Ultrafast optical manipulation of ordered phases in strongly correlated materials is a topic of significant theoretical, experimental, and technological interest. Inspired by a recent experiment on light-induced superconductivity in fullerenes [M. Mitrano et al., Nature (London) 530, 461 (2016)], we develop a comprehensive theory of light-induced superconductivity in driven electron-phonon systems with lattice nonlinearities. In analogy with the operation of parametric amplifiers, we show how the interplay between the external drive and lattice nonlinearities lead to significantly enhanced effective electron-phonon couplings. We provide a detailed and unbiased study of the nonequilibrium dynamics of the driven system using the real-time Green's function technique. To this end, we develop a Floquet generalization of the Migdal-Eliashberg theory and derive a numerically tractable set of quantum Floquet-Boltzmann kinetic equations for the coupled electron-phonon system. We study the role of parametric phonon generation and electronic heating in destroying the transient superconducting state. Finally, we predict the transient formation of electronic Floquet bands in time- and angle-resolved photoemission spectroscopy experiments as a consequence of the proposed mechanism.

We perform a systematic analysis of the influence of phonon driving on the superconducting Holstein model coupled to heat baths by studying both the transient dynamics and the nonequilibrium steady state (NESS) in the weak and strong electron-phonon coupling regimes. Our study is based on the nonequilibrium dynamical mean-field theory, and for the NESS we present a Floquet formulation adapted to electron-phonon systems. The analysis of the phonon propagator suggests that the effective attractive interaction can be strongly enhanced in a parametric resonant regime because of the Floquet side bands of phonons. While this may be expected to enhance the superconductivity (SC), our fully self-consistent calculations, which include the effects of heating and nonthermal distributions, show that the parametric phonon driving generically results in a suppression or complete melting of the SC order. In the strong coupling regime, the NESS always shows a suppression of the SC gap, the SC order parameter, and the superfluid density as a result of the driving, and this tendency is most prominent at the parametric resonance. Using the real-time nonequilibrium DMFT formalism, we also study the dynamics towards the NESS, which shows that the heating effect dominates the transient dynamics, and SC is weakened by the external driving, in particular at the parametric resonance. In the weak coupling regime, we find that the SC fluctuations above the transition temperature are generally weakened under the driving. The strongest suppression occurs again around the parametric resonances because of the efficient energy absorption.

Topological semimetals represent a new class of quantum materials hosting Dirac/Weyl fermions. The essential properties of topological fermions can be revealed by quantum oscillations. Here we present systematic de Haas–van Alphen (dHvA) oscillation studies on the recently discovered topological Dirac nodal-line semimetal ZrSiS. From the angular dependence of dHvA oscillations, we have revealed the anisotropic Dirac bands in ZrSiS and found surprisingly strong Zeeman splitting at low magnetic fields. The Landé $g$ factor estimated from the separation of Zeeman splitting peaks is as large as 38. From the analyses of dHvA oscillations, we also revealed nearly zero effective mass and exceptionally high quantum mobility for Dirac fermions in ZrSiS. These results shed light on the nature of novel Dirac fermion physics of ZrSiS.

The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Here we derive rigorously a description of inertia in the Landau-Lifshitz-Gilbert equation on the basis of the Dirac-Kohn-Sham framework. Using the Foldy-Wouthuysen transformation up to the order of $1/c^4$ gives the intrinsic inertia of a pure system through the second order time derivative of magnetization in the dynamical equation of motion. Thus, the inertial damping $\mathcal{I}$ is a higher order spin-orbit coupling effect, $\sim 1/c^4$, as compared to the Gilbert damping $\mathrm{\Gamma }$ that is of order $1/c^2$. Inertia is therefore expected to play a role only on ultrashort timescales (subpicoseconds). We also show that the Gilbert damping and inertial damping are related to one another through the imaginary and real parts of the magnetic susceptibility tensor, respectively.

