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Opening for submissions mid-2020, PRX Quantum will be a highly selective, open access journal featuring quantum information science and technology research with an emphasis on lasting and profound impact. The journal expands on the excellence and innovation of Physical Review X (PRX).

The year 2020 marks PRB’s 50^th anniversary. On this occasion, the editors launch a collection of select papers. These Milestone studies represent lasting contributions to physics by way of reporting significant discoveries, initiating new areas of research, or substantially enhancing the conceptual tools for making progress in the burgeoning field of condensed matter physics.

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A general three-stage model relates Raman spectroscopy in the visible range to the content of disordered carbons for all amorphous carbons, whether hydrogenated or hydrogen-free.

A. C. Ferrari and J. Robertson
Phys. Rev. B 61, 14095 (2000)

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When interactions support quantum fluctuations in frustrated magnets, one may hope to realize a quantum spin liquid. Here, the authors report a detailed experimental and theoretical study of the low-temperature phase diagram of the pyrochlore quantum magnet Yb${}_2$Ti${}_2$O${}_7$. Despite the geometric frustration associated with this structure, the material exhibits anisotropic ferromagnetism at low temperatures. The reentrant phase diagram and the reversed anisotropy of the upper critical field reported here are, however, inconsistent with a classical description and indicate that the reentrant low-field regime is driven by quantum fluctuations.

The rapidly developing collaboration of condensed matter and quantum optical physics opens up exciting new possibilities for creating novel photon-matter states that allow manipulating material properties using optical cavities. This paper addresses the important challenge of formulating consistent, accurate, and convenient forms of the light-matter coupling for the low-energy tight-binding-type models useful for theoretical analysis. It achieves this by constructing precise mappings from the fundamental $ab$ $initio$ description of the cavity-matter system to low-energy theories, producing a truncated Hamiltonian coupling to a quantum Peierls phase. In a simple model system, the authors demonstrate the quick convergence of the truncated Hamiltonian with the number of bands, paving the way for rigorous theoretical studies in this emerging research field.

The paradigm of topological band theory has been recently challenged by the discovery of band nodes with non-Abelian topological charges. Here, the authors simplify the description of the non-Abelian topology using elementary geometric rules, and they use them to prove a relation between the monopole charge and the linking structure of nodal-line rings. Furthermore, they establish a novel braiding phenomenon induced by topological interaction of the monopole charge with the Berry phase. The analysis suggests unexpected richness of topological structures in $\mathcal{P}\mathcal{T}$-symmetric systems.

Quantum many-body scarring is a recently discovered weak ergodicity-breaking phenomenon in quantum many-body systems. It was first seen in experiments with Rydberg atom array simulators, and characterized by nonthermalizing periodic revivals of certain initial-state configurations. Here, the authors extend our understanding of the physical origins of this phenomenon by showing through an explicit construction that scars can arise through the presence of entangled virtual degrees of freedom undergoing periodic dynamics. This demonstrates a novel analytic scenario, whereby intrinsically entangled states can exhibit periodic revivals in an otherwise chaotic many-body system, enriching the phenomenology of quantum many-body scarring.

Exact “towers” of quantum many-body scar states are recently discovered sets of states in some many-body nonintegrable systems. They are evenly spaced in energy and do not obey the eigenstate thermalization hypothesis. Here, the authors provide a unified framework of proving several towers of states known in the literature. This sheds light on the general structure of exact scar towers, and on the specific models themselves: the 1D AKLT models, spin-1 XY model, and a spin-½ domain wall conserving model. In particular, they provide a simple proof for the scars in the AKLT models and discover a “pyramid” of exact states in the spin-½ model.

The remarkable recent observation of a negative thermal Hall coefficient in the cuprate superconductors presents yet another puzzle about the nature of their ‘pseudogap’ regime. Here, the authors propose an emergent gauge theory framework to understand the sign change of the thermal Hall coefficient and the violation of the Wiedemann-Franz law. They compute the thermal Hall effect in Chern-Simons theories, and find that the emergent gauge fields yield a negative thermal Hall response. The theory captures the main observed trends, but the overall magnitude of the effect is smaller than that observed, suggesting that proper account of phonons could be the missing ingredient.

Recent advances in stochastic thermodynamics unraveled a thermodynamic uncertainty relation (TUR), which is a trade-off relation between cost (dissipation) and precision (fluctuations). In this study, the authors test its validity in nanoscale systems. Measurements of the electron current and its noise in gold atomic-scale junctions demonstrate that the TUR holds in this quantum coherent system. Based on calculations, the authors go on to argue that the TUR can identify transport mechanisms and pinpoint deviations from noninteracting-electron coherent dynamics.

There has been much recent interest in understanding topological band theory of non-Hermitian systems. Most work has focused on the Chern number in these systems, although interesting phenomena related to an eigenvalue winding number has also been reported. Here, the authors give a complete topological classification using homotopy theory. They find that the Chern number can be reduced modulo 2 depending on the winding number, and this is interpreted in terms of braiding of Weyl points and exceptional nodal rings.

Known routes to ergodicity breaking – integrability and many-body localization – require extensively many explicit or emergent conservation laws. Here we show that two conservation laws are sufficient to provably and robustly break ergodicity by “shattering” Hilbert space into exponentially many dynamically disconnected sectors. This represents a new paradigm for localization, which does not rely on disorder or energy conservation, which works in any number of spatial dimensions, and which should be realizable in near-term ultracold atom experiments.

Adiabatic changes in the Kohn-Sham Hamiltonian lead to polarization changes in the solid that can be computed within the context of first-principles total-energy calculations.

R. D. King-Smith and David Vanderbilt
Phys. Rev. B 47, 1651(R) (1993)

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Femtosecond optical lasers can be used to trigger drastic modifications of a material’s physical properties on the ultrafast timescale, revealing complex, otherwise inaccessible quantum mechanics as well as new processes with potential for creating novel devices with unprecedented high performance. Here, the authors report that the interaction of near-infrared laser light with a promising spin-orbitronics candidate, the layered mixed-valence manganite Nd${}_{1-x}$Sr${}_{1+x}$MnO${}_4$, produces unexpected results, triggering dramatic modifications in the spin state of the material while leaving its orbital state unperturbed. These observations are made with an x-ray free-electron laser, while optical spectroscopy further reveals the specific way electrons transfer within the system. The results open an opportunity to selectively control the spin state while keeping the orbital one intact, an extremely challenging task in strongly correlated electron systems. They pave an avenue for the next generation of multistate logic and memory devices.

Topological physics has introduced a modern design principle in optics, providing build-in protection to specific types of disorder. This work demonstrates how such principles could be used to create a topological resonator in nanophotonic crystals so as to enable chiral light-matter interaction in the context of cavity quantum electrodynamics. A moderate Purcell effect is shown as an experimental proof of principle.

Interacting electrons and excitons promise fascinating applications, such as exciton-mediated superconductors or semiconductor-based quantum simulators. A detailed understanding of the force, coupling electrons, and excitons is crucial to realize such applications in atomically thin semiconductors. The authors achieve this by developing an approach that simulates the physics of three charges in monolayer transition metal dichalcogenides (TMD). Their approach predicts novel rotational bound trions as well as collisional states. This allows for the derivation of an accurate model of exciton-electron interactions and, as an application, for the prediction of the photoabsorption properties of electron-doped TMD, revealing the existence of exciton-polarons.

The Rapid Communications section of Physical Review B is devoted to the accelerated publication of especially important new results. A Rapid Communication presents work that is important, interesting, or timely to those in a particular subfield.

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