A highly ordered $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}\left(100\right)\mathrm{c}(2\times 2)$ two-dimensional compound was found to form upon room-temperature adsorption of 1.0 monolayer (ML) of Au onto a $\mathrm{Tl}/\mathrm{Si}\left(100\right)2\times 1$ surface with 1.0 ML of Tl. The compound was determined to have a double-atomic-layer structure where Au and Tl atoms constitute the bottom and top layers, respectively. The compound is metallic due to the presence of the two dispersive surface-state bands crossing the Fermi level. The bands are spin split, and the peculiar feature of their spin texture is the occurrence of exclusively in-plane spin components that is originated from the ${\mathrm{C}}_{2v}$ symmetry of the system. One of the bands has a typical vortical spin texture, whereas the other exhibits an unusual nonvortical spin texture.

We study the normal vibrations of $2\times 2$ and $3\times 3$ potassium iodide (KI) crystalline nanowires embedded in single-walled carbon nanotubes (SWCNTs) by density functional theory calculations, accompanied by Raman spectroscopy measurements on SCWNTs infiltrated with KI. The calculated stand-alone KI nanowires are found to be fully stable, while their vibrational energies are very close to the additional low-frequency Raman lines experimentally detected in the KI@SCWNT systems and ascribed to the encapsulated KI crystals. The difference between the relaxed and observed geometries of the KI nanowires evidences an electron transfer from these nanowires to the surrounding SWCNTs. Calculated radial and tangential vibrational frequencies of KI nanowires compared to those of supplementary discovered Raman lines suggest that the nanowire-SWCNT interaction is of a van der Waals origin and exclude the possibility of functionalization between the KI nanocrystals and the nanotube walls.

A series of metamolecular structures is proposed to mimic chemical bonds in molecular physics. The coupling between different modes of each meta-atom can be achieved by adjusting their spacing and sizes. A gearlike $A{B}_6$ structure is then used to design a metamolecular crystal for intriguing band structures by simply rotating the metamolecule cell. In the process, Dirac degeneracy is achieved in higher-order compound modes, together with the edge states that take place at the interface between two topologically distinct metamolecular crystals in a metal background. Our design can help to analogize the orbital hybridization and related rich physics of polyatomic molecules in photonic systems.

Modern technological advances allow for the study of systems with additional synthetic dimensions. Using such approaches, higher-dimensional physics that was previously deemed to be of purely theoretical interest has now become an active field of research. In this work, we derive from first principles using a semiclassical equation-of-motion approach the bulk response of a six-dimensional Chern insulator. We find that in such a system a quantized bulk response appears with a quantization originating from a six-dimensional topological index: the third Chern number. Alongside this unique six-dimensional response, we rigorously describe the lower even-dimensional Chern-type responses that can occur due to nonvanishing first and second Chern numbers in subspaces of the six-dimensional space. Last, we propose how to realize such a bulk response using three-dimensional topological charge pumps in cold atomic systems.

Logarithmic relaxation is a unique relaxation process exhibited by a few molecular liquids and biomolecules. However, the microscopic origin of logarithmic relaxation is still unclear. To understand the origin of this process, we studied two liquids that exhibit logarithmic relaxation in a dissolved state using quasielastic neutron scattering (QENS) and depolarized dynamic light scattering (DDLS). Although the intermolecular potential of the liquids is drastically different in the dissolved state from the bulk liquids, we observed that the logarithmic relaxation still persists. Our results indicate that the intermolecular potential does not play a role in determining the logarithmic relaxation process. The coupling of rotational and translational relaxation processes could be the origin of the logarithmic relaxation process exhibited by the molecular liquids.

The orbital magnetic moment is still surprisingly not well understood, in contrast to the spin part. Its description in finite systems, such as isolated atoms and molecules, is not problematic, but it was only recently that a rigorous picture was provided for extended systems. Here we focus on an intermediate class of systems: magnetic adatoms placed on a nonmagnetic surface. We show that the essential quantity is the ground-state charge current density, in the presence of spin-orbit coupling, and set out its first-principles description. This is illustrated by studying the magnetism of the surface Pt electrons, induced by the presence of Cr, Mn, Fe, Co, and Ni adatoms. A physically appealing partition of the charge current is introduced. This reveals that there is an important interatomic contribution to the orbital moments of the Pt atoms, extending three times as far from each magnetic adatom as the induced spin and atomic orbital moments. We find that it is as sizable as the latter and attribute its origin to a spin-orbital susceptibility of the Pt surface, different from the one responsible for the formation of the atomic orbital moments.