The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound $\alpha -{\mathrm{NaFeO}}_2$ were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic $ab$-plane, $B_{ab}$, and along the $c^{*}$-axis, $B_c$) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in $B_{ab}$ and along $B_c$, respectively. In zero magnetic field, two spin-density-wave orderings with different $\mathit{k}$ vectors—$(0,q,\frac{1}{2})$ in phase I and ($q_a,q_b,q_c$) in phase II—appeared at $T=9.5$ and 8.25 K, respectively. Below $T=5$ K, a commensurate order with $\mathit{k}=(0.5,0,0.5)$ was stabilized as the ground state in phase III. Both $B_{ab}\ge 3$ T and $B_c\ge 5$ T were found to induce ferroelectric phases at the lowest temperature (2 K), with an electric polarization that was not confined to any highly symmetric directions in phases ${\mathrm{IV}}_{ab}$ ($3.3\le B_{ab}\le 8.5$ T), ${\mathrm{V}}_{ab}$ ($8.5\le B_{ab}\le 13.6$ T), ${\mathrm{IV}}_c$ ($5.0\le B_c\le 8.5$ T), and ${\mathrm{V}}_c$ ($8.5\le B_c\le 13.5$ T). In phase ${\mathrm{VI}}_c$, within a narrow temperature region in $B_c$, the polarization was confined to the $ab$ plane. For each of the ferroelectric phases, the $\mathit{k}$ vector was ($q_a,q_b,q_c$), and noncollinear structures were identified, including a general spiral in ${\mathrm{IV}}_{ab}$ an $ab$ cycloid in ${\mathrm{IV}}_c$ and ${\mathrm{V}}_c$, and a proper screw in ${\mathrm{VI}}_c$, along with a triclinic ${11}^{\prime }$ magnetic point group allowing polarization in the general direction. Comparing the polarization direction to the magnetic structures in the ferroelectric phases, we conclude that the extended inverse Dzyaloshinskii-Moriya mechanism expressed by the orthogonal components ${\mathit{p}}_1\propto {\mathit{r}}_{ij}\times ({\mathit{S}}_i\times {\mathit{S}}_j)$ and ${\mathit{p}}_2\propto {\mathit{S}}_i\times {\mathit{S}}_j$ can explain the polarization directions. Based on calculations incorporating exchange interactions up to fourth-nearest-neighbor (NN) couplings, we infer that competition among antiferromagnetic second NN interactions in the triangular lattice plane, as well as weak interplane antiferromagnetic interactions, are responsible for the rich phase diagrams of $\alpha -{\mathrm{NaFeO}}_2$.

The optical magnetoelectric effect, which produces the nonreciprocal directional dichroism, on the electromagnon resonances is investigated for multiferroic phases of ${\mathrm{YMn}}_2{\mathrm{O}}_5$ by terahertz spectroscopy. For the electromagnon driven by the exchange striction, the crucial role of the commensurability of spin order in the magnetoelectric coupling is manifested by the suppression of the directional dichroism in the incommensurate spin phase. Furthermore, the gapped electromagnon via the spin-current mechanism is identified in terms of the directional dichroism, irrespective of commensurability/incommensurability in the cycloidal spin order.

Do quantum correlations play a role in high-temperature dynamics of many-body systems? A common expectation is that thermal fluctuations lead to fast decoherence and make dynamics classical. In this paper we provide a striking example that a single particle created in a featureless, infinite temperature spin bath not only exhibits nonclassical dynamics but it also induces strong long-lived correlations between the surrounding spins. We study the nonequilibrium dynamics of a hole created in a Mott insulator in the atomic limit, which corresponds to a degenerate spin system. In the absence of interactions, the spin correlations arise purely from quantum interference. Furthermore, these correlations are both antiferromagnetic and ferromagnetic, in striking contrast to the equilibrium Nagaoka effect. These results are relevant for a number of condensed matter spin systems and should be observable using state of the art bosonic or fermionic quantum gas microscopes.

