Orthorhombic YMO${}_3$ ($o$-YMO) is currently being intensively investigated since a single-crystalline thin film of $o$-YMO recently showed a pronounced degree of magnetoelectric coupling. According to first-principles calculations, the ferroelectric ground state is represented by two degenerate $E$-type collinear spin configurations with the computed polarization $P$ of ∼1.4 μC/cm${}^2$. The $bc$-cycloidal spin phase is identified as the spin configuration that corresponds to the transition state in the $+P\leftrightarrow -P$ polarization switching. The remarkable coexistence of the $E$-type and cycloidal spin phases below ∼40 K is attributed to a small value of the Kohn-Sham energy difference between these two phases, 2.2 meV per formula unit. A modulated spin structure, which is characterized by the tilting of the Mn-spin vectors to the $a$-axis direction of Pbnm setting, is proposed to account for the observed strong magnetic-field-dependent polarization in $o$-YMO.

Under the action of an alternating perpendicular magnetic field, the polarity of the vortex state nanodisk can be efficiently switched. We predict the regular and chaotic dynamics of the vortex polarity and propose a simple analytical description in terms of a reduced vortex core model. Conditions for the controllable polarity switching are analyzed.

BaV${}_{\text{3}}$O${}_{\text{8}}$ contains magnetic V${}^{\text{4+}}$($S=1/2$) ions and also nonmagnetic V${}^{\text{5+}}$($S=0$) ions. The V${}^{4+}$ ions are arranged in a coupled Majumdar-Ghosh chainlike network. A Curie-Weiss fit of our magnetic susceptibility $\chi \left(T\right)$ data in the temperature region of $80–300$ K yields a Curie constant $C=0.39$ cm${}^{\text{3}}$K/mole V${}^{\text{4+}}$ and an antiferromagnetic Weiss temperature $\theta =-26\mathrm{K}$. The $\chi$(T) curve shows a broad maximum at $T\simeq 25$ K indicative of short-range order (SRO) and an anomaly corresponding to long-range order (LRO) at $T_{\mathrm{N}}$$\sim 6$ K. The value of the “frustration parameter” ($f=|\theta /T_{\text{N}}|\sim 5$) suggests that the system is moderately frustrated. Above the LRO temperature, the experimental magnetic susceptibility data match well with the coupled Majumdar-Ghosh (or $J_{\mathrm{nn}}-J_{\mathrm{nnn}}$ Heisenberg) chain model with the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic coupling $\alpha =2$ and $J_{\mathrm{nnn}}/k_B=40$ K. In a mean-field approach when considering the interchain interactions, we obtain the total interchain coupling to be about 16 K. The LRO anomaly at $T_{\mathrm{N}}$ is also observed in the specific heat $C_{\mathrm{P}}\left(T\right)$ data and is not sensitive to an applied magnetic field up to 90 kOe. A ${}^{\text{51}}$V NMR signal corresponding to the nonmagnetic vanadium was observed. Anomalies at 6 K were observed in the variation with temperature of the ${}^{51}$V NMR linewidth and the spin-lattice relaxation rate $1/T_1$ indicating that they are sensitive to the LRO onset and fluctuations at the magnetic V sites. The existence of two components (one short and another long) is observed in the spin-spin relaxation rate $1/T_2$ data in the vicinity of $T_{\mathrm{N}}$. The shorter component seems to be intimately connected with the magnetically ordered state. We suggest that both magnetically ordered and nonlong-range-ordered (non-LRO) regions coexist in this compound below the long-range-ordering temperature.

High-pressure in situ synchrotron x-ray diffraction experiments were performed on Ag${}_2$Te up to 42.6 GPa at room temperature, and four phases were identified. Phase I (β–Ag${}_2$Te) transformed into isostructural phase II at 2.4 GPa, and phase III and phase IV emerged at 2.8 and 12.8 GPa, respectively. Combined with first-principles calculations, we solved the phase II and phase III crystal structures and determined the compressional behavior of phase III. Electronic band structure calculations show that the insulating phase I with a narrow band gap first transforms into the semimetallic phase II with the perseverance of topologically nontrivial nature and then to the bulk metallic phase III. Density of states calculations indicate the contrasting transport behavior for Ag${}_{2-\delta }$Te and Ag${}_{2+\delta }$Te under compression. Our results highlight pressure's dramatic role in tuning Ag${}_2$Te's electronic band structure and its novel electrical and magnetotransport behaviors.

