We present an analytic microscopic theory showing that in a large class of spin-$\frac{1}{2}$ quasiperiodic quantum kicked rotors, a dynamical analog of the integer quantum Hall effect (IQHE) emerges from an intrinsic chaotic structure. Specifically, the inverse of the Planck's quantum ($h_e$) and the rotor's energy growth rate mimic the “filling fraction” and the “longitudinal conductivity” in conventional IQHE, respectively, and a hidden quantum number is found to mimic the “quantized Hall conductivity.” We show that for an infinite discrete set of critical values of $h_e$, the long-time energy growth rate is universal and of order of unity (“metallic” phase), but otherwise vanishes (“insulating” phase). Moreover, the rotor insulating phases are topological, each of which is characterized by a hidden quantum number. This number exhibits universal behavior for small $h_e$, i.e., it jumps by unity whenever $h_e$ decreases, passing through each critical value. This intriguing phenomenon is not triggered by the likes of Landau band filling, well known to be the mechanism for conventional IQHE, and far beyond the canonical Thouless-Kohmoto-Nightingale-Nijs paradigm for quantum Hall transitions. Instead, this dynamical phenomenon is of strong chaos origin; it does not occur when the dynamics is (partially) regular. More precisely, we find that a topological object, similar to the topological theta angle in quantum chromodynamics, emerges from strongly chaotic motion at microscopic scales, and its renormalization gives the hidden quantum number. Our analytic results are confirmed by numerical simulations. Our findings indicate that rich topological quantum phenomena can emerge from chaos and might point to a new direction of study in the interdisciplinary area straddling chaotic dynamics and condensed matter physics. This work is a substantial extension of a short paper published earlier by two of us [Y. Chen and C. Tian, Phys. Rev. Lett. 113, 216802 (2014)].

Frustrated quantum magnets can harbor unconventional spin-liquid ground states in which the elementary magnetic moments fractionalize into new emergent degrees of freedom. While the fractionalization of quantum numbers is one of the recurring themes in modern condensed matter physics, it often remains a challenge to devise a controlled analytical framework tracking this phenomenon. A notable exception is the exactly solvable Kitaev model, in which spin degrees of freedom fractionalize into Majorana fermions and a ${\mathbb{Z}}_2$ gauge field. Here, we discuss the physics of fractionalization in three-dimensional Kitaev models and demonstrate that the itinerant Majorana fermions generically form a (semi)metal which, depending on the underlying lattice structure, exhibits Majorana Fermi surfaces, nodal lines, or topologically protected Weyl nodes. We show that the nature of these Majorana metals can be deduced from an elementary symmetry analysis of the projective time-reversal and inversion symmetries for a given lattice. This allows us to comprehensively classify the gapless spin liquids of Kitaev models for the most elementary tricoordinated lattices in three dimensions. We further expand this classification by addressing the effects of time-reversal symmetry breaking and additional interactions.

We report photoluminescence experiments bringing the evidence for intervalley scattering in bulk hexagonal boron nitride. From a quantitative analysis of the defect-related emission band, we demonstrate that transverse optical phonons at the $K$ point of the Brillouin zone assist inter-$K$ valley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states. Time-resolved experiments highlight the different recombination dynamics of the phonon replicas implying either virtual excitonic states or real electronic states in the structural defects.

It is established that cyclotron resonance (CR) in a high-quality GaAs/AlGaAs two-dimensional electron system (2DES) originates as a pure resonance that does not hybridize with dimensional magnetoplasma excitations. The magnetoplasma resonances form a fine structure of the CR. The observed fine structure of the CR results from the interplay between coherent radiative and incoherent collisional mechanisms of two-dimensional plasma relaxation. We show that the range of 2DES filling factors from which the phenomenon arises is intimately connected to the fundamental fine-structure constant.

Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed phosphine was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature $\left({\mathrm{T}}_{\mathrm{C}}\right)$ of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. In this work, the phase diagram of ${\mathrm{PH}}_n (n=1,2,3,4,5,6)$ was extensively explored by means of ab initio crystal structure predictions using the minima hopping method (MHM). The results do not support the existence of thermodynamically stable ${\mathrm{PH}}_n$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of ${\mathrm{PH}}_{1,2,3}$ display ${\mathrm{T}}_{\mathrm{C}}$'s comparable to experiments, it remains uncertain if the measured values of ${\mathrm{T}}_{\mathrm{C}}$ can be fully attributed to a phase-pure compound of ${\mathrm{PH}}_n$.

We have studied a range of polar and nonpolar materials using x-ray absorption near-edge spectroscopy (XANES) in external electric fields. An energy shift of the XANES by a few meV/kV is found which scales linearly with the applied voltage, thus being reminiscent of the linear Stark effect. This is corroborated by the consistent presence of this energy shift in polar thin films and bulk crystals and its absence in nonpolar materials as well as in conducting films. The observed energy shift of the XANES is different between two atomic species in one specimen and appears to scale linearly with the atomic number of the studied element. Therefore, XANES in electrical fields opens the perspective to study atomic polarization with element specificity in a range of functional materials.

Phonon spectra of detwinned ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ crystals, as measured by inelastic x-ray scattering, show clear anisotropy accompanying the magnetostructural transition at 200 K. We model the mode splitting using magnetic DFT calculations, including a phenomenological reduction in force-constant anisotropy that can be attributed to magnetic fluctuations. This serves as a starting point for a general model of phonons in this material applicable to both the antiferromagnetically ordered phase and the paramagnetic phase. Using this model, the measured splitting in the magnetic phase below $T_N$, and the measured phonon linewidth, we set a lower bound on the mean magnetic fluctuation frequency above $T_N$ at 210 K.

In non-self-consistent calculations of the total energy within the random-phase approximation (RPA) for electronic correlation, it is necessary to choose a single-particle Hamiltonian whose solutions are used to construct the electronic density and noninteracting response function. Here we investigate the effect of including a $\text{Hubbard-}U$ term in this single-particle Hamiltonian, to better describe the on-site correlation of $3d$ electrons in the transition metal compounds ZnS, ${\mathrm{TiO}}_2$, and NiO. We find that the RPA lattice constants are essentially independent of $U$, despite large changes in the underlying electronic structure. We further demonstrate that the non-self-consistent RPA total energies of these materials have minima at nonzero $U$. Our RPA calculations find the rutile phase of ${\mathrm{TiO}}_2$ to be more stable than anatase independent of $U$, a result which is consistent with experiments and qualitatively different from that found from calculations employing $U\text{-corrected}$ (semi)local functionals. However we also find that the +$U$ term cannot be used to correct the RPA's poor description of the heat of formation of NiO.

Using single spins in semiconductor quantum dots as qubits requires full control over the spin state. As the $g$ tensor provides the coupling in a Hamiltonian between a spin and an external magnetic field, a deeper understanding of the $g$ tensor underlies magnetic-field control of the spin. The $g$ tensor is affected by the presence of spin-correlated orbital currents, of which the spatial structure has been recently clarified. Here we extend that framework to investigate the influence of the shape of quantum dots on the anisotropy of the electron $g$ tensor. We find that the spin-correlated orbital currents form a simple current loop perpendicular to the magnetic moment's orientation. The current loop is therefore directly sensitive to the shape of the nanostructure: for cylindrical quantum dots, the electron $g$-tensor anisotropy is mainly governed by the aspect ratio of the dots. Through a systematic experimental study of the size dependence of the separate electron and hole $g$ tensors of InAs/InP quantum dots, we have validated this picture. Moreover, we find that through size engineering it is possible to independently change the sign of the in-plane and growth direction electron $g$ factors. The hole $g$ tensor is found to be strongly anisotropic and very sensitive to the radius and elongation. The comparable importance of itinerant and localized currents to the hole $g$ tensor complicates the analysis relative to the electron $g$ tensor.

