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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The authors introduce a two-dimensional, chirally symmetric model of Dirac electrons subjected to a quasiperiodic modulation which harbors a rich phase diagram at the Dirac node energy: A semimetal phase with Dirac dispersion, a “chiral metal” phase with a nontrivial real-space structure, and – for strongest quasiperiodic modulation – subdiffusive dynamics and Chalker scaling. A natural link with higher order topological phases is identified and potential experimental realizations of the model are discussed.

All-in–all-out (AIAO) antiferromagnets demonstrate vividly the effects of correlated charge and spin with strong spin-orbit coupling. Here, using resonant magnetic x-ray diffraction, the authors directly track the evolution of AIAO order in the pyrochlore Sm${}_2$Ir${}_2$O${}_7$ across a pressure-tuned quantum critical point. The results demonstrate a powerful approach to the quantum critical state driven by electron correlations $U$/$t$ and spin orbit coupling $\lambda$/$t$. The authors find prominent Ising-type spin fluctuations under pressure instead of the Heisenberg-type seen via chemical doping. This work also circumscribes the region of interest for the putative magnetic Weyl semimetal state in Pressure-Temperature space.

Motion of an electron and precession of its spin are tightly interrelated via the spin-orbit interaction. In a two-dimensional magnetized system, the electron occupies a cyclotron orbit, which limits spin deviation and suppresses spin relaxation. Starting with this classical scenario at low magnetic fields, the authors unexpectedly encounter a peculiar spin relaxation at higher fields. It reveals an anomaly in the electron diffusion that was previously observed in magnetized plasmas and has found no unique explanation so far. In the quantum limit, the authors find a strong enhancement of the longitudinal spin relaxation time when Landau levels are fully occupied by electrons.

The Jaynes-Cummings (JC) model is at the heart of cavity QED and suitable to describe many different systems. Here, the authors use it to investigate a quantum dot strongly coupled to a micropillar cavity. Supported by experiments, they apply four-wave mixing spectroscopy to study the system’s dynamics. They push the limits of JC physics by injecting up to $\sim 20$ photons using ultrashort laser pulses. Starting with complicated dynamics, the signal evolves into a single-frequency oscillation monitoring the relaxation down the JC ladder.

Interlayer excitons present in van der Waals heterostructures of transition metal dichalcogenides exhibit remarkably distinct effective $g$ factors compared to their monolayer counterparts. Here, the authors employ first-principles calculations to investigate the interlayer exciton $g$ factors in MoSe${}_2$/WSe${}_2$ vdW heterostructures revealing that they are strongly spin and stacking dependent. Furthermore, within the present approach it is possible to unambiguously identify the nature of various previously measured interlayer exciton peaks. This first-principles approach can potentially be applied to calculate exciton $g$ factors of any semiconductor.

The experimental characterization of materials under high pressure is often limited to a few properties. Usually, this is insufficient for a detailed understanding of the underlying physics. Here, the authors report a suite of high-pressure experiments probing the ferromagnetic state in LaCrSb${}_3$. They assess the Curie temperature, crystal structure, and evolution of the magnetic moment as obtained from the integrated absolute difference in x-ray emission spectroscopy. The Curie temperature appears to be driven by the Cr moment, which decreases smoothly in contrast to the pair of steplike decrease features predicted by density functional theory.

In principle, the use of entangled photons provides enhanced precision in optical phase measurements. This precision significantly surpasses the classical shot-noise limit, and thereby has many possible applications in science and technology. In practice, however, entangled multiphoton states have been generated so far using various intrinsically probabilistic and unscalable processes, thereby counteracting the advantages that the entangled photon states might have. The authors overcome these limitations here by using a quantum knitting machine, based on a single semiconductor quantum dot, to generate a polarization-entangled multiphoton state in a deterministic manner. The multiphoton state is then used to demonstrate super-sensitive optical phase measurement. The results pave the way for realizing genuine quantum enhanced optical measurements in the very near future.

Maximally localized Wannier functions (MLWFs) form a valuable alternative to Bloch waves. They offer insight into chemical bonding and form an excellent starting point for a variety of numerical problems. Here, the authors present a robust and automated scheme for the construction of MLWFs from atomiclike orbitals in an all-electron first-principles framework. They demonstrate its applicability to a vast variety of materials covering semiconductors, (semi)metals, and complex organic-inorganic interfaces. This allows them to access electronic single-(quasi)particle energies and wavefunctions for both occupied and unoccupied states at several thousands of arbitrary points in reciprocal space at low computational cost, while retaining high precision.

Two-dimensional (2D) van der Waals magnetic structures provide precious opportunities to study excited-state properties of correlated materials under quantum confinement. This paper reveals enhanced excitonic effects in 2D and bulk CrCl${}_3$, in which the electron-hole (e-h) binding energy reaches more than 2 eV. The calculated exciton positions agree with measurements. Particularly, the e-h binding energy is more sensitive to quantum confinement than the self-energy correction. As a result, an unusual red shift of the “optical gap” is observed in thinner samples, which is opposite to the widely observed blue shift trend.

Electron-electron and electron-phonon interactions break valley symmetry in graphene, favoring a $\nu$ = 0 quantum Hall state. These interactions could result in a canted antiferromagnetic state or a spin-polarized ferromagnetic state with magnetic fields of 15-30 T. By placing graphene on the ferrimagnet Y${}_3$Fe${}_5$O${}_{12}$, the authors lower the magnetic field required to modulate the magnetic state at the zeroth Landau level, and demonstrate tunability of the energy gap of the edge modes in graphene with magnetic fields above 6 T.

For C${}_{60}$, the potential energy surface exhibits a driving force toward the ground state that allows it to be found rapidly. However, the potential energy surface of a cluster can also exhibit features that prevent us from finding its global minimum. In the case of decahedral Si${}_{20}$H${}_{20}$, which is a theoretically postulated stable ground state but was never synthesized, there is no driving force towards the global minimum. The system visits a huge number of low energy structures during its evolution without ever falling into the global minimum. This is exactly the scenario described by Levinthal in his famous paradox about protein folding.

Discovery of new quasiparticles with nontrivial topological field textures, such as polar vortices, skyrmions, and merons, holds promise in technological paradigms. Here, the authors demonstrate a ferroelectric quasiparticle with topological polarization vortices by engineering excess electron polarons. They further show that the electron polaron carries a magnetic moment coupled with ferrotoroidicity, i.e., the magnetoelectric effect. This result provides insight into the ultimate miniaturization of ferrotoroic materials and a new class of functional polaron families.

Helium-4 confined in disordered restricted geometries such as porous media exhibits a new intermediate phase between Mott insulator and superfluid.

Matthew P. A. Fisher, Peter B. Weichman, G. Grinstein, and Daniel S. Fisher
Phys. Rev. B 40, 546 (1989)

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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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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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