We present a quantitative analysis of the intramolecular origin of the inelastic electron tunneling signal of a molecular junction. We use density-functional theory to study a representative conjugated molecule with a low degree of symmetry and calculate, for all modes, the different contributions that give rise to the vibrational spectrum. These local contributions involve products of scattering states with electron-phonon matrix elements and thus encode information on both the vibrational modes and the electronic structure. We separate these intra- and interatomic terms and draw a pattern of addition or cancellation of these partial contributions throughout the inelastic spectrum. This allows for a quantitative relation between the degree of symmetry of each vibrational mode, its inelastic signal, and the locality of selection rules.

We study the nonlinear optical response of graphene in the quantum Hall regime to an intense laser pulse. In particular, we consider the harmonic generation process. We demonstrate that the generalized magneto-optical conductivity of graphene on the harmonics of a strong pump laser radiation has a characteristic Hall plateau feature. The plateau heights depend on the laser intensity and broadening of the Landau levels so that they are not quantized exactly. This nonlinear effect remains robust against the significant broadening of the Landau levels. We predict realization of an experiment through the observation of the third-harmonic signal and nonlinear Faraday effect, which are within the experimental feasibility.

We report on a theoretical study of the lifetime of the first excited state of spin chains made of an odd number of Fe atoms on $\mathrm{C}{\mathrm{u}}_2\mathrm{N}/\mathrm{Cu}\left(100\right)$. Yan, Choi, Burgess, Rolf-Pissarczyk, and Loth [Nat. Nanotech. 10, 40 (2015)] recently observed very long lifetimes in the case of $\mathrm{F}{\mathrm{e}}_3$ chains. We consider the decay of the first excited state induced by electron-hole pair creation in the substrate. For a finite magnetic field, the two lowest-lying states in the chain have a quasi-Néel state structure. Decay from one state to the other strongly depends on the degree of entanglement of the local spins in the chain. The weak entanglement in the chain accounts for the long lifetimes that increase exponentially with chain length. Despite their apparently very different properties, the behavior of odd and even chains is governed by the same kind of phenomena, in particular entanglement effects. The present results account quite well for the lifetimes recently measured by Yan et al. on $\mathrm{F}{\mathrm{e}}_3$.

We investigate the von Neumann entanglement entropy and Schmidt gap in the vortex-free ground state of the Kitaev model on the honeycomb lattice for square/rectangular and cylindrical subsystems. We find that, for both the subsystems, the free-fermionic contribution to the entanglement entropy $S_E$ exhibits signatures of the phase transitions between the gapless and gapped phases. However, within the gapless phase, we find that $S_E$ does not show an expected monotonic behavior as a function of the coupling $J_z$ between the suitably defined one-dimensional chains for either geometry; moreover, the system generically reaches a point of minimum entanglement within the gapless phase before the entanglement saturates or increases again until the gapped phase is reached. This may be attributed to the onset of gapless modes in the bulk spectrum and the competition between the correlation functions along various bonds. In the gapped phase, on the other hand, $S_E$ always monotonically varies with $J_z$ independent of the subregion size or shape. Finally, further confirming the Li-Haldane conjecture, we find that the Schmidt gap $\mathrm{\Delta }$ defined from the entanglement spectrum also signals the topological transitions but only if there are corresponding zero-energy Majorana edge states that simultaneously appear or disappear across the transitions. We analytically corroborate some of our results on entanglement entropy, the Schmidt gap, and the bulk-edge correspondence using perturbation theory.

We present a scanning tunneling microscopy (STM) study of the domain evolution across two first-order phase transitions of stripe modulations in ${\mathrm{IrTe}}_2$ that occur at $T_{\mathrm{C}}\approx 275$ K and $T_{\mathrm{S}}\approx 180$ K, respectively. Phase coexistence of the hexagonal $(1\times 1)$ structure and the $(5\times 1)$ stripe modulation is observed at $T_{\mathrm{C}}$, while various $(p\times 1)$ modulations ($p=3n+2$ with $2\le n\in \mathbb{N}$) are observed below $T_{\mathrm{S}}$. Using STM atomic resolution, we observe anisotropic propagation of domain boundaries along different directions, indicating significantly different kinetic energy barriers. These results are consistently explained by a theoretical analysis of the energy barrier for domain wall propagation as obtained by density functional theory. Individual switching processes observed by STM indicate that the wide temperature range of the transition from the $(5\times 1)$ stripes to the $(6\times 1)$-ordered ground state is probably caused by the numerically limited subset of switching processes that are allowed between a given initial and the final state. The observations on ${\mathrm{IrTe}}_2$ are discussed in terms of a “harmless staircase” with a finite number of first-order transitions between commensurate phases and within a “dynamical freezing” scenario.

