We study the role of electronegativity in sliding friction for five different two-dimensional (2D) monolayer systems using ab initio calculations within density functional theory with van der Waals corrections. We show that the friction depends strongly on the involved atoms’ electronegativity difference. All the studied systems exhibit almost the same magnitude of the friction force when sliding along the nonpolar path, independent of the material and the surface structures. In contrast, for sliding friction along a polar path, the friction force obeys a universal linear scaling law where the friction force is proportional to the electronegativity difference of its constituent atoms. We show that atomic dipoles in the 2D monolayers, induced by the electronegativity difference, enhance the corrugation of the charge distribution and increase the sliding barriers accordingly. Our studies reveal that electronegativity plays an important role in the friction of low-dimensional systems, and provides a strategy for designing nanoscale devices.

In a heavy metal (HM)/ferromagnet (FM)/HM multilayer with a symmetrical sandwich structure, the absence of an interfacial Dzyaloshinskii-Moriya interaction (iDMI) is generally expected. A zero iDMI constant is usually verified by the motion of a magnetic domain wall (DW) with a velocity that is symmetrical to the longitudinal magnetic field (symmetrical DW motion). However, asymmetrical DW motion was still experimentally observed in a symmetrical HM/FM/HM multilayer, and its mechanism is unclear. In this letter, we theoretically prove that even in a perfectly symmetrical HM/FM/HM multilayer with a zero total iDMI constant, asymmetrical DW motion is still possible owing to the exchange coupling between the neighboring FM atomic layers in the FM medium. This work paves a way to unraveling unique DW motion in HM/FM multilayers from a deep perspective of the subtle magnetic interactions within a FM medium.

We report time- and angle-resolved photoemission spectroscopy (trARPES) measurements on the Sb(111) surface. We observe band- and momentum-dependent binding-energy oscillations in the bulk and surface bands driven by $A_{1g}$ and $E_g$ coherent phonons. While the bulk band shows simultaneous $A_{1g}$ and $E_g$ oscillations, the surface bands show either $A_{1g}$ or $E_g$ oscillations. The observed behavior is reproduced by frozen-phonon calculations based on density functional theory. This evidences the connection between electron-phonon coupling and coherent binding-energy dynamics tied to lattice vibration and confirms that band-, momentum-, and mode-dependent electron-phonon coupling can indeed be probed by trARPES in the low fluence limit.

We investigate the effect of chemical disorder on the magnetic exchange couplings and the Curie temperature $\left(T_{\text{c}}\right)$ in $\mathrm{L}{1}_0$-ordered FeNi using first-principles-based calculations. We use supercells to model chemical disorder, to account for the specific symmetry-broken local chemical environments around the individual atoms. We find a very strong variation of the most dominant first-nearest neighbor Fe-Fe interaction for different inequivalent Fe-Fe pairs, ranging from around 5 to 37 meV, compared to a coupling strength of 27 meV in the ordered state. To estimate the influence of such strong variations of the magnetic coupling constants on the Curie temperature of the disordered or partially ordered state, we study a simple Heisenberg model with random Gaussian-distributed nearest-neighbor couplings on an fcc lattice. Our Monte Carlo simulations for this model indicate that strongly varying exchange couplings, such as those obtained for FeNi, can lead to a reduction of $T_{\text{c}}$ of around 10 % relative to the one obtained using only the average coupling.

Using ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) we investigate the recently discovered presaturation phase in the highly frustrated two-dimensional spin system $\mathrm{SrZnVO}{\left({\mathrm{PO}}_4\right)}_2$ [F. Landolt et al., Phys. Rev. B 104, 224435 (2021)]. Our data provide two pieces of evidence against the presumed spin-nematic character of this phase: (i) NMR spectra reveal that it hosts a dipolar spin order and (ii) the $T_1^{-1}$ relaxation rate data recorded above the saturation field can be fitted by the sum of a single-magnon term, exponential in the gap, and a critical second-order term, exponential in the triple gap, leaving no space for a nematic spin dynamics, characterized by a double-gap exponential. We explain the unexpectedly broad validity of the simple fit and the related critical spin dynamics.

In this work, we leverage atomistic spin-lattice simulations to examine how magnetic interactions impact the propagation of sound waves through a ferromagnetic material. To achieve this, we characterize the sound wave velocity in BCC iron, a prototypical ferromagnetic material, using three different approaches that are based on the oscillations of kinetic energy, finite-displacement derived forces, and corrections to the elastic constants, respectively. Successfully applying these methods within the spin-lattice framework, we find good agreement with the Simon effect including high-order terms. In analogy to experiments, morphic coefficients associated with the transverse and longitudinal waves propagating along the [001] direction are extracted from fits to the fractional change in sound velocity data. The present efforts represent an advancement in magnetoelastic modeling capabilities which can expedite the design of future magnetoacoustic devices.

In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}-{\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$, we are able to associate the magnetoelastic resonance with the presence of the exchange spin-spirals in both the ferromagnetic and antiferromagnetic layer. Our findings offer a complementary, integrative insight into emergent magnetic materials for applications of noncollinear spin textures in view of low-energy-consumption spintronic devices.