We investigate theoretically the electronic structure of tetragonally strained ${\mathrm{Hg}}_x{\mathrm{Cd}}_{1-x}\mathrm{Te}$. The topological phase diagram of the system was obtained by calculating both the topological invariants and the electronic structure of the (001) surface. We find strong topological-insulator and Weyl-semimetal phases that are caused by multiple inversions among the topmost valence bands. The topological character of the occupied bulk bands is fundamentally governed by the band inversion induced by spin-orbit coupling rather than by the $sp$ band inversion, which is usually considered as origin of the topological nontriviality.

Spin currents can be generated on an ultrafast time scale by excitation of a ferromagnetic (FM) thin film with a femtosecond laser pulse. Recently, it has been demonstrated that these ultrafast spin currents can transport angular momentum to neighboring FM layers, being able to change both the magnitude and orientation of the magnetization in the adjacent layer. In this paper, both the generation and absorption of these optically excited spin currents are investigated. This is done using noncollinear magnetic bilayers, i.e., two FM layers separated by a conductive spacer. Spin currents are generated in a Co/Ni multilayer with out-of-plane (OOP) anisotropy, and absorbed by a Co layer with an in-plane (IP) anisotropy. This behavior is confirmed by careful analysis of the laser-pulse induced magnetization dynamics, whereafter it is demonstrated that the transverse spin current is absorbed very locally near the injection interface of the IP layer ($90\%$ within the first $\approx 2$ nm). Moreover, it will also be shown that this local absorption results in the excitation of THz standing spin waves within the IP layer. The dispersion measured for these high-frequency spin waves shows a discrepancy with respect to the theoretical predictions, for which an explanation involving intermixed interface regions is proposed. Lastly, the spin current generation is investigated by using magnetic bilayers with a different number of repeats for the Co/Ni multilayer, which proves to be of great relevance for identifying the optical spin current generation mechanism.

We show that anisotropic biaxial stress can be used to tune the built-in dipole moment of excitons confined in In(Ga)As quantum dots up to complete erasure of its magnitude and inversion of its sign. We demonstrate that this phenomenon is due to piezoelectricity. We present a model to calculate the applied stress, taking advantage of the so-called piezotronic effect, which produces significant changes in the current-voltage characteristics of the strained diode-membranes containing the quantum dots. Finally, self-consistent $\mathbf{k}·\mathbf{p}$ calculations reveal that the experimental findings can be only accounted for by the nonlinear piezoelectric effect, whose importance in quantum dot physics has been theoretically recognized although it has proven difficult to single out experimentally.

A classification of SU(2)-invariant projected entangled paired states (PEPS) on the square lattice, based on a unique site tensor, has been recently introduced by Mambrini et al. [M. Mambrini, R. Orús, and D. Poilblanc, Phys. Rev. B 94, 205124 (2016)]. It is not clear whether such SU(2)-invariant PEPS can either (i) exhibit long-range magnetic order (such as in the Néel phase) or (ii) describe a genuine quantum critical point (QCP) or quantum critical phase (QCPh) separating two ordered phases. Here, we identify a specific family of SU(2)-invariant PEPS of the classification which provides excellent variational energies for the $J_1-J_2$ frustrated Heisenberg model, especially at $J_2=0.5$, corresponding to the approximate location of the QCP or QCPh separating the Néel phase from a dimerized phase. The PEPS are built from virtual states belonging to the ${\frac{1}{2}}^{\otimes N}\oplus 0$ SU(2) representation, i.e., with $N$ “colors” of virtual spin-$\frac{1}{2}$. Using a full-update infinite-PEPS approach directly in the thermodynamic limit, based on the corner transfer matrix renormalization algorithm supplemented by a conjugate gradient optimization scheme, we provide evidence of (i) the absence of magnetic order and of (ii) diverging correlation lengths (i.e., showing no sign of saturation with increasing environment dimension) in both the singlet and triplet channels, when the number of colors $N\ge 3$. We argue that such a PEPS gives a qualitative description of the QCP or QCPh of the $J_1-J_2$ model.