We theoretically establish mutual relations among magnetic momentum, heat, and fluctuations of propagating magnons in a ferromagnetic insulating junction in terms of noise and the bosonic Wiedemann-Franz (WF) law. Using the Schwinger-Keldysh formalism, we calculate all transport coefficients of a noise spectrum for both magnonic spin and heat currents, and establish Onsager relations between the thermomagnetic currents and the zero-frequency noise. Making use of the magnonic WF law and the Seebeck coefficient in the low-temperature limit, we theoretically discover universal relations, i.e., being independent of material parameters, for both the nonequilibrium and equilibrium noise, and show that each noise is described solely in terms of thermal conductance. Finally, we introduce a magnonic spin analog of the Fano factor, noise-to-current ratio, and demonstrate that the magnonic spin-Fano factor reduces to a universal value in the low-temperature limit and it remains valid even beyond a linear response regime.

The demonstration of a quasi-two-dimensional electron gas (2DEG) and superconducting properties in ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ heterostructures has stimulated intense research activity in the last ten years. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. The superconductivity in such heterostructures has been observed below 300 mK. For superconductivity applications it is desirable to have more wide temperature of the existence range and the ability to control superconducting properties by external stimulus. Based on first-principles calculations and theoretical consideration we show that all-oxide heterostructures incorporating a ferroelectric constituent, such as ${\mathrm{BaTiO}}_3$/${\mathrm{La}}_2{\mathrm{CuO}}_4$, allow creating 2DEG. We predict a possibility of a high-temperature quasi-two-dimensional superconducting state. This state could be switchable between superconducting and conducting states by ferroelectric polarization reversal. We also discuss that such structures must be more simple for preparation. The proposed concept of a ferroelectrically controlled interface superconductivity offers the possibility to design novel electronic devices.

When certain elemental metals such as Cu, Sr, and Nb are intercalated between the layers of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, a topological insulator, the intercalated systems superconduct with critical temperatures around 3 K. Naturally, in all these cases the possibility of topological superconductivity was suggested and explored. However, in cases of Cu and Sr intercalated systems, the low-temperature scanning tunneling microscopy (STM) experiments revealed fully formed gaps where no signature of low-energy states, a requisite for topological superconductivity, was found. Here, through STM spectroscopy down to 400 mK we show that in ${\mathrm{Nb}}_x\text{−}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ ($x=0.25$), the spectra deviate from a BCS-like behavior, and the tunneling conductance at low bias is large. Our observations are consistent with the idea that the order parameter of ${\mathrm{Nb}}_x\text{−}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ is nodal. Therefore, our results conclude that compared with other members of the family, ${\mathrm{Nb}}_x\text{−}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ has a stronger possibility of being a topological superconductor.

At ambient conditions, alkali metals adopt the body centered cubic structure, while if compressed up to tens of GPa and above, they exhibit complex low-symmetry modifications, due to the density-driven transition of the valence electrons from the $s$ state to states of higher angular momentum. These high-pressure, low-symmetry phases, whose unit cells may include up to tens of atoms, allow rich Raman activity, which was previously observed only in lighter alkalis Na and Li. Here we report an extensive study of the optical phonons of highly dense Rb up to 100 GPa in diamond anvil cells, conducted by challenging experimental Raman spectroscopy measurements and ab initio computer simulations. The relative (relative to the normal condition value) density behavior of Raman frequencies of Rb is compared to that of Na and Li, once the frequencies of the two light alkali elements have been rescaled by $\sqrt{M_{\mathrm{Na}}/M_{\mathrm{Rb}}}$ and $\sqrt{M_{\mathrm{Li}}/M_{\mathrm{Rb}}}$, respectively, where $M_{\mathrm{Na}}$, $M_{\mathrm{Li}}$, and $M_{\mathrm{Rb}}$ are the atomic masses of the here considered alkali elements. Importantly, while the rescaled density behaviors of Na and Li agree with each other, Rb significantly differs, which highlights the different nature of the valence electron transition being of the $s\text{−}d$ and of the $s\text{−}p$ type in heavy and light alkali metals, respectively, a result that calls for further similar investigations of K and Cs.

We study dynamics of antiferromagnets induced by simultaneous application of dc spin current and ac charge current, motivated by the requirement of all-electrically controlled devices in the terahertz (THz) gap (0.1–30 THz). We show that ac electric current, via Néel spin-orbit torques, can lock the phase of a steady rotating Néel vector whose precession is controlled by a dc spin current. In the phase-locking regime the frequency of the incoming ac signal coincides with the frequency of auto-oscillations, which for typical antiferromagnets falls into the THz range. The frequency of auto-oscillations is proportional to the precession-induced tilting of the magnetic sublattices related to the so-called dynamical magnetization. We show how the incoming ac signal can be detected and formulate the conditions of phase locking. We also show that the rotating Néel vector can generate ac electrical current via inverse Néel spin-orbit torque. Hence, antiferromagnets driven by dc spin current can be used as tunable detectors and emitters of narrow-band signals operating in the THz range.

