The London penetration depth $\lambda \left(T\right)$ was measured in a single crystal of ${\mathrm{V}}_3\mathrm{Si}$. The superfluid density obtained from this measurement shows a distinct signature of two almost decoupled superconducting gaps. This alone is insufficient to distinguish between $s_{\pm }$ and $s_{++}$ pairing states, but it can be achieved by studying the effect of controlled nonmagnetic disorder on the superconducting transition temperature $T_c$. For this purpose, the same ${\text{V}}_3\text{Si}$ crystal was sequentially irradiated by 2.5-MeV electrons three times, repeating the measurement between the irradiation runs. A total dose of $10\mathrm{C}/{\mathrm{cm}}^2$ ($6.24\times {10}^{19}$ $\text{electrons}/{\text{cm}}^2$) was accumulated, for which $T_c$ changed from 16.4 K in a pristine state to 14.7 K ($9.3\%$). Not only is this substantial suppression impossible for a single isotropic gap, but also it is not large enough for a sign-changing $s_{\pm }$ pairing state. Our electronic band structure calculations show how five bands crossing the Fermi energy can be naturally grouped to support two effective gaps, not dissimilar from the physics of iron pnictides. We analyze the results using two-gap models for both $\lambda \left(T\right)$ and $T_c$ which describe the data very well. Thus the experimental results and theoretical analysis provide strong support for an $s_{++}$ superconductivity with two unequal gaps, ${\mathrm{\Delta }}_1\left(0\right)\approx 2.53\text{meV}$ and ${\mathrm{\Delta }}_2\left(0\right)\approx 1.42\text{meV}$, and a very weak interband coupling in the ${\text{V}}_3\text{Si}$ superconductor.

We study the ground state of the doped Hubbard model on the honeycomb lattice in the small-doping and strongly interacting region. The nature of the ground state after doping holes into antiferromagnetic Mott insulating states on the honeycomb lattice remains a long-standing unsolved issue, even though tremendous effort has been spent investigating this challenging problem. In this work, we employ two complementary, state-of-the-art, many-body computational methods: constrained path (CP) auxiliary-field quantum Monte Carlo (AFQMC) with self-consistent constraint and density matrix renormalization group (DMRG) methods. Systematic and detailed cross validations are performed between these two methods for narrow systems for which DMRG can produce reliable results. AFQMC is then utilized to study wider systems to investigate the thermodynamic limit properties. The ground state is found to be a half-filled stripe state in the small-doping and strongly interacting region. The pairing correlation shows $d$-wave symmetry locally but decays exponentially with the distance between two pairs.

We report the electron dynamics of the recently discovered quasi-one-dimensional Kondo lattice compound ${\mathrm{CeCo}}_2{\mathrm{Ga}}_8$ by optical spectra measurements using polarized light. For an electric field along the $c$ axis, the high-temperature Drude-like peak is found to split into two distinct components below the coherence temperature $T^{*}$, including a narrower Drude-like peak at low frequencies characterizing the emergent heavy electron state with a large mass enhancement, and a midinfrared Lorentz-like peak originating from interband transitions across the hybridization gap. The splitting is, however, absent for an electric field along the perpendicular directions, suggesting that the hybridization only occurs along the $c$ axis, although the high-temperature optical spectra from the background conduction electrons are isotropic. Our observations are in agreement with the resistivity data that show a broad maximum at around 20 K only along the $c$ axis but which keep increasing with a lowering temperature below 50 K along the $ab$ plane. Our work confirms the quasi-one-dimensional nature of the heavy electron physics in ${\mathrm{CeCo}}_2{\mathrm{Ga}}_8$ and establishes a direct correspondence between the bulk transport properties and microscopic electronic structures. It also pushes the previous observation of anisotropic coherence temperature in many heavy fermion compounds to an extreme situation and reveals the importance of intersite magnetic correlations beyond the local Kondo picture.

Two-dimensional electron and hole gases separated by a few nm from each other are produced in $p-i-n$ diodes based upon molecular beam epitaxy-grown GaAs/AlGaAs heterostructures. At such interlayer distances, the exciton formation and possibly Bose-Einstein condensation (BEC) is expected. We measure the capacitance between the layers and find it to oscillate as a function of the bias voltage. The peak values exceed the geometric capacitance by up to a factor of 2. The surprisingly regular periods of the oscillations are of the order of 10 to 30 mV and scale linearly with the inverse of the thickness, between 60 and 150 nm, of the GaAs layer placed between the barrier and the $p$-doped AlGaAs. The oscillations are related to the resonant electron tunneling into high energy levels of this GaAs layer acting as a quantum well. We suggest that long lifetimes of the electrons in these levels are the origin of the large peak values of the capacitance. The possible relation of the capacitance oscillations with BEC is discussed.

