Strongly interacting systems with quantum many-body scars exhibit persistent fidelity oscillations when prepared in a certain class of initial states, but otherwise undergo ergodic dynamics. Previous examples of scars have interpreted the characteristic periodic dynamics in terms of the precession of a macroscopic SU(2) spin in an effective magnetic field. Here, the authors uncover a new exactly solvable example of many-body scars in a spin-\textonehalf{} model and show that the scarred dynamics in this model evades such an interpretation. This unusual coherent dynamics arises due to an emergent kinetic constraint that endows the time-evolving many-body state with constant area-law entanglement. The model studied in this work is relevant to experiments on Rydberg-atom quantum simulators in the antiblockade regime.

We study the eigenstate properties of a nonintegrable spin chain that was recently realized experimentally in a Rydberg-atom quantum simulator. In the experiment, long-lived coherent many-body oscillations were observed only when the system was initialized in a particular product state. This pronounced coherence has been attributed to the presence of special "scarred" eigenstates with nearly equally-spaced energies and putative nonergodic properties despite their finite energy density. In this paper we uncover a surprising connection between these scarred eigenstates and low-lying quasiparticle excitations of the spin chain. In particular, we show that these eigenstates can be accurately captured by a set of variational states containing a macroscopic number of magnons with momentum $\pi$. This leads to an interpretation of the scarred eigenstates as finite-energy-density condensates of weakly interacting $\pi$-magnons. One natural consequence of this interpretation is that the scarred eigenstates possess long-range order in both space and time, providing a rare example of the spontaneous breaking of continuous time-translation symmetry. We verify numerically the presence of this space-time crystalline order and explain how it is consistent with established no-go theorems precluding its existence in ground states and at thermal equilibrium.

We study the layered $J_1$-$J_2$ classical Heisenberg model on the square lattice using a self-consistent bond theory. We derive the phase diagram for fixed $J_1$ as a function of temperature $T$, $J_2$ and interplane coupling $J_z$. Broad regions of (anti)ferromagnetic and stripe order are found, and are separated by a first-order transition near $J_2\approx 0.5$ (in units of $|J_1|$). Within the stripe phase the magnetic and vestigial nematic transitions occur simultaneously in first-order fashion for strong $J_z$. For weaker $J_z$ there is in addition, for $J_2^*<J_2 < J_2^{**}$, an intermediate regime of split transitions implying a finite temperature region with nematic order but no long-range stripe magnetic order. In this split regime, the order of the transitions depends sensitively on the deviation from $J_2^*$ and $J_2^{**}$, with split second-order transitions predominating for $J_2^* \ll J_2 \ll J_2^{**}$. We find that the value of $J_2^*$ depends weakly on the interplane coupling and is just slightly larger than $0.5$ for $|J_z| \lesssim 0.01$. In contrast the value of $J_2^{**}$ increases quickly from $J_2^*$ at $|J_z| \lesssim 0.01$ as the interplane coupling is further reduced. In addition, the magnetic correlation length is shown to directly depend on the nematic order parameter and thus exhibits a sharp increase (or jump) upon entering the nematic phase. Our results are broadly consistent with predictions based on itinerant electron models of the iron-based superconductors in the normal-state, and thus help substantiate a classical spin framework for providing a phenomenological description of their magnetic properties.

We study the eigenstate properties of a nonintegrable spin chain that was recently realized experimentally in a Rydberg-atom quantum simulator. In the experiment, long-lived coherent many-body oscillations were observed only when the system was initialized in a particular product state. This pronounced coherence has been attributed to the presence of special "scarred" eigenstates with nearly equally-spaced energies and putative nonergodic properties despite their finite energy density. In this paper we uncover a surprising connection between these scarred eigenstates and low-lying quasiparticle excitations of the spin chain. In particular, we show that these eigenstates can be accurately captured by a set of variational states containing a macroscopic number of magnons with momentum π. This leads to an interpretation of the scarred eigenstates as finite-energy-density condensates of weakly interacting π-magnons. One natural consequence of this interpretation is that the scarred eigenstates possess long-range order in both space and time, providing a rare example of the spontaneous breaking of continuous time-translation symmetry. We verify numerically the presence of this space-time crystalline order and explain how it is consistent with established no-go theorems precluding its existence in ground states and at thermal equilibrium.

We study the spin-1 XY model on a hypercubic lattice in d dimensions and show that this well-known nonintegrable model hosts an extensive set of anomalous finite-energy-density eigenstates with remarkable properties. Namely, they exhibit subextensive entanglement entropy and spatiotemporal long-range order, both believed to be impossible in typical highly excited eigenstates of nonintegrable quantum many-body systems. While generic initial states are expected to thermalize, we show analytically that the eigenstates we construct lead to weak ergodicity breaking in the form of persistent oscillations of local observables following certain quantum quenches-in other words, these eigenstates provide an archetypal example of so-called quantum many-body scars. This Letter opens the door to the analytical study of the microscopic origin, dynamical signatures, and stability of such phenomena.

