SciPost Phys. Core 4, 011 (2021) ·
published 4 May 2021

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We derive an exact analytic expression for the hightemperature limit of the
Casimir interaction between two Drude spheres of arbitrary radii. Specifically,
we determine the Casimir free energy by using the scattering approach in the
planewave basis. Within a roundtrip expansion, we are led to consider the
combinatorics of certain partitions of the round trips. The relation between
the Casimir free energy and the capacitance matrix of two spheres is discussed.
Previously known results for the special cases of a sphereplane geometry as
well as two spheres of equal radii are recovered. An asymptotic expansion for
small distances between the two spheres is determined and analytical
expressions for the coefficients are given.
SciPost Phys. Core 4, 010 (2021) ·
published 29 April 2021

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We study an effective Hamiltonian generating time evolution of states on
intermediate time scales in the strongcoupling limit of the spin1/2 XXZ
model. To leading order, it describes an integrable model with local
interactions. We solve it completely by means of a coordinate Bethe Ansatz that
manifestly breaks the translational symmetry. We demonstrate the existence of
exponentially many jammed states and estimate their stability under the leading
correction to the effective Hamiltonian. Some ground state properties of the
model are discussed.
Anwesha Chattopadhyay, H. R. Krishnamurthy, Arti Garg
SciPost Phys. Core 4, 009 (2021) ·
published 28 April 2021

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We present a novel route for attaining unconventional superconductivity in a strongly correlated system without doping. In a simple model of a correlated band insulator at halffilling we demonstrate, based on a generalization of the projected wavefunctions method, that superconductivity emerges for a broad range of model parameters when ee interactions and the bare bandgap are both much larger than the kinetic energy, provided the system has sufficient frustration against the magnetic order. As the interactions are tuned, the superconducting phase appears sandwiched between the correlated band insulator followed by a paramagnetic metal on one side, and a ferrimagnetic metal, antiferromagnetic halfmetal, and Mott insulator phases on the other side
Corrado Rainone, Pierfrancesco Urbani, Francesco Zamponi, Edan Lerner, and Eran Bouchbinder
SciPost Phys. Core 4, 008 (2021) ·
published 16 April 2021

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Structural glasses feature quasilocalized excitations whose frequencies $\omega$ follow a universal density of states ${\cal D}(\omega)\!\sim\!\omega^4$. Yet, the underlying physics behind this universality is not yet fully understood. Here we study a meanfield model of quasilocalized excitations in glasses, viewed as groups of particles embedded inside an elastic medium and described collectively as anharmonic oscillators. The oscillators, whose harmonic stiffness is taken from a rather featureless probability distribution (of upper cutoff $\kappa_0$) in the absence of interactions, interact among themselves through random couplings (characterized by strength $J$) and with the surrounding elastic medium (an interaction characterized by a constant force $h$). We first show that the model gives rise to a gapless density of states ${\cal D}(\omega)\!=\!A_{\rm g}\,\omega^4$ for a broad range of model parameters, expressed in terms of the strength of stabilizing anharmonicity, which plays a decisive role in the model. Then  using scaling theory and numerical simulations  we provide a complete understanding of the nonuniversal prefactor $A_{\rm g}(h,J,\kappa_0)$, of the oscillators' interactioninduced mean square displacement and of an emerging characteristic frequency, all in terms of properly identified dimensionless quantities. In particular, we show that $A_{\rm g}(h,J,\kappa_0)$ is a nonmonotonic function of $J$ for a fixed $h$, varying predominantly exponentially with $(\kappa_0 h^{2/3}\!/J^2)$ in the weak interactions (small $J$) regime  reminiscent of recent observations in computer glasses  and predominantly decays as a powerlaw for larger $J$, in a regime where $h$ plays no role. We discuss the physical interpretation of the model and its possible relations to available observations in structural glasses, along with delineating some future research directions.
SciPost Phys. Core 4, 007 (2021) ·
published 15 April 2021

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We show that scattering from the boundary of static, higherorder topological
insulators (HOTIs) can be used to simulate the behavior of (timeperiodic)
Floquet topological insulators. We consider Ddimensional HOTIs with gapless
corner states which are weakly probed by external waves in a scattering setup.
We find that the unitary reflection matrix describing backscattering from the
boundary of the HOTI is topologically equivalent to a (D1)dimensional
nontrivial Floquet operator. To characterize the topology of the reflection
matrix, we introduce the concept of `nested' scattering matrices. Our results
provide a route to engineer topological Floquet systems in the lab without the
need for external driving. As benefit, the topological system does not to
suffer from decoherence and heating.