At variance with structural ferroic phase transitions which give rise to macroscopic tensors coupled to macroscopic fields, criteria defining antiferroelectric (AFE) phase transitions are still under discussion due to the absence of specific symmetry properties characterizing their existence. They are recognized by the proximity of a ferroelectric (FE) phase induced under applied electric field, with a double hysteresis loop relating the induced polarization to the electric field and a typical anomaly of the dielectric permittivity. Here, we show that there exist indeed symmetry criteria defining AFE transitions. They relate the local symmetry of the polar crystallographic sites emerging at an AFE phase transition with the macroscopic symmetry of the AFE phase. The dielectric properties of AFE transitions are deduced from a Landau theoretical model in which ferroelectric and ferrielectric phases are shown to stabilize as the result of specific symmetry-allowed couplings of the AFE order parameter with the field-induced polarization.

There is now copious direct experimental evidence of various forms of (short-range) charge order in underdoped cuprate high temperature superconductors, and spectroscopic signatures of a nodal-antinodal dichotomy in the structure of the single-particle spectral functions. In this context we analyze the Bogoliubov quasiparticle spectrum in a superconducting nematic glass. The coincidence of the superconducting “nodal points” and the nematic “cold-spots” on the Fermi surface naturally accounts for many of the most salient features of the measured spectral functions (from angle-resolved photoemission) and the local density of states (from scanning tunneling microscopy).

Submonolayer quantum dots combine the zero-dimensional charge-carrier confinement of self-assembled quantum dots with the large density of states of a quantum well. Electroluminescence and pump-probe experiments on a submonolayer-based optical amplifier show that the system exhibits a high gain of $90{\mathrm{cm}}^{-1}$ and an ultrafast gain recovery. We propose a rate equation system describing the microscopic carrier dynamics which quantitatively reproduces the observed behavior and provides deeper theoretical understanding of the material system. In contrast to Stranski-Krastanov quantum dots, the fast gain recovery is enhanced by a strong interdot coupling. Optically inactive submonolayer states form an efficient carrier reservoir and give rise to a large nonlinear optical response.

In half-doped $\mathrm{P}{\mathrm{r}}_{0.50}{A}_{0.50}\mathrm{Co}{\mathrm{O}}_3$ metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for $A=\mathrm{Ca}$ and $A=\mathrm{Sr}$ at temperatures close to ∼100 K. However, the ground magnetic properties of $\mathrm{P}{\mathrm{r}}_{0.50}\mathrm{S}{\mathrm{r}}_{0.50}\mathrm{Co}{\mathrm{O}}_3$ (PSCO) strongly differ from $\mathrm{P}{\mathrm{r}}_{0.50}\mathrm{C}{\mathrm{a}}_{0.50}\mathrm{Co}{\mathrm{O}}_3$ ones, where a partial $\mathrm{P}{\mathrm{r}}^{3+}$ to $\mathrm{P}{\mathrm{r}}^{4+}$ valence shift and Co spin transition makes the system insulating below the transition. This paper investigates and describes the relationship between the $Imma\to I4/mcm$ symmetry change [Padilla-Pantoja, García-Muñoz, Bozzo, Jirák, and Herrero-Martín, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition ($T_{S1}\sim 120K$) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry $I{m}^{\prime }{m}^{\prime }a$ ($m_x\ne 0, m_z=0$). The magnetic space group of the FM1 phase is $F{m}^{\prime }{m}^{\prime }m$ (with $m_x=m_y$). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by ${45}^{\circ }$ within the $ab$ plane across the transition, in which the system retains its metallic character. The presence below $T_{S1}$ of conjugated magnetic domains, both of $F{m}^{\prime }{m}^{\prime }m$ symmetry but having perpendicular spin orientations along the diagonals in the $xy$ plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field ${\mu }_{0}H\left[\perp z\right]\gtrsim 30\mathrm{mT}$ is able to reorient the magnetization within the $ab$ plane, whereas a higher field (${\mu }_{0}H[\parallel z]\gtrsim 1.2\mathrm{T}$ at 2 K) is necessary to align the Co moments perpendicular to the $ab$ plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by ${45}^{\circ }$, was not observed previously in analogous cobaltites without praseodymium.