Many-body localization (MBL) is a scenario wherein a many-body quantum system retains memory of its initial state under unitary dynamics out to infinite time, contrary to expectations based on statistical mechanics. Here, the authors argue that there is another, distinct way in which statistical mechanics can fail, namely, certain initial configurations of particles can relax in the way predicted by statistical mechanics, while others fail to do so in a manner reminiscent of MBL. They dub such a system a ``mobility emulsion'' (ME). The authors demonstrate that an ME exists in a simple model and constitutes a distinct phase from the usual thermal and MBL phases. Furthermore, they show that the strong dependence of the system's dynamics on its initial state can be understood as an emergent ``spin-charge separation'' resulting from the interplay of interactions and strong disorder, and has no analog in the case of noninteracting particles. The model for the ME discussed in this work can potentially be realized in synthetic quantum systems, such as cold atomic gases.

We propose utilizing the Cooper pair to induce magnetic frustration in systems of two-dimensional (2D) magnetic adatom lattices on $s$-wave superconducting surfaces. The competition between singlet electron correlations and the RKKY coupling is shown to lead to a variety of hidden-order states that break the point-group symmetry of the 2D adatom lattice at finite temperature. The phase diagram is constructed using a newly developed effective bond theory [M. Schecter et al., Phys. Rev. Lett. 119, 157202 (2017)], and exhibits broad regions of long-range vestigial nematic order.

Protected zero modes in quantum physics traditionally arise in the context of ground states of many-body Hamiltonians. Here we study the case where zero modes exist in the center of a reflection-symmetric many-body spectrum, giving rise to the notion of a protected ``infinite-temperature'' degeneracy. For a certain class of nonintegrable spin chains, we show that the number of zero modes is determined by a chiral index that grows exponentially with system size. We propose a dynamical protocol, feasible in ongoing experiments in Rydberg atom quantum simulators, to detect these many-body zero modes and their protecting spectral reflection symmetry. Finally, we consider whether the zero-energy states obey the eigenstate thermalization hypothesis, as is expected of states in the middle of the many-body spectrum. We find intriguing differences in their eigenstate properties relative to those of nearby nonzero-energy eigenstates at finite system sizes.

It is well known that spontaneous symmetry breaking in one spatial dimension is thermodynamically forbidden at finite energy density. Here we show that mirror-symmetric disorder in an interacting quantum system can invert this paradigm, yielding spontaneous breaking of mirror symmetry only at finite energy density and giving rise to "mirror-glass" order. The mirror-glass transition, which occurs via the energetic activation of a finite density of emergent Ising degrees of freedom, is enabled by many-body localization and appears to occur simultaneously with the localization transition. This counterintuitive manifestation of localization-protected order can be viewed as a quantum analog of inverse freezing, a phenomenon that occurs, e.g., in certain models of classical spin glasses.

We show that, in a many-body system, all particles can be strongly confined to the initially occupied sites for a time that scales as a high power of the ratio of the bandwidth of site energies to the hopping amplitude. Such time-domain formulation is complementary to the formulation of the many-body localization of all stationary states with a large localization length. The long localization lifetime is achieved by constructing a periodic sequence of site energies with a large period in a one-dimensional chain. The scaling of the localization lifetime is independent of the number of particles for a broad range of the coupling strength. The analytical results are confirmed by numerical calculations.

We study classical two-dimensional frustrated Heisenberg models with generically incommensurate ground states. A new theory for the lattice-nematic "order by disorder" transition is developed based on the self-consistent determination of the effective exchange coupling bonds. In our approach, fluctuations of the constraint field imposing conservation of the local magnetic moment drive nematicity at low temperatures. The critical temperature is found to be highly sensitive to the peak helimagnetic wave vector, and vanishes continuously when approaching rotation symmetric Lifshitz points. Transitions between symmetry distinct nematic orders may occur by tuning the exchange parameters, leading to lines of bicritical points.

We study a chain of magnetic moments exchange coupled to a conventional three dimensional superconductor. In the normal state the chain orders into a collinear configuration, while in the superconducting phase we find that ferromagnetism is unstable to the formation of a magnetic spiral state. Beyond weak exchange coupling the spiral wavevector greatly exceeds the inverse superconducting coherence length as a result of the strong spin-spin interaction mediated through the subgap band of Yu-Shiba-Rusinov states. Moreover, the simple spin-spin exchange description breaks down as the subgap band crosses the Fermi energy, wherein the spiral phase becomes stabilized by the spontaneous opening of a p-wave superconducting gap within the band. This leads to the possibility of electron-driven topological superconductivity with Majorana boundary modes using magnetic atoms on superconducting surfaces.