The Goldstone theorem states that there should be a massless mode for each spontaneously broken symmetry generator. There is no such rotational mode in crystals; however, superconducting quantum liquid crystals should carry rotational Goldstone modes. By generalization of thermal two-dimensional (2D) defect mediated melting theory into a (2+1)D quantum duality, the emergence of the rotational mode at the quantum phase transition from the solid to the $p$-atic liquid crystal arises as a deconfinement phenomenon, with the unusual property that the stiffness of the rotational mode originates entirely in the dual dislocation condensate.

The quantum $O\left(N\right)$ model in the infinite-$N$ limit is a paradigm for symmetry breaking. Qualitatively, its phase diagram is an excellent guide to the equilibrium physics for more realistic values of $N$ in varying spatial dimensions ($d>1$). Here, we investigate the physics of this model out of equilibrium, specifically its response to global quenches starting in the disordered phase. If the model were to exhibit equilibration, the late-time state could be inferred from the finite-temperature phase diagram. In the infinite-$N$ limit, we show that not only does the model not lead to equilibration on account of an infinite number of conserved quantities, it also does not relax to a generalized Gibbs ensemble (GGE) consistent with these conserved quantities. Instead, an infinite number of new conservation laws emerge at late times and the system relaxes to an emergent GGE consistent with these. Nevertheless, we still find that the late-time states following quenches bear strong signatures of the equilibrium phase diagram. Notably, we find that the model exhibits coarsening to a nonequilibrium critical state only in dimensions $d>2$, that is, if the equilibrium phase diagram contains an ordered phase at nonzero temperatures.

An unusual orbital state was recently proposed to explain the magnetic and transport properties of Ba${}_3$CoRu${}_2$O${}_9$ [H. Zhou et al., Phys. Rev. B. 85, 041201 (2012)]. We show that this state contradicts the first Hund's rule and does not realize in the system under consideration because of a too small crystal-field splitting in the $t_{2g}$ shell. A strong suppression of the local magnetic moment in Ba${}_3$CoRu${}_2$O${}_9$ is attributed to a strong hybridization between the Ru 4$d$ and O 2$p$ states.

Scanning probe microscopy lets us “see” atoms and molecules with unprecedented detail, particularly when the resolution is enhanced by functionalizing the tip of the microscope through deliberate adsorption of atomic or molecular species. However, interpreting the resultant images is often far from trivial as they contain features of both the tip and the sample. Here, a computationally simple theoretical approach is presented that allows the orientations of the tip and sample molecules to be determined from a single scanning tunneling microscopy (STM) image, which in turn reveals information on the bonding interaction between the molecules and the tip and surface. We use the approach to deconvolve the experimental STM images arising from the interaction between a C${}_{60}$-functionalized tip and a C${}_{60}$ molecule adsorbed on a Si(111)-($7\times 7$) surface. The results provide experimental verification of the surface orientations postulated theoretically by Rurali et al. [Phys. Rev. B 81, 075419 (2010)].

We model the quantum Hall effect in heterostructures made of two gapped graphene stripes with different gaps, ${\Delta }_1$ and ${\Delta }_2$. We consider two main situations, ${\Delta }_1=0,{\Delta }_2\ne 0$, and ${\Delta }_1=-{\Delta }_2$. They are different in a fundamental aspect: only the latter features kink states that, when intervalley coupling is absent, are protected against backscattering. We compute the two-terminal conductance of heterostructures with channel length up to 430 nm, in two transport configurations, parallel and perpendicular to the interface. By studying the effect of disorder on the transport along the boundary, we quantify the robustness of kink states with respect to backscattering. Transport perpendicular to the boundary shows how interface states open a backscattering channel for the conducting edge states, spoiling the perfect conductance quantization featured by the homogeneously gapped graphene Hall bars. Our results can be relevant for the study of graphene deposited on hexagonal boron-nitride, as well as to model graphene with an interaction-driven gapped phase with two equivalent phases separated by a domain wall.