We predict that specific group-IV elements and IV-IV, III-V, and II-VI compounds can form stable, freestanding two-dimensional (2D) monolayers consisting of octagon, hexagon, and square rings (ohs), in which the threefold coordination of atoms is preserved to allow $s{p}^2$-type hybridization. These monolayers can also construct bilayers, multilayers, three-dimensional (3D) layered van der Waals solids, and 3D crystals with strong vertical bonds between layers as well as quasi-one-dimensional nanotubes and nanoribbons with diverse edge geometries. All these ohs structures can constitute a large class of 2D materials ranging from good metals to wide bandgap semiconductors and display physical and chemical properties rather different from those of their counterparts in the hexagonal (honeycomb) network. The metallic state of freestanding 2D C, Si, and Ge ohs monolayers and 3D C ohs bulk contrast, respectively, with graphene, silicene, germanene, and graphite.

Tunneling two-level systems affect damping, noise, and decoherence in a wide range of devices, including nanoelectromechanical resonators, optomechanical systems, and qubits. Theoretically, this interaction is usually described within the tunneling state model. The dimensions of such devices are often small compared to the relevant phonon wavelengths at low temperatures, and extensions of the theoretical description to reduced dimensions have been proposed, but lack conclusive experimental verification. We have measured the intrinsic damping and the frequency shift in magnetomotively driven aluminum nanoelectromechanical resonators of various sizes at millikelvin temperatures. We find good agreement of the experimental results with a model where the tunneling two-level systems couple to flexural phonons that are restricted to one or two dimensions by geometry of the device. This model can thus be used as an aid when optimizing the geometrical parameters of devices affected by tunneling two-level systems.

We present a general approach to obtain effective field theories for topological crystalline insulators whose low-energy theories are described by massive Dirac fermions. We show that these phases are characterized by the responses to spatially dependent mass parameters with interfaces. These mass interfaces implement the dimensional reduction procedure such that the state of interest is smoothly deformed into a topological crystal, which serves as a representative state of a phase in the general classification. Effective field theories are obtained by integrating out the massive Dirac fermions, and various quantized topological terms are uncovered. Our approach can be generalized to other crystalline symmetry-protected topological phases and provides a general strategy to derive effective field theories for such crystalline topological phases.

The surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.

We suggest using spatially modulated strain for control of a spin wave propagating inside a bulk magnet. The modulation with the wave vector $\mathit{q}=2\mathit{k}$, by virtue of magnetoelasticity, mixes spin waves with wave vectors near $\mathit{k}$ and $-\mathit{k}$. This leads to lifting the degeneracy of the symmetric and antisymmetric eigenstate combinations of these waves. The resulting picture reminds one of a tunneling particle in a symmetric double-well potential. Here, a moving spin wave being subjected to $2\mathit{k}$-lattice modulation after some time alters its propagation direction to the opposite one, and so on. The effect can be utilized for the control of the spin-wave propagation, which can be useful for spintronic and magnonic applications. The control may include a delay line element, filtering, and waveguide of the spin waves.

We reveal giant proximity-induced magnetism and valley-polarization effects in Janus Pt dichalcogenides (such as SPtSe), when bound to the europium oxide (EuO) substrate. Using first-principles simulations, it is surprisingly found that the charge redistribution, resulting from proximity with EuO, leads to the formation of two $K$ and $K^{\prime }$ valleys in the conduction bands. Each of these valleys displays its own spin polarization and a specific spin texture dictated by broken inversion and time-reversal symmetries, and valley-exchange and Rashba splittings as large as hundreds of meV. This provides a platform for exploring spin-valley physics in low-dimensional semiconductors, with potential spin transport mechanisms such as spin-orbit torques much more resilient to disorder and temperature effects.

Quantifying domain population in multiferroics is required to understand domain nucleation/switching processes and achieve on-demand domain control. We report an approach based on nuclear magnetic resonance spectroscopy for the accurate measurement of volume fractions of multiferroic domains in bulk crystals. We demonstrate on a benchmarking system, ${\mathrm{GaV}}_4{\mathrm{Se}}_8$, that the electric quadrupole interaction of the ${}^{71}\mathrm{Ga}$ and the hyperfine field at the ${}^{51}\mathrm{V}$ nuclei are proper microscopic probes of the ferroelectric polarization and the ferromagnetic moment, respectively. We use the anisotropy of these local quantities to determine the multiferroic domain population, controlled here by both electric and magnetic fields. The sensitivity of this local-probe technique to site symmetries facilitates domain quantification in a wide range of anisotropic magnets, ferroelectrics, and multiferroics.