We present a fully ab initio study of the laser-induced ultrafast magnetization dynamics in cobalt-benzene clusters ${\mathrm{Co}}_m{\mathrm{Bz}}_n^{+/0}$ ($m,n=1,2$). It is found that different geometric configurations and $d-\pi$ interactions of the four structures cause their high-level electronic structures to exhibit distinct characteristics. By using well-tailored laser pulses to propagate the systems via a spin-orbit-coupling enabled $\mathrm{\Lambda }$ process, various local/global spin-flip scenarios are achieved within subpicosecond timescales. Besides, a spin-flip-transfer process within around 300 fs is predicted in ${\mathrm{Co}}_2\mathrm{Bz}$. Governed by the Goodenough-Kanamori rules, the simultaneous spin flip and transfer is favored when the two magnetic centers are antiferromagnetically coupled, reflected in the required lowest laser energy and intensity. Most interestingly, we obtain a novel, $W$-process facilitated spin-crossover scenario in the cluster ${\mathrm{CoBz}}^{+}$, which offers additional flexibility of designing relevant all spin-based functions such as molecular switches and sensors. The transferability of the latter two scenarios is also analyzed from the optical spectra of the initial and final states. These results provide the precursory guidance for further study of ultrafast magnetization control in more extended metal-aromatic polymers, and also show their great potential in future nanoscale spintronic applications and quantum information processing.

In noncentrosymmetric superconductors, spin-orbit coupling can induce an unconventional superconducting state with a mixture of $s$-wave spin-singlet and $p$-wave spin-triplet channels. It is commonly thought that inversion symmetry breaking is substantial for mixed-pairing superconducting states. In this paper, we theoretically propose that a new type of mixed-pairing state, namely the mixture of $s$-wave spin-singlet and $d$-wave spin-quintet channels, can occur even in the presence of inversion symmetry when electrons effectively carry “spin-3/2.” As a physical consequence of the singlet-quintet pairing mixing, topological nodal-line superconductivity is found in such a system and gives rise to flat-surface Majorana bands. Our work provides a possible explanation of unconventional superconducting behaviors observed in superconducting half-Heusler compounds and suggests that these superconducting materials provide a new platform for exploring unconventional and topological superconductivity.

We study nonchiral wave functions for systems with continuous spins obtained from the conformal field theory (CFT) of a free, massless boson. In contrast to the case of discrete spins, these can be treated as bosonic Gaussian states, which allows us to efficiently compute correlations and entanglement properties both in one (1D) and in two spatial dimensions (2D). In 1D, the computed entanglement entropy and spectra are in agreement with the underlying CFT. Furthermore, we construct a 1D parent Hamiltonian with a low-energy spectrum corresponding to that of a free, massless boson. In 2D, we find edge excitations in the entanglement spectrum, although the states do not have intrinsic topological order, as revealed by a determination of the topological entanglement entropy.

Using ab initio crystal structure prediction we study the high-pressure phase diagram of $A{\mathrm{BiO}}_3$ bismuthates ($A=\mathrm{Ba}$, Sr, Ca) in a pressure range up to 100 GPa. All compounds show a transition from the low-pressure perovskite structure to highly distorted, low-symmetry phases at high pressures (PD transition), and remain charge-disproportionated and insulating up to the highest pressure studied. The PD transition at high pressures in bismuthates can be understood as a combined effect of steric arguments and of the strong tendency of bismuth to charge-disproportionation. In fact, distorted structures permit to achieve a very efficient atomic packing, and at the same time, to have Bi-O bonds of different lengths. The shift of the PD transition to higher pressures with increasing cation size within the $A{\mathrm{BiO}}_3$ series can be explained in terms of chemical pressure.

Pairing in superconductors occurs in a variety of channels and can be produced by various mechanisms. Here, we show that, in the presence of strong correlations, a singlet-pair superconducting phase can occur in ladder geometries with the fermions in the pairs residing on different sides of a Mott insulating stripe. The antiferromagnet correlations in the Mott stripe provide the pairing “glue” in such a phase. We study, using the density matrix renormalization group method, the ground state of four-leg $t-J$ ladders with Mott insulating stripes in the inner two legs. Pairing and superconductivity are revealed by the presence of negative binding energies and by algebraically decaying interleg singlet-pair correlations, respectively.

We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and nonresonant part, associated, respectively, with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any three-dimensional inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of nontrivial optical torque, we derive closed-form expressions for the induced torque under the quasistatic approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nanomechanical and biomechanical systems.