We develop a linear-response transport theory of diffusive spin and heat transport by magnons in magnetic insulators with metallic contacts. The magnons are described by a position-dependent temperature and chemical potential that are governed by diffusion equations with characteristic relaxation lengths. Proceeding from a linearized Boltzmann equation, we derive expressions for length scales and transport coefficients. For yttrium iron garnet (YIG) at room temperature we find that long-range transport is dominated by the magnon chemical potential. We compare the model's results with recent experiments on YIG with Pt contacts [L. J. Cornelissen et al., Nat. Phys. 11, 1022 (2015)] and extract a magnon spin conductivity of ${\sigma }_m=5\times {10}^5$ S/m. Our results for the spin Seebeck coefficient in YIG agree with published experiments. We conclude that the magnon chemical potential is an essential ingredient for energy and spin transport in magnetic insulators.

We evaluate how thermal effects soften the magnon dispersion in 6 layers of Fe(001) on top of Ir(001). We perform a systematic study considering noncollinear spin arrangement and calculate configuration-dependent exchange parameters $J_{ij}^{nc}$ following the methodology described by Szilva et al. [Phys. Rev. Lett. 111, 127204 (2013)]. In addition, Monte Carlo simulations were performed in order to estimate the noncollinear spin arrangement as a function of temperature. Hence the $J_{ij}^{nc}$'s related to these configurations were calculated and used in an atomistic spin dynamics approach to evaluate the magnon spectra. Our results show good agreement with recent room-temperature measurements, and highlights how thermal effects produce magnon softening in this, and similar, systems.

Heterostructures made of a layer of a cuprate insulator $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$ on the top of a layer of a nonsuperconducting cuprate metal $\mathrm{L}{\mathrm{a}}_{1.55}\mathrm{S}{\mathrm{r}}_{0.45}\mathrm{Cu}{\mathrm{O}}_4$ show high-$T_c$ interface superconductivity confined within a single $\mathrm{Cu}{\mathrm{O}}_2$ plane. Given this extreme quasi-two-dimensional quantum confinement, it is of interest to find out how interface superconductivity behaves when exposed to an external magnetic field. With this motivation, we have performed contactless tunnel-diode-oscillator-based measurements in pulsed magnetic fields up to 56 T as well as measurements of the complex mutual inductance between a spiral coil and the film in static fields up to 3 T. Remarkably, we observe that interface superconductivity survives up to very high perpendicular fields, in excess of 40 T. In addition, the critical magnetic field $H_m\left(T\right)$ reveals an upward divergence with decreasing temperature, in line with vortex melting as in bulk superconducting cuprates.

We study the energy absorption in real time of a disordered quantum spin chain subjected to coherent monochromatic periodic driving. We determine characteristic fingerprints of the well-known ergodic (Floquet-Eigenstate thermalization hypothesis for slow driving/weak disorder) and many-body localized (Floquet–many-body localization for fast driving/strong disorder) phases. In addition, we identify an intermediate regime, where the energy density of the system—unlike the entanglement entropy a local and bounded observable—grows logarithmically slowly over a very large time window.

Current materials-related calculations employ density-functional theory (DFT), commonly using the (semi-)local-density approximations for the exchange-correlation (xc) functional. The difficulties in arriving at a reasonable description of van der Waals (vdW) interactions by DFT-based models is to date a big challenge. In this work, we use various flavors of vdW-corrected DFT xc functionals—ranging from the quasiempirical force-field add-on vdW corrections to self-consistent nonlocal correlation functionals—to study the bulk lattice and mechanical properties (including the elastic constants and anisotropic indices) of the coinage metals (copper, silver, and gold). We critically assess the reliability of the different vdW-corrected DFT methods in describing their anisotropic mechanical properties which have been less reported in the literature. In the context of this work, we regard that our results reiterate the fact that advocating a so-called perfect vdW-inclusive xc functional for describing the general physics and chemistry of these coinage metals could be a little premature. These challenges to modern-day functionals for anisotropically strained coinage metals (e.g., at the faceted surfaces of nanostructures) may well be relevant to other strained material systems.