We present a theoretical study of the image potential resonances (IPRs) at metal surfaces. We develop the Green's functions approach allowing us to calculate binding energies $E_n$ and lifetimes ${\tau }_n$ of IPRs with high quantum numbers $n$ (up to 10 in this work). A systematic study is performed at the $\overline{\Gamma }$ point for the close-packed metal surfaces: Cu(111), Ag(111), Au(111), Al(001), Al(111), Be(0001), Mg(0001), Na(110), Li(110), and also at the $\overline{\mathrm{Y}}$ point on Cu(110). The calculated lifetimes of IPRs on close-packed surfaces demonstrate the scaling law ${\tau }_n\propto n^3$. Our results are in agreement with available experimental data. We show that at the $\overline{\mathrm{Y}}$ point on Cu(110) each quantum number $n$ corresponds to a pair of IPRs $n^{+}$ and $n^{-}$, where the energy difference $E_{n+}-E_{n-}$ is proportional to $n^{-3}$. The lifetimes ${\tau }_{n+}$ and ${\tau }_{n-}$ differ significantly, however, they both obey the scaling law ${\tau }_{n\pm }\propto n^3$. Since the electrons trapped in the long-lived IPRs are strongly localized on the vacuum side, we argue that the inelastic electron-electron and electron-phonon scattering have a small contribution to the decay rate of these IPRs. The latter is dominated by the resonant electron transfer into the bulk.

Triplet exciton migration in the anthracene single crystal is modeled using a kinetic Monte Carlo scheme based on fully atomistic hopping rates. These account for the coupling of the triplet excitations to both intramolecular vibrations and lattice phonons from first principles. Thermal fluctuations in site energies and excitonic couplings obtained within the fragmented excitation difference approach yield triplet exciton diffusion constants in quantitative agreement with experiment.

We employ NMR techniques to investigate the nature of Mn spins in the I-II-V diluted magnetic semiconductor Li(Zn${}_{1-x}$Mn${}_x$)P ($x=0.1$, Curie temperature $T_C=25$ K). We successfully identify the ${}^7$Li NMR signals arising from the Li sites adjacent to Mn${}^{2+}$, and probe the static and dynamic properties of Mn spins. From the NMR spin-lattice relaxation data, we show that the Mn spin-spin interactions extend over many unit cells.

The existence of more than one steady state in a many-body quantum system driven out of equilibrium has been a matter of debate, both in the context of simple impurity models and in the case of inelastic tunneling channels. In this paper, we combine a reduced density matrix formalism with the multilayer multiconfiguration time-dependent Hartree method to address this problem. This allows us to obtain a converged numerical solution of the nonequilibrium dynamics. Considering a generic model for quantum transport through a quantum dot with electron-phonon interaction, we prove that a unique steady state exists regardless of the initial electronic preparation of the quantum dot, consistent with the converged numerical results. However, a bistability can be observed for different initial phonon preparations. The effects of the phonon frequency and strength of the electron-phonon couplings on the nonequilibrium dynamics and on the emergence of bistability is discussed.

By means of a comprehensive theoretical investigation, we show that external pressure can induce topological phase transitions in IV–VI semiconducting chalcogenides with a rocksalt structure. These materials satisfy mirror symmetries that are needed to sustain topologically protected surface states, at variance with time-reversal symmetry that is responsible for gapless edge states in ${\mathcal{Z}}_2$ topological insulators. The band inversions at high-symmetry points in the Brillouin zone that are related by mirror symmetry are brought about by an “asymmetric” hybridization between cation and anion $sp$ orbitals. By working out the microscopic conditions to be fulfilled in order to maximize this hybridization, we identify materials in the rocksalt chalcogenide class that are prone to undergo a topological phase transition induced by pressure and/or alloying. Our model analysis is fully confirmed by complementary advanced first-principles calculations and ab initio-based tight-binding simulations.

