The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a piezoelectric/magnetic thin film heterostructure yields potential for the realization of novel microwave devices and applications in magnonics. In the present work, we investigate the SAW-SW interaction in a Pt/Co($2\mathrm{nm}$)/Ru($0.85\mathrm{nm}$)/Co($4\mathrm{nm}$)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with different thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interfacial Dzyaloshinskii–Moriya interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode shows a large nondegenerate dispersion relation for oppositely propagating SWs. Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the piezoelectric/SAF hybrid device. The equilibrium magnetization directions of both Co layers are manipulated by an external magnetic field to set a ferromagnetic, canted, or antiferromagnetic configuration. This has a strong impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The experimental results are in agreement with a phenomenological SAW-SW interaction model, which considers the interlayer exchange coupling, iDMI, and interlayer dipolar coupling fields of the SWs.

In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to $20–25\%$ as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interference-induced enhancement can be much more pronounced, up to nearly $70\%$. Furthermore, here it is shown that such an interference enhancement persists over a wide range of microscopic parameters.

The Gross-Neveu-Heisenberg universality class describes a continuous quantum phase transition between a Dirac semimetal and an antiferromagnetic insulator. Such quantum critical points have originally been discussed in the context of Hubbard models on $\pi$-flux and honeycomb lattices, but more recently also in Bernal-stacked bilayer models, of potential relevance for bilayer graphene. Here, we demonstrate how the critical behavior of this fermionic universality class can be computed within an $\epsilon$ expansion around the lower critical space-time dimension of two. This approach is complementary to the previously studied expansion around the upper critical dimension of four. The crucial technical difference near the lower critical dimension is the presence of different four-fermion interaction channels at the critical point, which we take into account in a Fierz-complete way. By interpolating between the lower and upper critical dimensions, we obtain improved estimates for the critical exponents in $2+1$ space-time dimensions. For the situation relevant to single-layer graphene, we find an unusually small leading-correction-to-scaling exponent, arising from the competition between different interaction channels. This suggests that corrections to scaling may need to be taken into account when comparing analytical estimates with numerical data from finite-size extrapolations.

Flat-band topologies and localizations in noninteracting systems are extensively studied in different quantum and classical-wave systems. Recently, the exploration on the novel physics of flat-band localizations and topologies in interacting systems has aroused great interest. In particular, it is theoretically shown that the strong interaction could drive the formation of nontrivial topological flat bands; even dispersive trivial bands dominate the single-particle counterparts. However, the experimental observation of those interesting phenomena is still lacking. Here, we experimentally simulate the interaction-induced flat-band localizations and topological edge states in electrical circuit networks. We directly map the eigenstates of two correlated bosons in one-dimensional Aharonov-Bohm cages to modes of two-dimensional circuit lattices. In this case, by tuning the effective interaction strength through circuits’ groundings, the two-boson flat bands and topological edge states are detected by measuring frequency-dependent impedance responses and voltage dynamics in the time domain. Our finding suggests a flexible platform to simulate the interaction-induced flat-band topology, and may possess potential applications in designing novel electronic devices.

Although gauge invariance is a postulate in fundamental theories of nature such as quantum electrodynamics, in quantum-simulation implementations of gauge theories it is compromised by experimental imperfections. In a recent paper [Halimeh and Hauke, Phys. Rev. Lett. 125, 030503 (2020)], it has been shown in finite-size spin-$1/2$ quantum link lattice gauge theories that upon introducing an energy-penalty term of sufficiently large strength $V$, unitary gauge-breaking errors at strength $\lambda$ are suppressed $\propto {\lambda }^2/V^2$ up to all accessible evolution times. Here, we show numerically that this result extends to quantum link models in the thermodynamic limit and with larger spin $S$. As we show analytically, the dynamics at short times is described by an adjusted gauge theory up to a timescale that is at earliest ${\tau }_{\text{adj}}\propto \sqrt{V/V_0^3}$, with $V_0$ an energy factor. Moreover, our analytics predicts that a renormalized gauge theory dominates at intermediate times up to a timescale ${\tau }_{\text{ren}}\propto exp(V/V_0)/V_0$. In both emergent gauge theories, $V$ is volume independent and scales at worst $\sim S^2$. Furthermore, we numerically demonstrate that robust gauge invariance is also retained through a single-body gauge-protection term, which is experimentally straightforward to implement in ultracold-atom setups and NISQ devices.