We study the critical behavior of two-dimensional short-range quantum spin glasses by numerical simulations. Using a parallel tempering algorithm, we calculate the Binder cumulant for the Ising spin glass in a transverse magnetic field with two different short-range bond distributions, the bimodal and the Gaussian ones. Through an exhaustive finite-size analysis, we show that the cumulant probably follows an unconventional activated scaling, which we interpret as new evidence supporting the hypothesis that the quantum critical behavior is governed by an infinite randomness fixed point.

Coherent optical phonons in bulk solid systems play a crucial role in understanding and designing light-matter interactions and can be detected by the transient-reflectivity measurement. In this paper, we demonstrate spectrally resolved detection of coherent optical phonons in diamond from ultrashort infrared pump-probe measurements using optical bandpass filters. We show that this enhances the sensitivity approximately 35 times in measuring the coherent oscillations in the transient reflectivity compared with the commonly used spectrally integrated measurement. To explain this observation, we discuss its mechanism.

We report on broad-band electron-spin resonance measurements performed within the itinerant ferromagnetic phase of $R\mathrm{CoPO}$ $(R$ = La, Pr, Nd, and Sm). We reveal that the $R$ substitution is highly effective in gradually introducing a sizable easy-plane magnetocrystalline anisotropy within the Co sublattice. We explain our results in terms of a subtle interplay of structural effects and of indirect interactions between the $f$ and $d$ orbitals from $R$ and Co, respectively. Our observations provide interesting information on the crucial role of $R$ ions in affecting the electronic properties of the CoP layers, of possible interest also for the isostructural FeAs-based family of high-$T_c$ superconductors.

The dynamics of superfluid ${}^4\mathrm{He}$ at and above the Landau quasiparticle regime is investigated by high-precision inelastic neutron scattering measurements of the dynamic structure factor. A highly structured response is observed above the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for phonon-phonon, maxon-roton, and roton-roton coupling processes. The experimental dynamic structure factor is compared to the calculation of the same physical quantity by a dynamic many-body theory including three-phonon processes self-consistently. The theory is found to provide a quantitative description of the dynamics of the correlated bosons for energies up to about three times that of the Landau quasiparticles.

Pairing in one-atomic-layer-thick two-dimensional electron gas (2DEG) by a single flat band of high-energy longitudinal optical phonons is considered. The polar dielectric ${\mathrm{SrTiO}}_3$ (STO) exhibits such an energetic phonon mode and the 2DEG is created both when one unit cell FeSe layer is grown on its $\left(100\right)$ surface and on the interface with another dielectric like ${\mathrm{LaAlO}}_3$ (LAO). We obtain a quantitative description of both systems solving the gap equation for $T_c$ for arbitrary Fermi energy ${\epsilon }_F$, electron-phonon coupling $\lambda$, and the phonon frequency $\mathrm{\Omega }$, and direct (random-phase approximation) electron-electron repulsion strength $\alpha$. The focus is on the intermediate region between the adiabatic, ${\epsilon }_F>>\mathrm{\Omega }$, and the nonadiabatic, ${\epsilon }_F<<\mathrm{\Omega }$, regimes. The high-temperature superconductivity in single-unit-cell FeSe/STO is possible due to a combination of three factors: high-longitudinal-optical phonon frequency, large electron-phonon coupling $\lambda \sim 0.5$, and huge dielectric constant of the substrate suppression the Coulomb repulsion. It is shown that very low density electron gas in the interfaces is still capable of generating superconductivity of the order of 0.1 K in LAO/STO.

We study the decay of Bogoliubov quasiparticles in one-dimensional Bose gases. Starting from the hydrodynamic Hamiltonian, we develop a microscopic theory that enables one to systematically study both the excitations and their decay. At zero temperature, the leading mechanism of decay of a quasiparticle is disintegration into three others. We find that low-energy quasiparticles (phonons) decay with the rate that scales with the seventh power of momentum, whereas the rate of decay of the high-energy quasiparticles does not depend on momentum. In addition, our approach allows us to study analytically the quasiparticle decay in the whole crossover region between the two limiting cases. When applied to integrable models, including the Lieb-Liniger model of bosons with contact repulsion, our theory confirms the absence of the decay of quasiparticle excitations. We account for two types of integrability-breaking perturbations that enable finite decay: three-body interaction between the bosons and two-body interaction of finite range.