Ab initio theoretical simulations of Al(As${}_{1-x}$N${}_x$)Si${}_3$ alloys, a new class of optoelectronic materials, confirm that these compounds are likely to be disordered via a mechanism that preserves the integrity of the constituent III-V-Si${}_3$ tetrahedra but randomizes their orientation in the average diamond lattice of the compound. This type of disorder is consistent with experimental structural data and with the proposed growth mechanism for such alloys, according to which “III:V(SiH${}_3$)${}_3$” intermediate complexes are formed in the gas phase from reactions between group-III atomic beams and V(SiH${}_3$)${}_3$ molecules, delivering the entire III-V-Si${}_3$ tetrahedra to the growing film. Experimental optical studies of these Al(As${}_{1-x}$N${}_x$)Si${}_3$ alloys as well as more general [Al(As${}_{1-x}$N${}_x$)]${}_y$Si${}_{5-2y}$ compounds grown on Si substrates were carried out using spectroscopic ellipsometry. The resulting dielectric functions are found to be similar to broadened versions of their counterparts in pure Si. This broadening may have important practical applications, particularly in photovoltaics, because it dramatically enhances the optical absorption of Si in the visible range of the electromagnetic spectrum. A critical point analysis reveals the existence of direct optical transitions at energies as low as 2.5 eV, well below the lowest direct absorption edge of Si at 3.3 eV. Such transitions are predicted theoretically for perfectly ordered III-V-Si${}_3$ compounds, and the experimental results suggest that they are robust against tetrahedra orientational disorder.

We report a comprehensive investigation of carrier-transfer dynamics in a single InAs quantum dot (QD) based on second-order photon correlation measurements. The experimentally obtained auto and crosscorrelation functions as well as photoluminescence intensities are successfully explained on the basis of a series of rate equations with common excitation and also single-carrier-capture/escape rates to/from a QD. This approach enables us to understand the carrier-transfer dynamics responsible for the stability of a quantum two-level system formed in a single QD under various excitation conditions. We clarify that the transition between neutral and charged excitonic states is suppressed by one order of magnitude under quasiresonant excitation.

We study nonlinear propagation of electromagnetic waves in two closely spaced graphene layers and demonstrate that this double-layer graphene waveguide can operate as an efficient nonlinear optical coupler for both continuous plasmons and for subwavelength spatial optical plasmon solitons. We analyze the nonlinearity-induced effects of light localization and symmetry breaking in such a graphene coupler, and predict that the interlayer power-dependent coupling provides a mechanism for optical beam control and manipulation at realistic input power levels.

We use nonequilibrium molecular dynamics to study heat transfer across structures consisting of a few layers of graphene sandwiched between silicon crystals. We find that when heat transfers from a silicon lead on one side across the graphene layers to a silicon lead on the other side, the interfacial conductance is essentially independent of the number of layers, in agreement with recent experimental findings. By contrast, wave-packet simulations show that the transmission coefficient of individual vibrational modes depends strongly on frequency and the number of graphene layers, indicating significant interference effects. This apparent contradiction is resolved by a theoretical calculation, which shows that the integrated contribution of the phonons to the interfacial thermal conductance is essentially independent of the number of layers. When one atomic layer of graphene is heated directly, the effective interfacial conductance associated with heat dissipation to the silicon substrate is much smaller. We attribute this to the resistance associated with heat transfer between high and low frequency modes within heated graphene.

A new spinmotive force is predicted in ferromagnets with spin-orbit coupling. By extending the theory of spinmotive force, we show that a time-varying electric field can induce a spinmotive force with static and uniform magnetization. This spinmotive has two advantages; it can be detected free from the inductive voltage owing to the absence of dynamical magnetization and it can be tuned by electric fields. To observe the effect, we propose two experimental setups: electric voltage measurement in a single ferromagnet and spin injection from a ferromagnet into an attached nonmagnetic conductor.