results are obtained by approximating the ground state of the system in the thermodynamic limit using matrix product states via the infinite time-evolving block decimation method, as well as by effective field theory considerations. Our findings show that different large effective spin-1 SPT phases separated by continuous quantum phase transitions can be stabilized in a simple quantum spin chain.

We report on optical studies of coupled quantum dot–micropillar cavities using a ${90}^{\circ }$ excitation-and-detection scheme. This specific configuration allows us to excite the micropillar structures either in the axial direction or in the lateral direction and to simultaneously detect emission from both directions. That enables us to reveal correlations between emission into the cavity mode and the leaky modes in the regime of cavity quantum electrodynamics. In particular, we can access and distinguish between axial cavity emission and lateral emission consisting of emission of quantum dots into the leaky modes and losses due to sidewall scattering, respectively. In the multiemitter regime, this technique provides direct access to the respective loss channels and reveals a strong increase of sidewall losses in the low-diameter regime below about 3.0 $\mu \mathrm{m}$. Beyond that, in the single-emitter regime, we observe an anticorrelation between quantum dot emission coupled into the cavity mode and into the leaky modes which is controlled by light-matter interaction in the weak coupling regime. This anticorrelation is absent in the strong coupling regime due to the presence of entangled light-matter states. Moreover, excitation-power-dependent studies demonstrate that the intensity ratio between axial and lateral emission increases strongly above the lasing threshold due to enhanced directionality of emission into the lasing mode. In fact, theoretical studies confirm that this intensity ratio is an additional indicator of laser action in high-$\beta$ microlasers for which the onset of lasing is difficult to identify by the input-output characteristics.

We report on a temperature-induced transition from a conventional semiconductor to a two-dimensional topological insulator investigated by means of magnetotransport experiments on HgTe/CdTe quantum well structures. At low temperatures, we are in the regime of the quantum spin Hall effect and observe an ambipolar quantized Hall resistance by tuning the Fermi energy through the bulk band gap. At room temperature, we find electron and hole conduction that can be described by a classical two-carrier model. Above the onset of quantized magnetotransport at low temperature, we observe a pronounced linear magnetoresistance that develops from a classical quadratic low-field magnetoresistance if electrons and holes coexist. Temperature-dependent bulk band structure calculations predict a transition from a conventional semiconductor to a topological insulator in the regime where the linear magnetoresistance occurs.

We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two, and three dimensions, and their one-dimensional (1D) superlattice. We calculate the long-wavelength limit of the dynamical polarization function analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long-wavelength plasmon frequency in massive Dirac systems is found to be different compared to systems with parabolic and gapless Dirac dispersion. We also calculate the long-wavelength plasmon dispersion of a 1D metamaterial made from 1D and 2D massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy, as in the case of graphene [L. Ju et al., Nat. Nanotechnol. 6, 630 (2011)].

We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.

We have studied the polarization dependence of coherent phonons in the topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$. Using polarization-dependent femtosecond pump-probe spectroscopy, we measured coherent phonons as a function of angle when the pump and probe polarizations were fixed, and the crystal orientation was rotated. For isotropic detection, depending on the spot position, oscillations either from only low- and high-frequency phonons of $A_{1g}$ symmetry, or in addition from the mode at 3.6 THz were observed. All the modes were found to be independent of the orientation of electric field vector with respect to the crystal axes testifying to their full symmetry while no modes of lower symmetry appeared in any polarization geometry. For anisotropic detection both modes of $E_g$ symmetry could be detected, but their amplitudes were considerably smaller than those of $A_{1g}$ symmetry. To clarify the coherent phonon assignment and the process of coherent phonon generation in $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, the time-domain measurements were complemented by spontaneous Raman scattering. The comparison of frequency- and time-domain results and the polarization dependence suggest that the 3.6 THz mode belongs to crystalline Te arising due to tellurium segregation. A discrepancy between the time- and frequency-domain data is discussed.

The formation of a pair of dark solitons from a single nonlinear black spin-wave pulse was observed. The experiments were carried out with a long and narrow magnetic yttrium iron garnet film strip in a surface spin-wave configuration. The black spin-wave pulses were excited by the use of microwave black pulses—large-amplitude microwaves with narrow squarelike dips. Pairs of black solitons were observed in certain input power and input black pulse width ranges. For each pair, the two solitons show opposite $\pi$ phase jumps and an overall phase change of zero. Beyond those power and width ranges, one also observed pairs of gray solitons that showed opposite phase jumps and a zero total phase change. The formation of a single black soliton was also observed, but only for an input black pulse that was very narrow. The experimental observations were reproduced by numerical simulations based on the complex Ginzburg-Landau equation.

