Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped $p$-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped $p$-type Si(111) wafer is superconducting with a critical temperature of $4.7\pm 0.3\mathrm{K}$. While a phonon-mediated coupling scenario would be consistent with the observed $T_c$, Mott correlations in the Sn-derived dangling-bond surface state could suppress the $s$-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional.

In the quest to image the three-dimensional magnetization structure we show that the technique of magnetic small-angle neutron scattering (SANS) is highly sensitive to the details of the internal spin structure of nanoparticles. By combining SANS with numerical micromagnetic computations we study the transition from single-domain to multidomain behavior in nanoparticles and its implications for the ensuing magnetic SANS cross section. Above the critical single-domain size we find that the cross section and the related correlation function cannot be described anymore with the uniform particle model, resulting, e.g., in deviations from the well-known Guinier law. In the simulations we identify a clear signature for the occurrence of a vortexlike spin structure at remanence. The micromagnetic approach to magnetic SANS bears great potential for future investigations, since it provides fundamental insights into the mesoscale magnetization profile of nanoparticles.

Motivated by multiple possible physical realizations, we study the $\mathrm{SU}(4)$ quantum antiferromagnet with a fundamental representation on each site of the triangular lattice. We provide evidence for a gapless liquid ground state of this system with an emergent Fermi surface of fractionalized fermionic partons coupled with a U(1) gauge field. Our conclusions are based on numerical simulations using the density matrix renormalization group method, which we support with a field theory analysis.

Using the ab initio local force method, we investigate the formation mechanism of the helical spin structure in ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ and ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$. We calculate the paramagnetic spin susceptibility and find that the Fermi surface nesting is not the origin of the incommensurate modulation, in contrast to the naive scenario based on the Ruderman-Kittel-Kasuya-Yosida mechanism. We then decompose the exchange interactions between the Gd spins into each orbital component, and show that spin-density-wave type interaction between the $\mathrm{Gd}\text{−}5d$ orbitals is ferromagnetic, but the interaction between the $\mathrm{Gd}\text{−}4f$ orbitals is antiferromagnetic. We conclude that the competition of these two interactions, namely, the interorbital frustration, stabilizes the finite-$\mathit{Q}$ structure.

We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-$\frac{1}{2}$ system: ${\mathrm{Y}}_2{\mathrm{CuTiO}}_6$. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie-Weiss scale $(-{\theta }_{\mathrm{CW}})$ of $\sim 134\mathrm{K}$. We observe scaling collapses of the magnetic field and temperature dependent magnetic heat capacity and magnetization data, respectively, in conformity with expectations from the random singlet physics. Our experiments establish the suppression of any freezing scale, if at all present, by more than 3 orders of magnitude, opening a plethora of interesting possibilities such as disorder stabilized long range quantum entangled ground states.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic ${\mathrm{BiFe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ solid solutions with $0.25\le x\le 0.50$ (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1–1.0 GPa, depending on the composition, the electric polarization of bulk BFCO increases by $\sim 150\%$. The origin of such an exceptional behavior is a phase transformation involving a low-$T$ rhombohedral ($\mathcal{R}$) phase and a high-$T$ supertetragonal ($\mathcal{T}$) phase. Both $\mathcal{R}$ and $\mathcal{T}$ phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 ${\mu }_B$ per formula unit. Contrary to what occurs in either bulk ${\mathrm{BiFeO}}_3$ or ${\mathrm{BiCoO}}_3$, the $\mathcal{T}$ phase is stabilized over the $\mathcal{R}$ by increasing temperature due to its higher vibrational entropy. This extraordinary $T$-induced $\mathcal{R}\to \mathcal{T}$ phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Solids built out of active components can exhibit nonreciprocal elastic coefficients that give rise to non-Hermitian wave phenomena. Here, we investigate non-Hermitian effects present at the boundary of two-dimensional active elastic media obeying two general assumptions: their microscopic forces conserve linear momentum and arise only from static deformations. Using continuum equations, we demonstrate the existence of the non-Hermitian skin effect in which the boundary hosts an extensive number of localized modes. Furthermore, lattice models reveal non-Hermitian topological transitions mediated by exceptional rings driven by the activity level of individual bonds.

We consider the problem of local operations and classical communication (LOCC) discrimination between two bipartite pure states of fermionic systems. We show that, contrary to the case of quantum systems, for fermionic systems it is generally not possible to achieve the ideal state discrimination performances through LOCC measurements. On the other hand, we show that an ancillary system made of two fermionic modes in a maximally entangled state is a sufficient additional resource to attain the ideal performances via LOCC measurements. The stability of the ideal results is studied when the probability of preparation of the two states is perturbed, and a tight bound on the discrimination error is derived.

The NV-NMR spectrometer is a promising candidate for detection of NMR signals at the nanoscale. Field inhomogeneities, however, are a major source of noise that limits spectral resolution in state of the art NV-NMR experiments and constitutes a major bottleneck in the development of nanoscale NMR. Here we propose, a route in which this limitation could be circumvented in NV-NMR spectrometer experiments, by utilizing the nanometric scale and the quantumness of the detector.