Localization transitions as a function of temperature require a many-body mobility edge in energy, separating localized from ergodic states. We argue that this scenario is inconsistent because local fluctuations into the ergodic phase within the supposedly localized phase can serve as mobile bubbles that induce global delocalization. Such fluctuations inevitably appear with a low but finite density anywhere in any typical state. We conclude that the only possibility for many-body localization to occur is lattice models that are localized at all energies. Building on a close analogy with a model of assisted two-particle hopping, where interactions induce delocalization, we argue why hot bubbles are mobile and do not localize upon diluting their energy. Numerical tests of our scenario show that previously reported mobility edges cannot be distinguished from finite-size effects.

We propose and analyze a simple mesoscopic quantum heat engine that exhibits both high power and high efficiency. The system consists of a biased Josephson junction coupled to two microwave cavities, with each cavity coupled to a thermal bath. Resonant Cooper pair tunneling occurs with the exchange of photons between cavities, and a temperature difference between the baths can naturally lead to a current against the voltage, and hence work. As a consequence of the unique properties of Cooper-pair tunneling, the heat current is completely separated from the charge current. This combined with the strong energy selectivity of the process leads to an extremely high efficiency.

We investigated the effect of an external magnetic field on the diffusive spin transport by magnons in the magnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, using a nonlocal magnon transport measurement geometry. We observed a decrease in magnon spin diffusion length ${\lambda }_m$ for increasing field strengths, where ${\lambda }_m$ is reduced from $9.6\pm 1.2\mu \mathrm{m}$ at 10 mT to $4.2\pm 0.6\mu \mathrm{m}$ at 3.5 T at room temperature. In addition, we find that there must be at least one additional transport parameter that depends on the external magnetic field. Our results do not allow us to unambiguously determine whether this is the magnon equilibrium density or the magnon diffusion constant. These results are significant for experiments in the more conventional longitudinal spin Seebeck geometry, since the magnon spin diffusion length sets the length scale for the spin Seebeck effect as well and is relevant for its understanding.

This paper provides a review and fundamental physical interpretation for the effective densities and moduli of acoustic metamaterials. We introduce the terminology of hidden force and hidden source of volume: the effective density or modulus is negative when the hidden force or source of volume is larger than, and operates in antiphase to, respectively, the force or volume change that would be obtained in their absence. We demonstrate this ansatz for some established acoustic metamaterials with elements based on membranes, Helmholtz resonators, springs, and masses. The hidden force for membrane-based acoustic metamaterials, for instance, is the force from the membrane tension. The hidden source for a Helmholtz-resonator-based metamaterial is the extra air volume injected from the resonator cavity. We also explain the analogous concepts for pure mass-and-spring systems, in which case, hidden forces can arise from masses and springs fixed inside other masses, whereas hidden sources—more aptly termed hidden expanders of displacement in this case—can arise from light rigid trusses coupled to extra degrees of freedom for mechanical motion such as the case of coupling to masses that move at right angles to the wave-propagation direction. This overall picture provides a powerful tool for conceptual understanding and design of new acoustic metamaterials, and avoids common pitfalls involved in determining the effective parameters of such materials.