Entanglement growth and out-of-time-order correlators (OTOC) are used to assess the propagation of information in isolated quantum systems. In this work, using large scale exact time evolution we show that for weakly disordered nonintegrable systems information propagates behind a ballistically moving front, and the entanglement entropy growths linearly in time. For stronger disorder the motion of the information front is algebraic and subballistic and is characterized by an exponent, which depends on the strength of the disorder, similarly to the sublinear growth of the entanglement entropy. We show that the dynamical exponent associated with the information front coincides with the exponent of the growth of the entanglement entropy for both weak and strong disorder. We also demonstrate that the temporal dependence of the OTOC is characterized by a fast nonexponential growth, followed by a slow saturation after the passage of the information front. Finally, we discuss the implications of this behavioral change on the growth of the entanglement entropy.

Effects of short-range electronic interactions in a three-dimensional line-node semimetal that supports linearly dispersing quasiparticles around an isolated loop in the Brillouin zone are discussed. Due to vanishing density of states $\left[\varrho \right(E)\sim |E\left|\right]$, various orderings in the bulk of the system, such as the antiferromagnet and charge-density wave, set in for sufficiently strong on-site ($U$) and nearest-neighbor ($V$) repulsions, respectively. While onset of these two orderings from the semimetallic phase takes place through continuous quantum phase transitions, a first-order transition separates two ordered phases. By contrast, topologically protected drumhead shaped surface states can undergo charge or spin orderings, depending on relative strength of $U$ and $V$, even when they are sufficiently weak. Such surface orderings as well as weak long-range Coulomb interaction can be conducive to spontaneous symmetry breaking in the bulk for weaker interactions. We numerically establish such proximity effect driven spontaneous symmetry breaking in the bulk for subcritical strength of interactions due to flat surface band and also discuss possible superconducting orders in this system.

Under tensile loading the ideal strength of a solid is governed by mechanical instabilities corresponding to failure in tension or shear, indicative of intrinsically brittle or ductile behavior, respectively. Ideal-strength first-principles calculations are performed in this work on several hexagonal-close-packed (hcp) and body-centered-cubic (bcc) metals. It is shown that some metals fail in tension under uniaxial loading, whereas others fail in shear. The observed behavior is rationalized with a simple analytical model based on second-order and third-order elastic constants. This formalism correctly predicts the failure mode of all but one of the metals studied in this work and leads to fundamental insights into why some classes of metals are intrinsically brittle or ductile. Further, for the transition metals, filling of the $d$ bands is shown to correlate with the type of mechanical instability encountered, thus providing insights into the effect of alloying on the intrinsic mechanical behavior of hcp and bcc metals.

Two-dimensional (2D) materials have recently been the focus of extensive research. By following a similar trend as graphene, other 2D materials, including transition metal dichalcogenides ($\mathrm{M}{\mathrm{X}}_2$) and metal mono-chalcogenides (MX), show great potential for ultrathin nanoelectronic and optoelectronic devices. Despite the weak nature of interlayer forces in semiconducting MX materials, their electronic properties are highly dependent on the number of layers. Using scanning tunneling microscopy and spectroscopy, we demonstrate the tunability of the quasiparticle energy gap of few-layered gallium selenide (GaSe) directly grown on a bilayer graphene substrate by molecular beam epitaxy. Our results show that the band gap is about 3.50 ± 0.05 eV for single-tetralayer, $3.00\pm 0.05\mathrm{eV}$ for bi-tetralayer, and $2.30\pm 0.05\mathrm{eV}$ for tri-tetralayer GaSe. This band-gap evolution of GaSe, particularly the shift of the valence band with respect to the Fermi level, was confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and our theoretical calculations. Moreover, we observed a charge transfer in the GaSe/graphene van der Waals (vdW) heterostructure using ARPES. These findings demonstrate the high impact on the GaSe electronic band structure and electronic properties that can be obtained by the control of 2D materials layer thickness and the graphene induced doping.