We theoretically study a stabilization mechanism of the skyrmion crystal in centrosymmetric magnets with a bilayer structure. We show that the interplay between a layer-dependent staggered Dzyaloshinskii-Moriya interaction that arises from the absence of local inversion symmetry and the interlayer exchange interaction gives rise to a plethora of multiple-$Q$ states including the skyrmion crystal with a quantized topological number. By performing the simulated annealing for the bilayer triangular-lattice model under an external magnetic field, we demonstrate that the skyrmion forms the triangular-shaped crystals with different helicities in each layer owing to the staggered Dzyaloshinskii-Moriya interaction. Although the relative positions of the skyrmion core in each layer are different depending on the sign of the interlayer exchange interactions, the skyrmion crystal phases robustly appear under both ferromagnetic and antiferromagnetic interlayer interactions. We also find another two triple-$Q$ states with a uniform scalar chirality but without a quantized topological number in the low- and high-field regions. Especially, the low-field triple-$Q$ state exhibits the opposite sign of the scalar chirality to the skyrmion crystal, which is not found in the single-layer system. Our results indicate that the lack of local inversion symmetry in the lattice structure is another key ingredient to induce topological spin textures in centrosymmetric magnets.

Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in an external magnetic field applied along the film plane. The film thickness of $\sim 5$ nm was comparable with the coherence length, so vortices should not form. This was confirmed by the fact that no marked difference between the spectra with a terahertz electric field set perpendicular and parallel to the external magnetic field was observed. Simultaneous use of Maxwell-Garnett effective medium theory and the model of optical conductivity by Herman and Hlubina [Phys. Rev. B 96, 014509 (2017)] proved to correctly reproduce the terahertz spectra obtained experimentally in a magnetic field of up to 7 T. This let us conclude that the magnetic field tends to suppress the superconductivity, resulting in an inhomogeneous state where superconducting domains are enclosed within a normal-state matrix. The scattering rate due to pair-breaking effects was found to linearly increase with the magnetic field.

Since the successful synthesis of bulk single crystals ${\mathrm{MoN}}_2$ and ${\mathrm{ReN}}_2$, which have a layered structure, transition-metal dinitrides have attracted considerable attention in recent years. Here, we focus on rare-earth metal (Rem) elements, and propose seven stable Rem dinitride monolayers with a 1T structure, namely, $1\mathrm{T}\text{−}{\mathrm{RemN}}_2$. We use first-principles calculations, and find that these monolayers have a ferromagnetic ground state with in-plane magnetization. Without spin-orbit coupling (SOC), the band structures are spin-polarized with Dirac points at the Fermi level. Remarkably, the $1\mathrm{T}\text{−}{\mathrm{LuN}}_2$ monolayer exhibits an isotropic magnetocrystalline anisotropy energy in the $xy$ plane with in-plane magnetization, indicating easy tunability of the magnetization direction. When rotating the magnetization vector in the $xy$ plane, we propose a model that accurately describes the variation of the SOC band gap and the two possible topological states (Weyl-like semimetal and Chern insulator states) whose properties are tunable. The Weyl-like semimetal state is a critical point between the two Chern insulator states with opposite sign of the Chern numbers (±1). The nontrivial band gap (up to 60.3 meV) and the Weyl-like semimetal state are promising for applications in spintronic devices.

Novel structural building blocks in compounds could induce interesting physical and chemical properties. Although phosphorus tends to form very different motifs, the existence of lone pair electrons has always prevented the formation of graphenelike structures. Here, the application of first-principles swarm structural calculations has allowed us to predict the stability of pressure-induced hexagonal ${\mathrm{LaP}}_2{\mathrm{H}}_2$ containing graphenelike phosphorus, which derives from the trigonal bipyramid configuration of P atoms regulated by symmetric hydrogen bonds. ${\mathrm{LaP}}_2$ in ${\mathrm{LaP}}_2{\mathrm{H}}_2$ has the same configuration as ${\mathrm{MgB}}_2$, and P and H atoms form a three-dimensional framework as ${\mathrm{H}}_3\mathrm{S}$. Interestingly, ${\mathrm{LaP}}_2{\mathrm{H}}_2$ shows a superconductivity dominated by the graphenelike phosphorus layer and its coupling with La atoms. On the other hand, ${\mathrm{LaP}}_2{\mathrm{H}}_2$ is not only superconducting at a lower pressure than the H-rich ${\mathrm{LaPH}}_6$, but it also shows a superconducting transition temperature three times higher. Our work provides an example which extends the landscape of conventional superconductors at lower pressures.

