Light emission from the gap cavity formed by the tip of a scanning tunneling microscope (STM) and a flat metallic sample allows us to probe the dielectric response of metals at the atomic scale and presents a way to distinguish between different materials. The excitation mechanism of the charge carrier oscillations, which ultimately decay into light, is linked to inelastic electron tunneling as opposed to the mostly semiclassical picture of the electromagnetic resonance of the gap cavity. Thus, the observed light emission does not only reflect the electromagnetic resonance of the cavity but also involves the electronic density of states. In this paper, we compare light emission from Cu(111) and Co nanoislands on Cu(111). We find a strong intensity contrast but almost no alteration of the resonance wavelength except close to step edges. Our results show that the light emission from the STM junction is highly sensitive to a few atomic layers of alien material mostly due to the dielectric properties of the layer.

The 203 K superconductivity of ${\mathrm{SH}}_3$ stimulates enormous interest in searching for high-temperature superconductors in compressed hydrides. While several hydrides show high-temperature superconductivity ($>180\mathrm{K}$) at extremely high pressure, it is highly desired to design hydrides that can achieve similar superconductivity at relatively lower pressure. In this work we identify a cubic structure in ternary ${\mathrm{CSH}}_7$ that can be viewed as methane (${\mathrm{CH}}_4$) molecules inserted into the ${\mathrm{SH}}_3$ host lattice as a guest. Ionic bonding in this special host-guest arrangement acts as a chemical precompression for ${\mathrm{SH}}_3$, leading to a much lower predicted dynamically stable pressure of 100 GPa for ${\mathrm{CSH}}_7$ than that of ${\mathrm{SH}}_3$ at 170 GPa. Given the superhigh superconductivity of the parent ${\mathrm{SH}}_3$ sublattice, we performed electron-phonon coupling calculations to estimate the $T_c$ of ${\mathrm{CSH}}_7$ at high pressure, which could reach 181 K at 100 GPa. Our results may shed light on the design principle for other multinary host-guest high-temperature superconducting hydrides with low stable pressure.

We consider the quasiperiodic Aubry-André chain in the insulating regime with localized single-particle states. Adding local interaction leads to the emergence of extended correlated two-particle bound states. We analyze the nature of these states, including their multifractality properties. We use a projected Green function method to compute numerically participation numbers of eigenstates and analyze their dependence on the energy and the system size. We then perform a scaling analysis. We observe multifractality of correlated extended two-particle bound states, which we confirm independently through exact diagonalization.

A theory of the coherent control of optical phonons by double-pulse excitation was modified for the calculation of a large detuning case such as diamond; we calculated the amplitude of the optical phonons without using a rotating-wave approximation. Additionally, we took the frequency chirping and non-Gaussian pulse shape into consideration. The coherent control of the amplitude of 40 THz optical phonons in diamond was demonstrated by using a pair of infrared pulses at a pump-pump delay between $-10$ and 120 fs, and analyzed by using the modified theory.

We study the dynamics of a double quantum dot (DQD) system interacting with a Gaussian white noise (GWN) environment which is measured by a quantum point contact (QPC) device. With both the transverse and longitudinal noise taken into account, we utilize an effective method by adding an additional Bloch vector to calculate the cumulant generating functions of the electron transfer in the QPC detector based on the full counting statistics. We study the average detector current, Fano factor, and average waiting time of the electron transfer in the presence of decoherence effects of the DQD system caused by both the QPC and the GWN environment. It indicates that the decoherence effects arising from the QPC and the GWN environment have obviously different influences on the electron transfer detected by the QPC device in both short-time and long-time limits. It is shown that the measurement process would localize the electron in a DQD in a short time and that the distribution of the average current and Fano factor versus level displacement in long-time limit are broadened due to the interaction between the system and GWN environment, which provides a reliable method to explore the dynamical behavior of an open quantum system and to extract the characteristics of the environment by analyzing the detector outcome. Our results provide theoretical support for studies of quantum measurement in a semiconductor device affected by a fluctuant environment.