Pseudospin plays a central role in many novel physical properties of graphene and other artificial systems which have pseudospins of $1/2$. Here we show that in certain photonic crystals (PCs) exhibiting conical dispersions at $\mathbf{k}=0$, the eigenmodes near the “Dirac-like point” can be described by an effective spin-orbit Hamiltonian with a higher dimension value $S=1$, treating the wave propagation in positive index (upper cone), negative index (lower cone), and zero index (flat band) media within a unified framework. The three-component spinor gives rise to boundary conditions distinct from those of pseudospin $1/2$, leading to wave transport behaviors as manifested in super Klein tunneling and supercollimation. For example, collimation can be realized more easily with pseudospin 1 than pseudospin $1/2$. The effective medium description of the PCs allows us to further understand the physics of pseudospin-1 electromagnetic (EM) waves from the perspective of complementary materials. The special wave scattering properties of pseudospin-1 EM waves, in conjunction with the discovery that the effective photonic potential can be varied by a simple change of length scale, offer ways to control photon transport. As a useful platform to study pseudospin-1 physics, dielectric PCs are much easier to fabricate and characterize than ultracold atom systems proposed previously. The system also provides a platform to realize the concept of “complementary medium” using dielectric materials and has the unique advantage of low loss.

The spin transport and spin-to-charge current conversion properties of bismuth are investigated using permalloy/copper/bismuth (Py/Cu/Bi) lateral spin valve structures. The spin current is strongly absorbed at the surface of Bi, leading to ultrashort spin-diffusion lengths. A spin-to-charge current conversion is measured, which is attributed to the inverse Rashba-Edelstein effect at the Cu/Bi interface. The spin-current-induced charge current is found to change direction with increasing temperature. A theoretical analysis relates this behavior to the complex spin structure and dispersion of the surface states at the Fermi energy. The understanding of this phenomenon opens novel possibilities to exploit spin-orbit coupling to create, manipulate, and detect spin currents in two-dimensional systems.

The peculiar features of quantum magnetism sometimes forbid the existence of gapped “featureless” paramagnets which are fully symmetric and unfractionalized. The Lieb-Schultz-Mattis theorem is an example of such a constraint, but it is not known what the most general restriction might be. We focus on the existence of featureless paramagnets on the spin-1 square lattice and the spin-1 and spin-1/2 honeycomb lattice with spin rotation and space group symmetries in 2+1 dimensions. Although featureless paramagnet phases are not ruled out by any existing theorem, field theoretic arguments disfavor their existence. Nevertheless, by generalizing the construction of Affleck, Kennedy, Lieb, and Tasaki to a class we call “slave-spin” states, we propose featureless wave functions for these models. The featurelessness of the spin-1 slave-spin states on the square and honeycomb lattice are verified both analytically and numerically, but the status of the spin-1/2 honeycomb state remains unclear.

We study theoretically electromagnetic radiation emitted by inelastic Cooper-pair tunneling. We consider a dc-voltage-biased superconducting transmission line terminated by a Josephson junction. We show that the generated continuous-mode electromagnetic field can be expressed as a function of the time-dependent current across the Josephson junction. The leading-order expansion in the tunneling coupling, similar to the $P\left(E\right)$ theory, has previously been used to investigate the photon emission statistics in the limit of sequential (independent) Cooper-pair tunneling. By explicitly evaluating the system characteristics up to the fourth order in the tunneling coupling, we account for dynamics between consecutively tunneling Cooper pairs. Within this approach we investigate how temporal correlations in the charge transport can be seen in the first- and second-order coherences of the emitted microwave radiation.

Hydrogen sulfides have recently received a great deal of interest due to the record high superconducting temperatures of up to 203 K observed on strong compression of dihydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$). A joint theoretical and experimental study is presented in which decomposition products and structures of compressed ${\mathrm{H}}_2\mathrm{S}$ are characterized, and their superconducting properties are calculated. In addition to the experimentally known ${\mathrm{H}}_2\mathrm{S}$ and ${\mathrm{H}}_3\mathrm{S}$ phases, our first-principles structure searches have identified several energetically competitive stoichiometries that have not been reported previously: ${\mathrm{H}}_2{\mathrm{S}}_3, {\mathrm{H}}_3{\mathrm{S}}_2, {\mathrm{HS}}_2$, and ${\mathrm{H}}_4{\mathrm{S}}_3$. In particular, ${\mathrm{H}}_4{\mathrm{S}}_3$ is predicted to be thermodynamically stable within a large pressure range of 25–113 GPa. High-pressure x-ray diffraction measurements confirm the presence of ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{H}}_4{\mathrm{S}}_3$ through decomposition of ${\mathrm{H}}_2\mathrm{S}$ that emerges at 27 GPa and coexists with residual ${\mathrm{H}}_2\mathrm{S}$, at least up to the highest pressure of 140 GPa studied in our experiments. Electron-phonon coupling calculations show that ${\mathrm{H}}_4{\mathrm{S}}_3$ has a small $T{}_c$ of below 2 K, and that ${\mathrm{H}}_2\mathrm{S}$ is mainly responsible for the observed superconductivity of samples prepared at low temperature ($<100$ K).