Recently, it was proposed that the orbital ordering of ${\pi }_{x,y}^{*}$ molecular orbitals in the superoxide ${\mathrm{CsO}}_2$ compound leads to the formation of spin-1/2 chains below the structural phase transition occurring at $T_{\mathrm{s}1}=61 \mathrm{K}$ on cooling. Here we report a detailed $X$-band electron paramagnetic resonance (EPR) study of this phase in ${\mathrm{CsO}}_2$ powder. The EPR signal appears as a broad line below $T_{\mathrm{s}1}$, which is replaced by the antiferromagnetic resonance below the Néel temperature $T_{\mathrm{N}}=8.3 \mathrm{K}$. The temperature dependence of the EPR linewidth between $T_{\mathrm{s}1}$ and $T_{\mathrm{N}}$ agrees with the predictions for the one-dimensional Heisenberg antiferromagnetic chain of $S=1/2$ spins in the presence of symmetric anisotropic exchange interaction. Complementary analysis of the EPR line shape, linewidth, and the signal intensity within the Tomonaga-Luttinger liquid (TLL) framework allows for a determination of the TLL exponent $K=0.48$. Present EPR data thus fully comply with the quantum antiferromagnetic state of spin-1/2 chains in the orbitally ordered phase of ${\mathrm{CsO}}_2$, which is therefore a unique $p$-orbital system where such a state could be studied.

We investigate the finite-temperature properties of an Anderson lattice with regularly depleted impurities. The physics of this model is ruled by two different magnetic exchange mechanisms: conventional Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction at weak hybridization strength $V$ and an inverse indirect magnetic exchange (IIME) at strong $V$, both favoring a ferromagnetic ground state. The stability of ferromagnetic order against thermal fluctuations is systematically studied by static mean-field theory for an effective low-energy spin-only model emerging perturbatively in the strong-coupling limit as well as by dynamical mean-field theory for the full model. The Curie temperature is found at a maximum for a half-filled conduction band and at intermediate hybridization strengths in the crossover regime between RKKY and IIME.

Absorption and scattering of electromagnetic waves by dielectric media are of fundamental importance in many branches of physics. Here we analytically derived the ultimate upper limits for the absorbed and scattered powers by any system of coupled particles and give sufficient conditions to reach these limits paving a way for a rational design of optimal metamaterials.

We study the problem of wave transport in a one-dimensional disordered system, where the scatterers of the chain are $n$ barriers and wells with statistically independent intensities and with a spatial extension $l_c$ which may contain an arbitrary number $\delta /2\pi$ of wavelengths, where $\delta =k{l}_c$. We analyze the average Landauer resistance and transmission coefficient of the chain as a function of $n$ and the phase parameter $\delta$. For weak scatterers, we find: (i) a regime, to be called I, associated with an exponential behavior of the resistance with $n$; (ii) a regime, to be called II, for $\delta$ in the vicinity of $\pi$, where the system is almost transparent and less localized; and (iii) right in the middle of regime II, for $\delta$ very close to $\pi$, the formation of a band gap, which becomes ever more conspicuous as $n$ increases. In regime II, both the average Landauer resistance and the transmission coefficient show an oscillatory behavior with $n$ and $\delta$. These characteristics of the system are found analytically, some of them exactly and some others approximately. The agreement between theory and simulations is excellent, which suggests a strong motivation for the experimental study of these systems. We also present a qualitative discussion of the results.

Electronic and magnetic properties of ${\mathrm{Ga}}_{1-}{}_x\text{Mn}{}_x\text{As}$, obtained from first-principles calculations employing the hybrid HSE06 functional, are presented for $x=6.25\%$ and 12.5% under pressures ranging from 0 to 15 GPa. In agreement with photoemission experiments at ambient pressure, we find for $x=6.25\%$ that nonhybridized Mn $3d$ levels and Mn-induced states reside about 5 and 0.4 eV below the Fermi energy, respectively. For elevated pressures, the Mn $3d$ levels, Mn-induced states, and the Fermi level shift toward higher energies, however the position of the Mn-induced states relative to the Fermi energy remains constant due to hybridization of the Mn $3d$ levels with the valence As $4p$ orbitals. We also evaluate, employing Monte Carlo simulations, the Curie temperature $\left({\mathit{T}}_{\mathrm{C}}\right)$. At zero pressure, we obtain ${\mathit{T}}_{\mathrm{C}}=181$ K, whereas the pressure-induced changes in ${\mathit{T}}_{\mathrm{C}}$ are $d{T}_{\mathrm{C}}/dp=+4.3$ K/GPa for $x=12.5\%$ and an estimated value of $d{T}_{\mathrm{C}}/dp\approx +2.2$ K/GPa for $x=6.25\%$ under pressures up to 6 GPa. The determined values of $d{T}_{\mathrm{C}}/dp$ compare favorably with $d{T}_{\mathrm{C}}/dp=+(2$–3) K/GPa at $p$ $\le 1.2$ GPa found experimentally and estimated within the $p\text{−}d$ Zener model for ${\mathrm{Ga}}_{0.93}{\mathrm{Mn}}_{0.07}\text{As}$ in the regime where hole localization effects are of minor importance [M. Gryglas-Borysiewicz et al., Phys. Rev. B 82, 153204 (2010)].