Instanton crystal is a fascinating phase which is encountered when the minimum of the free energy corresponds to a configuration with an imaginary-time-dependent order parameter in the form of a chain of alternating instantons and anti-instantons. We present the results of the investigation of the real-time correlation functions of the order parameter in the instanton crystal phase. In order to obtain the correlation functions in real time, we formulate an original method of analytic continuation from imaginary times, which is easily adapted into an efficient numerical scheme for the computations. The resulting correlation functions exhibit nontrivial slowly decaying oscillations in real time, which is reminiscent of prethermal time crystals.

We studied the low-temperature specific heats of two antiferromagnets with the two-dimensional square-kagome structure, i.e., ${\mathrm{KCu}}_6{\mathrm{AlBiO}}_4{\left({\mathrm{SO}}_4\right)}_5\mathrm{Cl}$ $\left({\mathrm{KCu}}_6\right)$ and ${\mathrm{Na}}_6{\mathrm{Cu}}_7{\mathrm{BiO}}_4{\left({\mathrm{PO}}_4\right)}_4{[\mathrm{Cl},(\mathrm{OH}\left)\right]}_3$ $\left({\mathrm{NaCu}}_7\right)$, with the structural difference that there are interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$. Both materials show no magnetic ordering down to 50 mK. At zero field, the $C/T$ of ${\mathrm{KCu}}_6$ has a finite value when the temperature is close to 0 K. Under the magnetic field, an apparent $T^2$ dependence appears and its coefficient is progressively suppressed by the field. For ${\mathrm{NaCu}}_7$, the specific heat exhibits the $T^2$ dependence at zero field and under fields. The ratio of the quadratic coefficients of ${\mathrm{KCu}}_6$ and ${\mathrm{NaCu}}_7$ at high fields is inversely proportional to that of the squared Weiss temperatures, which indicates these two compounds host the same ground state under fields. Our results suggest that the interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$ play a negligible role in determination of its ground state. We discuss the possible quantum-spin-liquid states in these compounds and further directions to pursue based on our results.

We consider hydrodynamic electron transport in the Hall-bar geometry. The theory is developed for systems with non-Galilean-invariant electron liquids. We show that inhomogeneity of the electron density induced by long-range disorder and gating leads to mixing between the hydrodynamic transport mode and transport relative to the electron liquid. For graphene systems near charge neutrality, these effects lead to strong coupling of the hydrodynamic flow to charge transport. As a result, the effective electrical conductivity of the system may significantly exceed the intrinsic conductivity of the electron liquid. We obtain analytic expressions for the thermoelectric transport coefficients of the system as a function of density in the full crossover region between clean and disorder-dominated regimes.

The linewidths and the energy shifts of the resonant states of the impurity electron in GaAs-based quantum wells (QWs) with infinite barriers are calculated. The two-dimensional Schrödinger equation for the charge impurity in the QW is solved by the developed finite-difference method combined with the complex-scaling technique. A dependence of the linewidths and energy shifts on the QW width for the impurity localized in the center of the QW is studied. The calculated results extend and improve theoretical estimations of these quantities in the GaAs-based QW by Monozon and Schmelcher [Phys. Rev. B 71, 085302 (2005)]. In particular, in agreement with their studies we obtain that resonant states originating from the second quantum-confinement subband have negligibly small linewidths. In contrast to previous estimations, we show that for the QW widths of the order of the electron impurity's Bohr radius the linewidths of resonant states associated with the third quantum-confinement subband linearly depend on the thickness of the QW. We show how the previous theoretical predictions can be improved for such QW widths. We also calculate linewidths for the case when the electron impurity is localized away from the center of the QW.

From first-principles calculations, we investigate the structural and electronic properties of the kagome metals ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{Cs}, \mathrm{K}, \mathrm{Rb})$ under isotropic and anisotropic pressure. Charge-ordering patterns are found to be unanimously suppressed, while there is a significant rearrangement of $p$-type and $m$-type Van Hove point energies with respect to the Fermi level. Already for moderate tensile strain along the V plane and compressive strain normal to the V layer, we find that a Van Hove point can be shifted to the Fermi energy. Such a mechanism provides an invaluable tuning knob to alter the correlation profile in the kagome metal, and suggests itself for further experimental investigation. It might allow us to reconcile possible multidome superconductivity in kagome metals not only from phonons but also from the viewpoint of unconventional pairing.

We conducted ultrasonic measurements to clarify whether a phase transition exists in ErNiAl, a hexagonal compound, below the antiferromagnetic transition temperature, $T_{\mathrm{N}}\sim 6$ K. We discovered a significant elastic softening of the transverse modulus, $C_{66}$, accompanied by a significant ultrasonic attenuation toward $T_{\mathrm{Q}}=3.4$ K, which is the temperature of a sharp downward peak in other moduli, indicating a phase transition at $T_{\mathrm{Q}}$. The crystal field analysis reveals that the softening of $C_{66}$ below 80 K is due to an interlevel $O_{xy}$-type quadrupole interaction with a positive quadrupole-quadrupole coupling constant between the ground and excited Kramers doublets. A spontaneous expectation value of $O_{xy}$ emerges at $T_{\mathrm{Q}}$ in our crystal field model with the mean-field approximation. No quadrupolar ordering occurs in a magnetically ordered state because the degeneracy of quadrupoles is lifted by an internal magnetic field of magnetic ordering. However, our experimental and calculated results suggest that the driving force of the phase transition at $T_{\mathrm{Q}}$ is $O_{xy}$-type ferroquadrupolar ordering, implying a quadrupolar ordering in a magnetically ordered state at zero magnetic field.