A suite of experimental tools, including high-field magnetization and electron spin resonance (ESR) studies in magnetic fields of up to 50 T and heat capacity studies up to 9 T, have revealed antiferromagnetic order in single crystals of the Heisenberg $S=2$ chain compound ${\text{MnCl}}_3$(bpy), where bpy is $2,2^{\prime }\text{-bipyridine}$. The Néel temperature, which depends on the strength of the applied magnetic field and its orientation with respect to the crystalline axes that was revealed by heat capacity measurements, is near 11.5 K in zero field. The spin-flop transition is identified in the magnetization curve acquired at 1.7 K and at ${\mu }_{\mathrm{o}}{H}_{\mathrm{SF}}^c=24$ T along the $c$ axis. The transition field $H_{\mathrm{SF}}$ is lower than that expected from the previous antiferromagnetic resonance (AFMR) studies on a powder sample. The identification of the long-range antiferromagnetic order resolves an earlier report by Granroth et al. [Phys. Rev. Lett. 77, 1616 (1996)] that identified ${\text{MnCl}}_3$(bpy) as an $S=2$ Haldane system down to 40 mK. The ESR studies identify a wide range of antiferromagnetic resonance modes that provide additional microscopic information about the $g$ values ($g_{a^{*}}=2.09, g_b=1.92$, and $g_c=2.07$), the zero-field splitting constants, $D/k_{\mathrm{B}}=-1.5$ K and $E/k_{\mathrm{B}}=-0.17$ K when the nearest-neighbor spin interaction $J/k_{\mathrm{B}}=31.2$ K, which is evaluated from fitting the susceptibility, and the anisotropy of this compound (easy axis is the $c$ axis, the second easy-axis is the $b$ axis, and the hard axis is the $a^{*}$ axis), when using a standard (two-sublattice) AFMR analysis that does not quantitatively reproduce the observed $H_{\mathrm{SF}}^c$ value. The observed resonance mode indicates the frequency minimum at $H_{\mathrm{SF}}^c$.

The Kadanoff-Baym equations, which allow for the calculation of time-dependent expectation values of all one-particle observables, are found to yield unphysical electron density dynamics in the linear and nonlinear response, for $\mathrm{\Phi }$-derivable approximations, irrespective of interaction strength or type. In particular, we show that when calculated from the Kadanoff-Baym equations using correlated self-energy approximations, the linear response dynamics of isolated electron systems damps to an unphysical homogeneous density-distribution. The damping is also present for Hartree or Hartree-Fock self-energies. These surprising results supplement previous findings on the nonlinear response, and complement them by showing that the linear response is also plagued by unphysical dynamics. Being universal, this additional feature indicates the possible presence of an attractor that leads to amplitude death and a subsequent tendency to a homogeneous charge and density distribution. This unveils a scenario in which the Kadanoff-Baym dynamics simply breaks down, drastically restricting the parameter space for which the method can give physically meaningful insights. In addition to their relevance to the field of ultrafast electron dynamics in isolated and open systems, these findings may also impact the results obtained with the Bethe-Salpeter equation in linear response, due to the well-known equivalency between the two methods. This suggests the need for a different approach to the dynamics of quantum systems.