Two-dimensional (2D) ferromagnetism at room temperature is highly desired in spintronic devices and applications. Recently, ${\mathrm{CrTe}}_2$ film has been successfully synthesized. Bulk ${\mathrm{CrTe}}_2$ has in-plane magnetic anisotropy (MA), and the Curie temperature reaches up to 300 K. When the dimensions are down to two, MA switches to out-of-plane. However, the mechanism of MA transition from in-plane to out-of-plane remains unknown. Here, we investigate the switching of MA in ${\mathrm{CrTe}}_2$ by second-order perturbation theory. The MA energy (MAE) originates from competition between $p_z/p_y$ and $p_x/p_y$ orbital hybridization, which contributes to out-of-plane and in-plane MA, respectively. The $p_x/p_y$ orbital hybridization, induced by van der Waals force, is absent in the monolayer. Moreover, the out-of-plane MA of monolayer ${\mathrm{CrTe}}_2$ breaks mirror symmetry, producing an anomalous Hall effect. Slight charge doping is significant to achieve a tunable anomalous Hall effect. Based on charge doping, we design a spintronic anomalous Hall device by taking advantage of the easily controllable properties of ${\mathrm{CrTe}}_2$. This highly thickness-dependent magnetism provides a perspective to control the magnetic properties of 2D materials.

We perform a theoretical investigation about the valley dynamics of interlayer excitons (IXs) in $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{W}{\mathrm{S}}_2$ heterostructure with AB stacking configuration grown on ferromagnetic substrate. We find that the interlayer charge transfer process changes dramatically the IX emission as well as its valley polarization (VP). The magnetic proximity effect (MPE) exerted by ferromagnetic substrate, on the other hand, generates a zero-field valley Zeeman splitting, which suppresses the depolarization induced by electron-hole exchange interaction. Remarkably, unlike usual exciton-phonon scattering, which is harmful to the VP, the phonon-assisted intervalley scatterings between two split IX states in the different valleys foster an IX population imbalance in these states, giving rise to a giant VP. A combination of these experimentally tunable physical quantities (interlayer charge transfer rate, MPE intensity and exciton-phonon coupling strength) provides a promising tool for intriguing emerging magneto-optical emissions and their VPs. Considering a large family of layered materials, this study sheds light on the path for development of van der Waals heterostructures with detectable IX emissions and giant VP.

The superconductor ${\mathrm{SnTaS}}_2$ is theoretically predicted to be an intriguing topological nodal line semimetal without consideration of spin-orbit coupling. By carrying out angle-resolved photoemission (ARPES) and spin-resolved ARPES measurements combined with band structure calculations, we have provided a complete picture of the electronic structure and spin polarization property for the prominent surface states of ${\mathrm{SnTaS}}_2$. The low-energy electronic states are dominated by surface states; two of them are from the S-terminated surface, while four of them are from the Sn-terminated surface. These give rise to interesting Fermi surface topology of ${\mathrm{SnTaS}}_2$: three pockets located at $\overline{\mathrm{\Gamma }}, \overline{M}$ and $\overline{K}$ for the S-terminated surface and two pockets surrounding $\overline{\mathrm{\Gamma }}$ and $\overline{K}$ for the Sn-terminated surface. We further reveal that two surface states that cross the Fermi level are spin-polarized. Since ${\mathrm{SnTaS}}_2$ is also a superconductor, our observations indicate that it may provide a new platform to explore topological superconductivity and other exotic properties.

In this paper, we consider a path integral formulation of the Hubbard model based on a Hubbard-Stratonovich transformation that couples the auxiliary field to the local electronic density. This decoupling is known to have a saddle-point structure that shows a remarkable regularity: The field configuration at each saddle point can be understood in terms of a set of elementary field configurations localized in space and imaginary time which we coin instantons. The interaction between instantons is short ranged. Here, we formulate a classical partition function for the instanton gas that has predictive power. For a given set of physical parameters, we can predict the distribution of instantons and show that the instanton number is sharply defined in the thermodynamic limit, thereby defining a unique dominant saddle point. Decoupling in the charge channel conserves SU(2) spin symmetry for each field configurations. Hence, the instanton approach provides an SU(2) spin-symmetric approximation to the Hubbard model. It fails, however, to capture the magnetic transition inherent to the Hubbard model on the honeycomb lattice despite being able to describe local moment formation. In fact, the instanton itself corresponds to local moment formation and concomitant short-ranged antiferromagnetic correlations. This aspect is also seen in the single particle spectral function that shows clear signs of the upper and lower Hubbard bands. Our instanton approach bears remarkable similarities to local dynamical approaches, such as dynamical mean-field theory, in the sense that it has the unique property of allowing for local moment formation without breaking the SU(2) spin symmetry. In contrast to local approaches, it captures short-ranged magnetic fluctuations. Furthermore, it also offers possibilities for systematic improvements by taking into account fluctuations around the dominant saddle point. Finally, we show that the saddle point structure depends upon the choice of lattice geometry. For the square lattice at half filling, the saddle-point structure reflects the itinerant to localized nature of the magnetism as a function of the coupling strength. The implications of our results for Lefschetz thimble approaches to alleviate the sign problem are also discussed.