The recent discovery of single-ion magnets with magnetic hysteresis above liquid-nitrogen temperatures placed these compounds among the best candidates to realize high-density storage devices. Starting from a prototypical dysprosocenium molecule, showing hysteresis up to 60 K, we derive here a general recipe to design high-blocking-temperature rare-earth single-ion magnets. The complex magnetic relaxation is unraveled by combining magnetization and nuclear magnetic resonance measurements with inelastic neutron scattering experiments and ab initio calculations, thus disentangling the different mechanisms and identifying the key ingredients behind slow relaxation.

The magnetic nature of the quasi-one-dimensional ${\mathrm{BaVS}}_3$ has been studied as a function of temperature down to 0.25 K and pressure up to 1.97 GPa on a powder sample using the positive muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) technique. At ambient pressure, ${\mathrm{BaVS}}_3$ enters an incommensurate antiferromagnetic ordered state below the $\mathrm{N}\acute{\mathrm{e}}\mathrm{el}$ temperature $\left(T_{\mathrm{N}}\right)31\mathrm{K}$. $T_{\mathrm{N}}$ is almost constant as the pressure ($p$) increases from ambient pressure to 1.4 GPa, then $T_{\mathrm{N}}$ decreases rapidly for $p>1.4\mathrm{GPa}$, and finally disappears at $p\sim 1.8\mathrm{GPa}$, above which a metallic phase is stabilized. Hence, $T_{\mathrm{N}}$ is found to be equivalent to the pressure-induced metal-insulator transition temperature ($T_{\mathrm{MI}}$) at $p>1.4\mathrm{GPa}$.

Inspired by the recent results regarding whether the Harris criterion is valid for quantum spin systems, we have simulated a two-dimensional spin-1/2 Heisenberg model on the square lattice with a specific kind of quenched disorder using the quantum Monte Carlo calculations. In particular, the considered quenched disorder has a tunable parameter $0\le p\le 1$ which can be considered as a measure of randomness. Interestingly, when the magnitude of $p$ increases from 0 to 0.95, at the associated quantum phase transitions the numerical value of the correlation length exponent $\nu$ grows from a number compatible with the $O\left(3\right)$ result 0.7112(5) to a number slightly greater than 1. In other words, by varying $p$, $\nu$ can reach an outcome between 0.7112(5) and 1 (or greater). Furthermore, among the studied values of $p$, all the associated $\nu$ violate the Harris criterion except the ones corresponding to $p\ge 0.9$. Considering the form of the employed disorder here, the above described scenario should remain true for other randomness if it is based on an idea similar to the one used in this study. This is indeed the case according to our preliminary results stemming from investigating another disorder distribution.

The point-contact-spectroscopy measurement is a powerful method to detect the superconducting gap and the spin polarization of materials. However, it is difficult to get a stable and clean point contact by conventional techniques. In this work, we fabricate multiple point contacts by depositing Au nanoparticle arrays on the surface of a superconductor through an anodic aluminum oxide patterned shadow mask. We obtained the superconducting gaps of niobium nitride thin film (NbN, $T_c=16$ K) and iron superconductors ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$ single crystals (Ca-1111, $T_c=21.3$ K) by fitting the point-contact spectroscopy with the Blonder-Tinkham-Klapwijk theory. We found that NbN's gap ($\mathrm{\Delta }$) exhibits the BCS-like temperature dependence with $\mathrm{\Delta }\approx 2.88$ meV at 0 K and $2\mathrm{\Delta }/k_B{T}_c\approx 4.22$ in agreement with previous reports. By contrast, Ca-1111 has a multigap structure with ${\mathrm{\Delta }}_1\approx 1.99$ meV and ${\mathrm{\Delta }}_2\approx 5.01$ meV at 0 K, and the ratio between the superconducting gap and $T_c$ is $2{\mathrm{\Delta }}_1/k_B{T}_c=2.2$ and $2{\mathrm{\Delta }}_2/k_B{T}_c=5.5$, suggesting an unconventional paring mechanism of Ca-1111 also in agreement with previous reports on other Fe-based superconductors. Our multiple point-contacts method thus provides an alternative way to perform measurements of the superconducting gap.