We overview the main features of mobile impurities moving in one-dimensional superfluid backgrounds by modeling it as a mobile Josephson junction, which leads naturally to the periodic dispersion of the impurity. The dissipation processes, such as radiative friction and quantum viscosity, are shown to result from the interaction of the collective phase difference with the background phonons. We develop a more realistic depleton model of an impurity-hole bound state that provides a number of exact results interpolating between the semiclassical weakly interacting picture and the strongly interacting Tonks-Girardeau regime. We also discuss the physics of a trapped impurity, relevant to current experiments with ultra cold atoms.

We investigate the formation of a new type of composite topological excitation -- the skyrmion-vortex pair (SVP) -- in hybrid systems consisting of coupled ferromagnetic and superconducting layers. Spin-orbit interaction in the superconductor mediates a magnetoelectric coupling between the vortex and the skyrmion, with a sign (attractive or repulsive) that depends on the topological indices of the constituents. We determine the conditions under which a bound SVP is formed, and characterize the range and depth of the effective binding potential through analytical estimates and numerical simulations. Furthermore, we develop a semiclassical description of the coupled skyrmion-vortex dynamics and discuss how SVPs can be controlled by applied spin currents.

We study the magnetic and electronic phases of a one-dimensional (1D) magnetic adatom chain on a 2D superconductor. In particular, we confirm the existence of a "self-organized" 1D topologically nontrivial superconducting phase within the set of subgap Yu-Shiba-Rusinov states formed along the magnetic chain. This phase is stabilized by incommensurate spiral correlations within the magnetic chain that arise from the competition between short-range ferromagnetic and long-range antiferromagnetic electron-induced exchange interactions, similar to a recent study for a 3D superconductor [M. Schecter et al., Phys. Rev. B 93, 140503(R) (2016)]. The exchange interactions along diagonal directions are also considered and found to display behavior similar to a 1D substrate when close to half filling. We show that the topological phase diagram is robust against local superconducting order parameter suppression and weak substrate spin-orbit coupling. Lastly, we study the effect of a direct ferromagnetic exchange coupling between the adatoms, and find the region of spiral order in the phase diagram to be significantly enlarged in a wide range of the direct exchange coupling.

We study theKondo chain in the regime of high spin concentrationwhere the low energy physics is dominated by the Ruderman–Kittel–Kasuya–Yosida interaction. As has been recently shown (Tsvelik andYevtushenko 2015 Phys. Rev. Lett. 115 216402), thismodel has two phases with drastically different transport properties depending on the anisotropy of the exchange interaction. In particular, the helical symmetry of the fermions is spontaneously brokenwhen the anisotropy is of the easy plane type. This leads to a parametrical suppression of the localization effects. In the present paper we substantially extend the previous theory, in particular, by analyzing a competition of forwardand backwardscattering, including into the theory short range electron interactions and calculating spin correlation functions.We discuss applicability of our theory and possible experiments which could support the theoretical findings.

We study potential and electron density depth profiles in accumulation, inversion, and depletion layers in crystals with a large and nonlinear dielectric response such as ${\mathrm{SrTiO}}_{3}$. We describe the lattice dielectric response using the Landau-Ginzburg free energy expansion. In accumulation and inversion layers we arrive at new nonlinear dependencies of the width $d$ of the electron gas on an applied electric field ${D}_{0}$. Particularly important is the predicted electron density profile of accumulation layers (including the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface) $n(x)\ensuremath{\propto}{(x+d)}^{\ensuremath{-}12/7}$, where $d\ensuremath{\propto}{D}_{0}^{\ensuremath{-}7/5}$. We compare this profile with available data and find satisfactory agreement. For a depletion layer we find an unconventional nonlinear dependence of the capacitance on voltage. We also evaluate the role of spatial dispersion in the dielectric response by adding a gradient term to the Landau-Ginzburg free energy.

We investigate magnetic order in a lattice of classical spins coupled to an isotropic gas of one-dimensional conduction electrons via local exchange interactions. The frequently discussed Ruderman-Kittel-Kasuya-Yosida effective exchange model for this system predicts that spiral order is always preferred. Here we consider the problem nonperturbatively, and find that such order vanishes above a critical value of the exchange coupling that depends strongly on the lattice spacing. The critical coupling tends to zero as the lattice spacing becomes commensurate with the Fermi wave vector, signaling the breakdown of the perturbative Ruderman-Kittel-Kasuya-Yosida picture, and spiral order, even at weak coupling. We provide the exact phase diagram for arbitrary exchange coupling and lattice spacing, and discuss its stability. Our results shed new light on the problem of utilizing a spiral spin-lattice state to drive a one-dimensional superconductor into a topological phase.