Suspended graphene is difficult to image by scanning probe microscopy due to the inherent van der Waals and dielectric forces exerted by the tip, which are not counteracted by a substrate. Here, we report scanning tunneling microscopy data of suspended monolayer graphene in constant-current mode, revealing a surprising honeycomb structure with amplitude of 50–200 pm and lattice constant of 10–40 nm. The apparent lattice constant is reduced by increasing the tunneling current $I$, but does not depend systematically on tunneling voltage $V$ or scan speed $v_{\mathrm{scan}}$. The honeycomb lattice of the rippling is aligned with the atomic structure observed on supported areas, while no atomic corrugation is found on suspended areas down to the resolution of about $3\text{–}4$ pm. We rule out that the honeycomb structure is induced by the feedback loop using a changing $v_{\mathrm{scan}}$, that it is a simple enlargement effect of the atomic lattice, as well as models predicting frozen phonons or standing phonon waves induced by the tunneling current. Although we currently do not have a convincing explanation for the observed effect, we expect that our intriguing results will inspire further research related to suspended graphene.

We consider the dynamics of an impurity atom immersed in an ideal Fermi gas at zero temperature. We focus on the coherent quantum evolution of the impurity following a quench to strong impurity-fermion interactions, where the interactions are assumed to be short range like in cold-atom experiments. To approximately model the many-body time evolution, we use a truncated basis method, where at most two particle-hole excitations of the Fermi sea are included. When the system is initially noninteracting, we show that our method exactly captures the short-time dynamics following the quench, and we find that the overlap between initial and final states displays a universal nonanalytic dependence on time in this limit. We further demonstrate how our method can be used to compute the impurity spectral function, as well as describe many-body phenomena involving coupled impurity spin states, such as Rabi oscillations in a medium or highly engineered quantum quenches.

We examine the dynamics after a sudden quench in the magnetic field of the Lipkin-Meshkov-Glick model. Starting from the ground state and by employing the time-dependent fidelity, we see manifestly different dynamics are present if the system is quenched through the critical point. Furthermore, we show that the average work shows no sensitivity to the quantum phase transition; however, the free energy and irreversible work show markedly different rates of change in each phase. Finally, we assess the spectral function showing the fundamental excitations that dictate the dynamics of the postquenched system, further highlighting the qualitative differences between the dynamics in the two phases.

We have studied the magnetic properties and the underlying type of orbital ordering in the layered hybrid organic-inorganic compound $\left({\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{NH}}_3\right){}_2{\mathrm{CuCl}}_4$ by using ac-magnetic susceptibility and polarization-dependent soft-x-ray absorption spectroscopy at the Cu-$L_2$ and Cu-$L_3$ edges. We have established that the compound has a long-range ferromagnetic ordering both in plane and out of plane and we found from the analysis of the absorption spectra that the orbital ordering of the ${\mathrm{Cu}}^{2+}$ holes involves the cross-type $d_{x^2-z^2}/d_{y^2-z^2}$ arrangement which is different from the $d_{x^2-y^2}$ arrangement found in the parent compounds of the high-$T_c$ cuprate superconductors.

We theoretically investigate spin transport in half-metallic ferromagnets at finite temperatures. The side-jump and skew-scattering contributions to spin Hall conductivity are derived using the Kubo formula. The electron-magnon interaction causes a finite density of states in the energy gap of the minority-spin band and induces spin Hall conductivity. We show that spin Hall conductivity is proportional to $T^{3/2}$, with $T$ being temperature, and is sensitive to $T$. We propose that spin Hall conductivity may be a tool to study the minority-spin state.

We study interactions of perpendicular and longitudinal magnetic fields in niobium films of different thickness in a wide range of temperatures below the superconducting transition temperature $\left(T_C\right)$. In 100-nm Nb film at all temperatures the longitudinal field $H_{\left|\right|}$ practically does not influence the dynamics of the normal flux. However, in 200-nm Nb film, a considerable anisotropy in the vortex motion is found with advanced propagation of the normal flux along $H_{\left|\right|}$ at $T>T_C/2$ and the preferential jumpwise growth of the thermomagnetic flux dendrites across $H_{\left|\right|}$ at $T<T_C$. Appearance of the in-plane vortices and their cutting reconnection with tilted vortices induced by the normal field $H_{\perp }$ is the reason for the observed anisotropy in the thicker film. Absence of the in-plane vortices and much smaller tilt of vortices generated by $H_{\perp }$ explain the isotropic normal flux dynamics in the thinner film. Our results open an alternative way of manipulating both slow vortex motion and fast thermomagnetic avalanches.