We construct exact steady states of unitary nonequilibrium time evolution in the gapless XXZ spin-1/2 chain where integrability preserves ballistic spin transport at long times. We characterize the quasilocal conserved quantities responsible for this feature and introduce a computationally effective way to evaluate their expectation values on generic matrix product initial states. We employ this approach to reproduce the long-time limit of local observables in all quantum quenches which explicitly break particle-hole or time-reversal symmetry. We focus on a class of initial states supporting persistent spin currents and our predictions remarkably agree with numerical simulations at long times. Furthermore, we propose a protocol for this model where interactions, even when antiferromagnetic, are responsible for the unbounded growth of a macroscopic magnetic domain.

Roughness in graphene is known to contribute to scattering effects which lower carrier mobility. Encapsulating graphene in hexagonal boron nitride (hBN) leads to a significant reduction in roughness and has become the de facto standard method for producing high-quality graphene devices. We have fabricated graphene samples encapsulated by hBN that are suspended over apertures in a substrate and used noncontact electron diffraction measurements in a transmission electron microscope to measure the roughness of encapsulated graphene inside such structures. We furthermore compare the roughness of these samples to suspended bare graphene and suspended graphene on hBN. The suspended heterostructures display a root mean square (rms) roughness down to 12 pm, considerably less than that previously reported for both suspended graphene and graphene on any substrate and identical within experimental error to the rms vibrational amplitudes of carbon atoms in bulk graphite. Our first-principles calculations of the phonon bands in graphene/hBN heterostructures show that the flexural acoustic phonon mode is localized predominantly in the hBN layer. Consequently, the flexural displacement of the atoms in the graphene layer is strongly suppressed when it is supported by hBN, and this effect increases when graphene is fully encapsulated.

We calculate superfluid density for a dirty $d$-wave superconductor. The effects of impurity scattering are treated within the self-consistent $t$-matrix approximation, in weak-coupling BCS theory. Working from a realistic tight-binding parametrization of the Fermi surface, we find a superfluid density that is both correlated with $T_c$ and linear in temperature, in good correspondence with recent experiments on overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$.

Using optical measurements, we demonstrate that the rotation of micron-scale graphene nanoplatelets levitated in a quadrupole ion trap in high vacuum can be frequency-locked to an applied radiofrequency electric field ${\mathit{E}}_{\mathrm{rf}}$. Over time, frequency-locking stabilizes the nanoplatelet so that its axis of rotation is normal to the nanoplatelet and perpendicular to ${\mathit{E}}_{\mathrm{rf}}$. We observe that residual slow dynamics of the direction of the axis of rotation in the plane normal to ${\mathit{E}}_{\mathrm{rf}}$ is determined by an applied magnetic field. We present a simple model that accurately describes our observations. From our data and model, we can infer both a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation, which we compare to theoretical values. Our results establish that trapping technologies have applications for materials measurements at the nanoscale.

We report on a systematic study of type-II Dirac fermions in a layered crystal of $\mathrm{PdT}{\mathrm{e}}_2$. De Haas–van Alphen oscillations show a small Fermi-surface pocket with a cross section of $0.077\mathrm{n}{\mathrm{m}}^{-2}$ with a nontrivial Berry phase. First-principles calculation reveals that the nontrivial Berry phase originates from a hole pocket formed by the tilted Dirac cone. In addition, the band dispersion measured with angle-resolved photoemission spectroscopy is found to be consistent with that of a type-II Dirac cone dispersion. We propose that $\mathrm{PdT}{\mathrm{e}}_2$ is an improved platform to host topological superconductors.