We report a $\mu \mathrm{SR}$ study on the antiperovskite oxide superconductor ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$. By using transverse-field $\mu \mathrm{SR}$, we observed an increase of the muon relaxation rate upon cooling below the superconducting transition temperature $T_{\mathrm{c}}=5.4$ K, evidencing bulk superconductivity. The exponential temperature dependence of the relaxation rate $\sigma$ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth $\lambda \left(0\right)\propto 1/\sqrt{\sigma \left(0\right)}$ to be around 320–1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal. Moreover, we established that the ratio $T_{\mathrm{c}}/\lambda {\left(0\right)}^{-2}$ is larger than those of ordinary superconductors and is comparable to those of unconventional superconductors. The relatively high $T_{\mathrm{c}}$ for small carrier density may hint at an unconventional pairing mechanism beyond the ordinary phonon-mediated pairing. In addition, zero-field $\mu \mathrm{SR}$ did not provide evidence of broken time-reversal symmetry in the superconducting state. These features are consistent with the theoretically proposed topological superconducting state in ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$, as well as with ordinary $s$-wave superconductivity.

The phase of the wave function of charged matter is sensitive to the value of the electric potential, even when the matter never enters any region with nonvanishing electromagnetic fields. Despite its fundamental character, this archetypal electric Aharonov-Bohm effect has evidently never been observed. We propose an experiment to detect the electric potential through its coupling to the superconducting order parameter. A potential difference between two superconductors will induce a relative phase shift that is observable via the DC Josephson effect even when no electromagnetic fields ever act on the superconductors, and even if the potential difference is later reduced to zero. This is a type of electromagnetic memory effect, and would directly demonstrate the physical significance of the electric potential.

We study the temporal evolution of the mutual information (MI) in a one-dimensional Kitaev chain, coupled to a fermionic Markovian bath, subsequent to a global quench of the chemical potential. In the unitary case, the MI (or equivalently the bipartite entanglement entropy) saturates to a steady-state value (obeying a volume law) following a ballistic growth. On the contrary, we establish that in the dissipative case the MI is exponentially damped both during the initial ballistic growth as well as in the approach to the steady state. We observe that even in a dissipative system, postquench information propagates solely through entangled pairs of quasiparticles having a finite lifetime; this quasiparticle picture is further corroborated by the out-of-equilibrium analysis of two-point fermionic correlations. Remarkably, in spite of the finite lifetime of the quasiparticles, a finite steady-state value of the MI survives in asymptotic times which is an artifact of nonvanishing two-point correlations. Further, the finite lifetime of quasiparticles renders to a finite length scale in these steady-state correlations.

We demonstrate an anomalous spin-orbit torque induced by the broken magnetic symmetry in the antiferromagnet IrMn. We study the magnetic structure of three phases of IrMn thin films using neutron diffraction technique. The magnetic mirror symmetry $\mathcal{M}{}^{\prime }$ is broken laterally in both $\mathrm{L}{1}_0\text{−}\mathrm{IrMn}$ and $\mathrm{L}{1}_2\text{−}\mathrm{IrM}{\mathrm{n}}_3$ but not $\gamma \text{−}\mathrm{IrM}{\mathrm{n}}_3$. We observe an out-of-plane dampinglike spin-orbit torque in both $\mathrm{L}{1}_0\text{−}\mathrm{IrMn}$/permalloy and $\mathrm{L}{1}_2\text{−}\mathrm{IrM}{\mathrm{n}}_3$/permalloy bilayers but not in $\gamma \text{−}\mathrm{IrM}{\mathrm{n}}_3$/permalloy. This is consistent with both the symmetry analysis on the effects of a broken $\mathcal{M}{}^{\prime }$ on spin-orbit torque and the theoretical predictions of the spin Hall effect and the Rashba-Edelstein effect. In addition, the measured spin-orbit torque efficiencies are 0.61 ± 0.01, 1.01 ± 0.03, and 0.80 ± 0.01 for the $\mathrm{L}{1}_0,\mathrm{L}{1}_2$, and γ phases, respectively. Our work highlights the critical roles of the magnetic asymmetry in spin-orbit torque generation.