To understand the nature of the brightest $\gamma$-ray binary system LS 5039, hard x-ray data of the object, taken with the Suzaku and NuSTAR observatories in 2007 and 2016, respectively, were analyzed. The two data sets jointly gave tentative evidence for a hard x-ray periodicity, with a period of $\sim 9\mathrm{s}$ and a period increase rate by $\sim 3\times {10}^{-10}\mathrm{s}{\mathrm{s}}^{-1}$. Therefore, the compact object in LS 5039 is inferred to be a rotating neutron star, rather than a black hole. Furthermore, several lines of arguments suggest that this object has a magnetic field of several times $\sim {10}^{10}\mathrm{T}$, two orders of magnitude higher than those of typical neutron stars. The object is hence suggested to be a magnetar, which would be the first to be found in a binary. The results also suggest that the highly efficient particle acceleration process, known to be operating in LS 5039, emerges through interactions between dense stellar winds from the massive primary star, and ultrastrong magnetic fields of the magnetar.

The no-hair theorem by Mayo and Bekenstein states that there exists no nonextremal static and spherical charged black hole endowed with hair in the form of a charged scalar field with a self-interaction potential. In our recent work [Phys. Lett. B 803, 135324 (2020)], we showed that the effect of a scalar mass term is important at an asymptotic infinity, which was omitted to prove the no-hair theorem. In this Letter, we demonstrate that there actually exists static and spherical charged scalar hair, dubbed as $Q$ hair, around charged black holes, by taking into account the backreaction to the metric and gauge field. We also discuss that $Q$ cloud, which is constructed without the backreaction around a Reissner-Nordström black hole, is a good approximation to $Q$ hair under a certain limit.

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of ${\theta }_{\mathrm{lab}}=7.9°$ and a mean energy of 1.149 GeV. The asymmetry result is $B_n=-5.194\pm 0.067(\mathrm{stat})\pm 0.082$ (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (${\theta }_{\mathrm{lab}}\to 0$) where they should be most reliable.

A recently modified method to enable low-energy nuclear scattering results to be extracted from the discrete energy levels of the target-projectile clusters confined by harmonic potential traps is tested. We report encouraging results for neutron-$\alpha$ and neutron-${}^{24}\mathrm{O}$ elastic scattering from analyzing the trapped levels computed using two different ab initio nuclear structure methods. The $n\text{−}\alpha$ results have also been checked against a direct ab initio reaction calculation. The $n\text{−}{}^{24}\mathrm{O}$ results demonstrate the approach’s applicability for a large range of systems provided their spectra in traps can be computed by ab initio methods. A key ingredient is a rigorous understanding of the errors in the calculated energy levels caused by inevitable Hilbert-space truncations in the ab initio methods.

The backreaction of dispersed rigid fibers to turbulence is analyzed by means of a state-of-the-art fully coupled immersed boundary method. The following universal scenario is identified: turbulence at large scales looses a consistent part of its kinetic energy (via a Darcy friction term), which partially reappears at small scales where a new range of energy-containing scales does emerge. Large-scale mixing is thus depleted in favor of a new mixing mechanism arising at the smallest scales. Anchored fibers cause the same backreaction to turbulence as moving fibers of large inertia. Our results thus provide a link between two apparently separated realms: the one of porous media and the one of suspension dynamics.

Ions in ionic liquids and concentrated electrolytes reside in a crowded, strongly interacting environment, leading to the formation of discrete layers of charges at interfaces and spin-glass structure in the bulk. Here, we propose a simple theory that captures the coupling between steric and electrostatic forces in ionic liquids. The theory predicts the formation of discrete layers of charge at charged interfaces. Further from the interface, or at low polarization of the electrode, the model outputs slowly decaying oscillations in the charge density with a wavelength of a single ion diameter, as shown by analysis of the gradient expansion. The gradient expansion suggests a new structure for partial differential equations describing the electrostatic potential at charged interfaces. We find quantitative agreement between the theory and molecular simulations in the differential capacitance and concentration profiles.

The exciton-phonon coupling in highly oriented pyrolytic graphite is studied using resonant inelastic x-ray scattering (RIXS) spectroscopy. With $\sim 70\mathrm{meV}$ energy resolution, multiple low energy excitations associated with coupling to phonons can be clearly resolved in the RIXS spectra. Using resonance dependence and the closed form for RIXS cross section without considering the intermediate state mixing of phonon modes, the dimensionless coupling constant $g$ is determined to be 5 and 0.35, corresponding to the coupling strength of $0.42\mathrm{eV}+/-20\mathrm{meV}$ and $0.20\mathrm{eV}+/-20\mathrm{meV}$, for zone center and boundary phonons, respectively. The reduced $g$ value for the zone-boundary phonon may be related to its double resonance nature.

We propose a diagnostic for finite temperature topological order using “topological entanglement negativity,” the long-range component of a mixed-state entanglement measure. As a demonstration, we study the toric code model in $d$ spatial dimensions for $d=2$,3,4, and find that when topological order survives thermal fluctuations, it possesses a nonzero topological entanglement negativity, whose value is equal to the topological entanglement entropy at zero temperature. Furthermore, we show that the Gibbs state of 2D and 3D toric code at any nonzero temperature, and that of 4D toric code above a certain critical temperature, can be expressed as a convex combination of short-range entangled pure states, consistent with the absence of topological order.

An odd-occupied quantum dot in a Josephson junction can flip transmission phase, creating a $\pi$ junction. When the junction couples topological superconductors, no phase flip is expected. We investigate this and related effects in a full-shell hybrid interferometer, using gate voltage to control dot-junction parity and axial magnetic flux to control the transition from trivial to topological superconductivity. Enhanced zero-bias conductance and critical current for odd parity in the topological phase reflects hybridization of the confined spin with zero-energy modes in the leads.