Numerical calculations of anisotropic hopping transport based on the resistor network model are presented. Conductivity is shown to follow the stretched exponential dependence on temperature with exponents increasing from $\frac{1}{4}$ to 1 as the wave functions become anisotropic and their localization length in the direction of charge transport decreases. For sufficiently strong anisotropy, this results in nearest-neighbor hopping at low temperatures due to the formation of a single conduction path, which adjusts in the planes where the wave functions overlap strongly. In the perpendicular direction, charge transport follows variable-range hopping, a behavior that agrees with experimental data on organic semiconductors.

The electrical conductivity of graphene with a nonzero mass-gap parameter is investigated starting from the first principles of quantum electrodynamics in (2+1)-dimensional space time at any temperature. The formalism of the polarization tensor defined over the entire plane of complex frequency is used. At zero temperature we reproduce the results for both real and imaginary parts of the conductivity, obtained previously in the local approximation, and generalize them taking into account the effects of nonlocality. At nonzero temperature the exact analytic expressions for real and imaginary parts of the longitudinal and transverse conductivities of gapped graphene are derived, as well as their local limits and approximate expressions in several asymptotic regimes. Specifically, a simple local result for the real part of conductivity of gapped graphene valid at any temperature is obtained. According to our results, the real part of the conductivity is not equal to zero for frequencies exceeding the width of the gap and goes to the universal conductivity with increasing frequency. The imaginary part of conductivity of gapped graphene varies from infinity at zero frequency to minus infinity at the frequency defined by the gap parameter and then goes to zero with further increase of frequency. The analytic expressions are accompanied by the results of numerical computations. Possible future generalization of the used formalism is discussed.

Reentrant charge-ordering transition (RCOT) in the bilayered perovskite manganite $\mathrm{L}{\mathrm{a}}_{2-2x}\mathrm{S}{\mathrm{r}}_{1+2x}\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_7$ can yield observable changes in both the structural and physical properties associated with phase separation. Our recent measurements show that laser illumination can result in persistent modifications of both the resistance and microstructure in the phase-separated (PS) states. Measurements of photoinduced effects on an $x=0.6$ sample reveal a persistent increase of the resistance by as much as 40%. Low-temperature laser in situ transmission electron microscope observations clearly show that in situ laser irradiation can modify the PS nature and strengthen the charge-ordered state. We attribute these photoinduced phenomena to the optical modulation of the hole concentration in the $\mathrm{Mn}{\mathrm{O}}_2$ layers and the alteration of the local Mn orbital configurations in the PS states.

We show how quasi-one-dimensional correlated insulating states arise at two-thirds filling in organometallic multinuclear coordination complexes described by layered decorated honeycomb lattices. The interplay of spin-orbit coupling and electronic correlations leads to pseudospin-one moments arranged in weakly coupled chains with highly anisotropic exchange and a large trigonal splitting. We show that the in-plane exchange coupling is very different from the interlayer coupling; in particular the latter is much larger, despite the underlying hopping integrals being close to isotropic. Surprisingly, the effective dimensionality of the pseudospin model is strongly dependent on the strength of the electronic correlations: With increasing Hubbard $U$ the pseudospin-one model becomes increasingly one dimensional, even though the crystal is almost isotropic. We predict that the trigonal splitting leads to a quantum phase transition from a Haldane phase to a topologically trivial phase as the relative strength of the spin-orbit coupling increases.

We calculate orbital spin-dependent magnetization in a two-dimensional electron gas with a Rashba-type spin-orbit interaction. Such an orbital magnetization is admitted by the time-reversal symmetry of the system, and gives rise to spin currents when the system is not in thermal equilibrium. The theoretical approach is based on the linear response theory and the Matsubara Green's function formalism. To account for the spin-resolved orbital magnetization, a spin-dependent vector potential has been introduced. The spin currents which appear in thermal nonequilibrium due to spin-resolved orbital magnetization play an important role in the spin Nernst effect, and have to be included in order to correctly describe the low-temperature spin Nernst conductivity.

We consider a simple spin-membrane model for rippling in graphene. The model exhibits transitions from a flat but rippled membrane to a buckled one. At high temperature the transition is second order, but it is first order at low temperature for appropriate strength of the spin-spin coupling. Driving the system across the first-order phase transition in nonequilibrium conditions that mimic interaction of the graphene membrane with a scanning tunneling microscopy (STM) tip explains recent experiments. In particular, we observe a reversible behavior for small values of the STM current and an irreversible transition from a flat rippled membrane to a rigid buckled membrane when the current surpasses a critical value. This work makes it possible to test the mechanical properties of graphene under different temperature and electrostatic conditions.