We study the topological phase in a dipolar-coupled two-dimensional breathing square lattice of magnetic vortices. By evaluating the quantized Chern number and ${\mathbb{Z}}_4$ Berry phase, we obtain the phase diagram and identify that the second-order topological corner states appear only when the ratio of alternating bond lengths goes beyond a critical value. Interestingly, we uncover three corner states at different frequencies ranging from subgigahertz to tens of gigahertz by solving the generalized Thiele's equation, which has no counterpart in natural materials. We show that the emerging corner states are topologically protected by a generalized chiral symmetry of the quadripartite lattice, leading to particular robustness against disorder and defects. Full micromagnetic simulations confirm theoretical predictions with great agreement. A vortex-based display device is designed as a demonstration of the real-world application of the second-order magnetic topological insulator. Our findings provide a route for realizing symmetry-protected multiband corner states that are promising to achieve spintronic higher-order topological devices.

We study theoretically the Josephson effect between two two-band superconductors respecting time-reversal symmetry, where we assume a spin-singlet $s$-wave pair potential in each conduction band. The superconducting phase at the first band ${\phi }_1$ and that at the second band ${\phi }_2$ characterize a two-band superconducting state. We consider a Josephson junction where an insulating barrier separates two such two-band superconductors. By applying the tunnel Hamiltonian description, the Josephson current is calculated in terms of the anomalous Green's function on either side of the junction. We find that the Josephson current consists of three components which depend on three types of phase differences across the junction: the phase difference at the first band $\delta {\phi }_1$, the phase difference at the second band $\delta {\phi }_2$, and the difference at the center-of-mass phase $(\delta {\phi }_1+\delta {\phi }_2)/2$. A Cooper pair generated by the band hybridization carries the last current component. We discuss the relation between the Josephson current calculated in theories and that observed in experiments.

We present resistivity measurements of bilayer and trilayer films with layer-dependent Rashba spin-orbit interactions. A sharp upturn is observed in the temperature dependence of the parallel upper critical field. We show that it corresponds to a transition from a complex-stripe phase to a helical phase. Moreover, we propose the phase diagram of a multilayer system, which is obtained from experimental observations with the support of numerical calculations. These results pave the way for the clarification of non-trivial superconducting states in multilayer systems composed of two-dimensional Rashba superconductors.

In this paper, we study the statistics of permutation cycles of ground-state hardcore lattice bosons described by various two-dimensional Bose-Hubbard-type models on both square and kagome lattices. We find that it is possible to differentiate quantum phases by the statistics of permutations cycles. Indeed, features in the permutation cycles statistics can be used to uniquely identify certain insulating phases and are consistent with local resonances of occupation numbers in the ground-state expansion of the phase. We also confirm that suitable quantities derived from the probability distribution of the length of permutation cycles can be used to detect superfluid to insulator phase transitions.

Metamaterials exhibit optical properties not observed in traditional materials. Such behavior emerges from the interaction of light with precisely engineered subwavelength features built from different constituent materials. Recent research into the design and fabrication of metamaterial-based devices has established a foundation for the next generation of functional materials. Of particular interest is the all-dielectric metasurface, a two-dimensional metamaterial exploiting shape-dependent resonant features while avoiding losses through the use of dielectric building blocks. However, even this simple metamaterial class has a nearly infinite number of possible configurations; researchers now require new methods to efficiently explore these types of design spaces. In this work, we employ rigorous coupled wave analysis to calculate reflection and transmission spectra associated with a class of open-cylinder all-dielectric metasurface. By altering the geometric parameters of open-cylinder metasurfaces, we generate a sparse training data set and construct artificial neural networks capable of relating metasurface geometries to reflection and transmission spectra. Here, we successfully demonstrate that pseudo autodecoder neural networks can suggest device geometries based on a requested optical performance—inverting the design process for this metasurface class. As an example, we query for and discover a particular open-cylinder metasurface displaying a reflection band $R\ge 99\%$ centered at ${\lambda }_0=1550\mathrm{nm}$ that is much broader $\mathrm{\Delta }\lambda =450\mathrm{nm}$ than anything reported for optical metasurfaces. We then analyze the modal interplay in the open-cylinder metasurface to better understand the underlying physics driving the broadband behavior. Ultimately, we conclude that neural networks are ideally suited for generally approaching these types of complex inverse design problems.