Motivated by the recent experimental observation of a large pressure effect on $T_c$ in ${\mathrm{Pr}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$, we study the electronic properties of this compound as a function of pressure for $x=0$ and 0.2 doping using self-consistent $GW+\mathrm{EDMFT}$. Our numerical results demonstrate a nontrivial interplay between chemical doping and physical pressure, and small but systematic changes in the orbital occupations, local level energies, and interaction parameters with increasing pressure. The proper treatment of correlation effects, beyond density functional theory, is shown to play an important role in revealing these trends. While the pressure-dependent changes in the electronic structure of the undoped compound suggest a more single-band-like behavior in the high-pressure regime, a qualitatively different behavior is found in the doped system. We also point out that the fluctuations in the orbital occupations and spin states are not consistent with a single-band picture, and that at least a two-band model is necessary to reproduce the full result. This multiorbital nature manifests itself most clearly in the doped compound.

Recent experiments have reported that inserting metal atoms or small molecules in between the FeSe layers of $\beta$-FeSe can significantly enhance the superconducting transition temperature. Here, based on first-principles electronic structure calculations, we propose a stable compound SnOFeSe by alternatively stacking the SnO and $\beta$-FeSe layers. The predicted SnOFeSe has the same tetragonal structure as the well-known FeAs-based compound LaOFeAs, meanwhile their electronic structures in the nonmagnetic state are quite similar. The magnetic ground state of SnOFeSe is predicted to be the dimer antiferromagnetic (AFM) state, which is energetically only 2.77 (2.15) meV/Fe lower than the trimer (dimer-trimer-dimer-trimer) AFM state, indicating that strong magnetic fluctuations might be induced via slight modulation. Interestingly, SnOFeSe is at the verge of metal-insulator transition in these low-energy magnetic states, hence bridging the metallic parent compounds of iron-based superconductors and the insulating ones of cuprate superconductors. With the reduced dimensionality, monolayer SnOFeSe also shows great similarities in the electronic and magnetic properties to its bulk phase. Given that SnOFeSe is adjacent to magnetic frustration and resembles LaOFeAs in both crystal and electronic structures, we suggest that SnOFeSe is a possible superconductor parent compound, which may provide a promising platform to study the interplay between magnetism and unconventional superconductivity in FeSe-derived materials.

The effect of Anderson diagonal disorder on the topological surface (drumhead) states of a Weyl nodal loop semimetal (SM) is addressed. Since diagonal disorder breaks chiral symmetry, a winding number cannot be defined. Seen as a perturbation, the weak random potential mixes the clean exponentially localized drumhead states of the SM, thereby producing two effects: (i) the algebraic decay of the surface states into the bulk and (ii) a broadening of the low energy density of surface states of the open system due to degeneracy lifting. This behavior persists with increasing disorder, up to the bulk SM-to-metal transition at the critical disorder $W_c$. Above $W_c$, the surface states hybridize with bulk states and become extended into the bulk.

Bidimensional materials are ideally viewed as having no thickness, as their name suggests. Their optical response have been previously modelled by a purely bidimensional surface current or by a very thin film with some contradictory results. The advent of multilayer stacks of bidimensional materials and combinations of different materials in vertical van der Waals heterostructures highlights, however, that these materials have a finite thickness. In this article, we propose a new model that reconciles both approaches and we show how volume properties of stacked bidimensional layers can be calculated from the bidimensional response of each individual layer, and conversely. In our approach, each bidimensional layers is surrounded by vacuum and described as a kind of transfer matrix with intrinsic parameters that do not depend on the external medium. This provides a link between continuous thin films and discrete layers. We show how to model heterostructures of bidimensional materials and identify the parameters of the current sheet that represents the bidimensional material in the zero-thickness limit, namely the in-plane surface susceptibility and the out-of-plane displacement susceptibility. We show that our unified model is perfectly compatible with existing ellipsometric data with the same reliability as the existing interface model but with different values of the surface susceptibility or bulk dielectric function. We discuss in detail the origin of the discrepancies and show that our approach allows to determine intrinsic properties of the bidimensional materials with the advantage that multilayer and monolayer systems are described in a same framework.