Using ab initio electronic structure calculations we have investigated the effect of the thickness of a heavy-metal (HM) cap on the magnetic anisotropy of the $\mathrm{Cu}/\mathrm{FeCo}/\mathrm{HM}\left(n\right)$ thin film where HM = Hf and Ta with thicknesses of $n=0\text{–}10$ monolayers (MLs). We find that the Hf cap results in a large perpendicular magnetic anisotropy (PMA), which exhibits quasiperiodic oscillation with a period of two MLs. In contrast, the Ta-capped heterostructure exhibits a spin reorientation from out-of-plane to in-plane magnetization orientation at two MLs of Ta. Moreover, the MA remains negative and depends weakly on the Ta-cap thickness beyond the critical thickness. The underlying mechanism of the PMA oscillation is the periodic change in spin-flip spin-orbit coupling between the minority-spin Fe $d(xz,yz)$ and majority Fe $d\left(z^2\right)$ at $\overline{\mathrm{\Gamma }}$, which is induced by the hybridization with Hf at the FeCo/Hf interface. Our results help resolve the contradictory experiments regarding the role of the FeCo/Ta interface on the PMA of the MgO/FeCo/Ta junction. The calculations reveal that the ferromagnet/Hf is promising for spintronic applications and that the capping material and thickness are additional parameters for optimizing the functional properties of spintronic devices.

We study spin Hall magnetoresistance (SMR) and magnon spin transport (MST) in $\mathrm{Pt}/{\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}\left(\mathrm{YIG}\right)$-based devices with intentionally modified interfaces. Our measurements show that the surface treatment of the YIG film results in a slight enhancement of the spin-mixing conductance and an extraordinary increase in the efficiency of the spin-to-magnon conversion at room temperature. The surface of the YIG film develops a surface magnetic frustration at low temperatures, causing a sign change of the SMR and a dramatic suppression of the MST. Our results evidence that SMR and MST could be used to explore magnetic properties of surfaces, including those with complex magnetic textures, and stress the critical importance of the nonmagnetic/ferromagnetic interface properties in the performance of the resulting spintronic devices.

In this work, we present a neutron Larmor diffraction study of the rare-earth pyrochlores $R_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ with $R=\mathrm{Tb}$, Dy, Ho. We measured the temperature dependence of the lattice parameter with precision ${10}^{-5}$, between 0.5 and 300 K in each of the three compounds. The lattice parameter of the spin ices ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ enters into the derivation of the charge of the emergent magnetic monopole excitations suggested to exist in these materials. We found that throughout the range of applicability of the theory of emergent monopoles in the spin ices there will be no renormalization of the monopole charge due to lattice contraction. In ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, strong magnetoelastic interactions have been reported. We found no sign of the previously reported negative thermal expansion, but did observe anomalies in the thermal expansion that can be correlated with previously observed interactions between phonon and crystal-field excitations. Other features in the thermal expansion of all three compounds can be related to previously observed anomalies of the elastic constants, and explained by the phonon band structure of the rare-earth titanates. The temperature dependence of the lattice strain in all three compounds can be correlated with the thermal population of excited crystal-field levels.

To understand the recently observed high temperature superconductivity in the monolayer ${\mathrm{CuO}}_2$ grown on the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ substrates, we propose a two-band model of the hybridized oxygen $p_x$ and $p_y$ orbitals with the proximity effect of the substrate. We demonstrate that both the nodal and nodeless superconducting states can be induced by the proximity effect, depending on the strengths of the pairing parameters.

The effect of atomic short-range order on the macroscopic and microscopic properties of the prototype of relaxor ferroelectrics, that is, lead magnesium niobate $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$ (PMN), is studied via the combination of an annealing technique and a large-scale effective Hamiltonian method. The investigated short-range order gradually varies from the case of fully disordered solid solutions to the situation for which the first three nearest-neighboring shells of the $B$ lattice of PMN adopt a rocksalt ordering between a sublattice made of pure Nb ions and a randomly distributed sublattice consisting of $\frac{2}{3}$ of Mg and $\frac{1}{3}$ of Nb. The characteristic temperatures of relaxor ferroelectrics (namely, the Burns, so-called $T^{*}$, and depolarizing temperatures) significantly increase when strengthening this short-range chemical order, which is accompanied by an overall enhancement of the size of the polar nanoregions as well as of some antiferroelectric interactions. These results can be understood by the fact that chemical short-range order strongly modifies the internal electric fields felt by the Pb ions.

As a guideline for experimental tests of the ideal glass transition (random-pinning glass transition, RPGT) that shall be induced in a system by randomly pinning particles, we performed first-principle computations within the hypernetted chain approximation and numerical simulations of a hard-sphere model of a glass former. We obtain confirmation of the expected enhancement of glassy behavior under the procedure of random pinning. We present the analytical phase diagram as a function of $c$ and of the packing fraction $\varphi$, showing a line of RPGT ending in a critical point. We also obtain microscopic results on cooperative length scales characterizing medium-range amorphous order in hard-sphere glasses and indirect quantitative information on a key thermodynamic quantity defined in proximity to ideal glass transitions, the amorphous surface tension. Finally, we present numerical results of pair correlation functions able to differentiate the liquid and the glass phases, as predicted by the analytic computations.