Superconductivity and electron topology are two quantum phenomena that have attracted much interest, but no causal relationship between them has been reported because superconductivity is a many-body effect due to electron-electron interaction, while electron topology is a single-particle manifestation of electron states. Here, we demonstrate that electron topology can induce Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing in Ising Bardeen-Cooper-Schrieffer (IBCS) superconductors. Specifically, we predict that the nonmagnetic metals of the ${MA}_2{Z}_4$ family, including ${\alpha }_1\text{−}{\mathrm{TaSi}}_2{\mathrm{P}}_4, {\alpha }_1\text{−}{\mathrm{TaSi}}_2{\mathrm{N}}_4, {\alpha }_2\text{−}{\mathrm{TaGe}}_2{\mathrm{P}}_4, {\alpha }_1\text{−}{\mathrm{NbSi}}_2{\mathrm{P}}_4$, and ${\alpha }_2\text{−}{\mathrm{NbGe}}_2{\mathrm{P}}_4$ monolayers, are all IBCS superconductors with a transition temperature ranging from a few to tens of degrees Kelvin. The intrinsic IBCS pairing alone will enhance the in-plane critical field $B_{\mathrm{c}}$ to ∼20–60 times the Pauli limit $B_{\mathrm{p}}$, and the extrinsic FFLO pairing evoked by topological Weyl nodal lines under a magnetic field can further double the $B_{\mathrm{c}}/B_{\mathrm{p}}$ ratio. Our findings not only enrich the fundamental relationship between superconductivity and electron topology, but they also yield an effective approach to enhance the robustness of superconductivity.

Square-net-layered materials have attracted attention as an extended research platform of Dirac fermions and of exotic magnetotransport phenomena. In this study, we investigated the magnetotransport properties of $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$, which has Sb-square-net layers and shows charge density wave (CDW) transitions at ambient pressure. The application of pressure suppresses the CDWs, and above a pressure of 3.2 GPa a normal metallic phase with no CDWs is realized. By utilizing a mechanical rotator combined with a high-pressure cell, we observed the angular dependence of the Shubnikov–de Haas (SdH) oscillation up to 3.5 GPa, and we confirmed the notable two-dimensional nature of the Fermi surface. In the normal metallic phase, we also observed a remarkable field-angular-dependent magnetoresistance (MR), which exhibited a “butterflylike” polar pattern. To understand these results, we theoretically calculated the Fermi surface and conductivity tensor at the normal metallic phase. We showed that the SdH frequency and Hall coefficient calculated based on the present Fermi surface model agree well with the experiment. The transport properties in the normal metallic phase are mostly dominated by the anisotropic Dirac band, which has the highest conductivity due to linear energy dispersions. We also proposed that momentum-dependent relaxation time plays an important role in the large transverse MR and negative longitudinal MR in the normal metallic phase, which is experimentally supported by the considerable violation of Kohler's scaling rule. Although quantitatively complete reproduction was not achieved, the calculation showed that the elemental features of the butterfly MR could be reasonably explained as the geometrical effect of the Fermi surface.

New theoretical proposals and experimental findings on transition metal dichalcogenide $1T-{\mathrm{TaS}}_2$ have revived interest in its possible Mott insulating state. We perform a comprehensive scanning tunneling microscopy and spectroscopy experiment on different single-step areas in pristine $1T-{\mathrm{TaS}}_2$. After accurately determining the relative displacement of the Star of David superlattices in two layers, we find that different stacking orders can correspond to a similar large-gap spectrum on the upper terrace. When the measurement is performed away from the step edge, the large-gap spectrum can always be maintained. The stacking order seems to rarely disturb the large-gap spectrum in the ideal bulk material. We conclude that the large insulating gap is from the single-layer property, which is a correlation-induced Mott gap based on the single-band Hubbard model. Specific stacking orders can perturb the state and induce a small-gap or metallic spectrum for a limited area around the step edge, which we attribute to a surface and edge phenomenon. Our work provides more evidence about the stacking-order effect on the electronic state and deepens our understanding of the Mott insulating state in $1T-{\mathrm{TaS}}_2$.

The origin of different electronic states of $1T-{\mathrm{TaS}}_2$ and $1T-{\mathrm{TaSe}}_2$ remains controversial due to the complicated correlated electronic properties. We apply scanning tunneling microscopy to study the electronic state of bulk $1T-{\mathrm{TaSe}}_2$. Both insulating and metallic states are identified in different areas of the same sample. The insulating state is similar to that in $1T-{\mathrm{TaS}}_2$, concerning both the $dI/dV$ spectrum and the orbital texture. With detailed investigations in single-step areas, the electronic state measured on the upper-layer surface is found to be associated with different stacking orders and the lower layer's electronic state. The insulating state is most possibly a single-layer property, perturbed to a metallic state by particular stacking orders. Both the metallic and large-gap insulating spectra, together with their corresponding stacking orders, are stable states in $1T\text{−}{\mathrm{TaSe}}_2$. The connected metallic areas lead to the metallic transport behavior. We then reconcile the bulk metallic and surface insulating state in $1T-{\mathrm{TaSe}}_2$. The rich phenomena in $1T-{\mathrm{TaSe}}_2$ deepen our understanding of the correlated electronic state in bulk $1T-{\mathrm{TaSe}}_2$ and $1T-{\mathrm{TaS}}_2$.