In recent years, the application of mechanical stress has become a widespread experimental method to tune the electronic and optical properties of two-dimensional (2D) materials. In this work, we investigate the impact of uniaxial tensile strain along zigzag and armchair directions on the excitonic properties of graphene-like ${\mathrm{C}}_3\mathrm{N}$, a single-layer indirect-gap material with relevant mechanical and optical properties. To do that, we develop a tight-binding Bethe-Salpeter equation framework based on a Wannier-function description of the frontier bands of the system, and use it to compute both dark and bright excitons of ${\mathrm{C}}_3\mathrm{N}$ for different applied strain configurations. Then, we use this model approach to classify excitons of pristine and strained ${\mathrm{C}}_3\mathrm{N}$ according to the crystal symmetry and to explain the appearance of bright excitons with intense optical anisotropy in strained ${\mathrm{C}}_3\mathrm{N}$, even at small strains. Finally, the effect of strain on the exciton dispersion at small center-of-mass momenta is discussed, with special focus on the implications for 2D linear-nonanalytic dispersions.

In the search for high-performance thermoelectrics, materials such as clathrates have drawn attention due to having both glasslike low phonon thermal conductivity and crystal-like high electrical conductivity. ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ (BGG) has a loosely bound guest Ba atom trapped inside rigid Ga-Ge cage structures. Avoided crossings between acoustic phonons and the flat guest atom branches have been proposed to be the source of the low lattice thermal conductivity of BGG. Ga-Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory to elucidate the effect of the disorder on heat-carrying phonons. Experimental results agreed much better with the calculation assuming the disorder than with the calculation assuming the ordered configuration. Although the atomic masses of Ga and Ge are nearly identical, we found that disorder strongly reduces phonon group velocities, which significantly reduces thermal conductivity. Our work points to a path towards optimizing thermoelectrics.

${\mathrm{BaNi}}_2{\mathrm{As}}_2$ is a nonmagnetic analog of the iron pnictide superconductors, and exhibits an incommensurate charge density wave (IC-CDW) and a sizable elastoresistance. In this Letter, phonons in ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ associated with the IC-CDW and uniform in-plane lattice distortions are investigated using high-resolution inelastic x-ray scattering. The in-plane transverse acoustic phonons reveal no softening at temperatures where the elastoresistance increases strongly, indicating the latter to be electronically driven. Systematic phonon measurements suggest the IC-CDW occurs in two stages upon cooling: Underdamped phonons first soften to zero energy well above the IC-CDW ordering temperature, then the resulting quasielastic IC-CDW fluctuations gradually slow down and coalesce into the static IC-CDW order. A possible origin for our observations is the IC-CDW in ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ being uniaxial, which provides an additional Ising degree of freedom favorable for disordered IC-CDW modulations, and accounts for the elastoresistance through a weak coupling to the lattice.

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

In two-dimensional (2D) disordered superconductors prior to superconducting (SC) transition, the appearance of a resistance peak in the temperature dependent resistance $R\left(T\right)$ measurements indicates the presence of weak localization (WL) and electron-electron interaction (EEI) in the diffusion channel and SC fluctuations in the Cooper channel. Here, we demonstrate an interplay between SC fluctuations and EEI by low-temperature magnetotransport measurements for a set of 2D disordered TiN thin films. While cooling the sample, a characteristic temperature $T^{*}$ is obtained from the $R\left(T\right)$ at which SC fluctuations start to appear. The upturn in $R\left(T\right)$ above $T^{*}$ corresponds to WL and/or EEI. By the temperature and field dependences of the observed resistance, we show that the upturn in $R\left(T\right)$ originates mainly from EEI with a negligible contribution from WL. Further, we have used the modified Larkin's electron-electron attraction strength $\beta \left(T/T_{\mathrm{c}}\right)$, containing a field-induced pair-breaking parameter, in the Maki-Thompson SC fluctuation term. Here, the temperature dependence of $\beta \left(T/T_{\mathrm{c}}\right)$ obtained from the magnetoresistance (MR) analysis shows a diverging behavior close to $T_{\mathrm{c}}$, and it remains almost constant at higher temperature within the limit of $\ln \left(T/T_{\mathrm{c}}\right)<1$. Interestingly, the variation of $\beta \left(T/T_{\mathrm{c}}\right)$ on the reduced temperature $\left(T/T_{\mathrm{c}}\right)$ offers a common trend which has been closely followed by all the concerned samples presented in this paper. Finally, the temperature dependence of inverse phase scattering time (${\tau }_{\varphi }^{-1}$), as obtained from the MR analysis, clearly shows two different regimes; the first one close to $T_{\mathrm{c}}$ follows the Ginzburg-Landau relaxation rate $\left({\tau }_{\mathrm{GL}}^{-1}\right)$, whereas the second one at high temperature varies almost linearly with temperature, indicating the dominance of inelastic electron-electron scattering for the dephasing mechanism. These two regimes are followed in a generic way by all the samples despite being grown under different growth conditions.