The photon-assisted tunneling of Majorana bound states in a Majorana nanowire driven by the periodic field is studied both theoretically and numerically. We find that Majorana bound states exhibit an anomalous photon-assisted tunneling signal, which is different from an ordinary fermionic state: the height of photonic sidebands is related to the degree of Majorana nonlocality. Moreover, we show that the Bardeen-–Cooper-–Schrieffer (BCS) charge and spin components of subgap states can be well revealed by the local conductance. Our work illuminates the effect of driving field on Majorana bound states and provides a systematic scheme to directly measure BCS information of overlapping Majorana bound states.

Transition metal dichalcogenide (TMD) shows layer-dependent electrical conductivity. However, there are conflicting experimental reports on the trend of conductivity, which is attributed to originating from point defects as a function of the number of layers (NLs). Using density functional theory, we analyze the layer-dependent defect thermodynamics of $n$- and $p$-type defects in ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$. The shallow donor levels of hydrogen defects systematically become deep with the increasing NLs to six or seven-layers and hence reduces the $n$-type conductivity. Moreover, the deep acceptor $V_{\text{S}}$ in a one-layer turns into a weak shallow acceptor for six-layers. Interestingly, from eight-layers onwards, the thermodynamic defect transition levels shift towards the conduction band due to the interplay between the bonding characteristic of the localized defect state and Coulombic repulsion of the added charge in the changing dielectric environment. The study uncovers the plausible cause of the layer-dependent electrical conductivity of TMDs.

We analyze a general method for the dissipative preparation and stabilization of volume-law entangled states of fermionic and qubit lattice systems in one dimension (and higher dimensions for fermions). Our approach requires minimal resources: nearest-neighbor Hamiltonian interactions that obey a suitable chiral symmetry, and the realization of just a single, spatially localized dissipative pairing interaction. In the case of a qubit array, the dissipative model we study maps to an interacting fermionic problem. Nonetheless, we analytically show the existence of a unique pure entangled steady state (a so-called rainbow state). Our ideas are compatible with a number of experimental platforms, including superconducting circuits and trapped ions.

Thermalization in many-body systems can be inhibited by the application of a linearly increasing potential, which is known as Stark many-body localization. Here we investigate the fate of this phenomenon on a two-dimensional disorder-free lattice with up to $24\times 6$ sites. Similar to the one-dimensional case, “density-polarized” regions can act as bottlenecks for transport and thermalization on laboratory timescales. However, compared to the one-dimensional case, a substantially stronger potential gradient is needed to prevent thermalization when an extra spatial dimension is involved. The origin of this difference and implications for experiments are discussed. We argue that delocalization is generally favored for typical states in two-dimensional Stark many-body systems, although nonergodicity can still be observed for a specific choice of initial states, such as those probed in experiments.

In recent years, charge-two Weyl point phonons (WPPs) have attracted increasing attention. Charge-two WPPs not only provide a new platform for realizing phonon-based unconventional Weyl points (WPs) but also help in realizing specific phonon-based transport behaviors. Herein, based on the first-principles calculations and symmetry analysis, we propose a realistic material with the $P{3}_{1}21$ space group, ${\mathrm{BaZnO}}_2$, which has a charge-two WPP and two charge-one WPPs at high-symmetry points. These three unpaired WPPs form a unique triangular Weyl complex. Unlike previously reported material candidates with type-I or type-II charge-two WPPs, the proposed ${\mathrm{BaZnO}}_2$ has a type-III charge-two WPP, which has a constant frequency surface that contains two electronlike or holelike states connected at the charge-two WP. ${\mathrm{BaZnO}}_2$ can support double-helicoid phonon surface states that cover the entire (001) surface Brillouin zone. The clean type-III WPP, unique triangular Weyl complex, and clear and long surface states in ${\mathrm{BaZnO}}_2$ suggest that it is an excellent platform for further research into the physics and applications of type-III charge-two WPPs. Furthermore, we pointed out that charge-two WPPs with type-III band dispersion may appear at high-symmetry points in space groups 75–80, 89–98, 143–146, 149–155, 168–173, 177–182, 196, 207–210. Besides trigonal ${\mathrm{BaZnO}}_2$, some material candidates, including tetragonal ${\mathrm{MgTiO}}_4$, trigonal ${\mathrm{Li}}_2{\mathrm{GeF}}_6$, hexagonal ${\mathrm{CaSO}}_4$, and cubic ${\mathrm{Li}}_{10}{\mathrm{B}}_{14}{\mathrm{Cl}}_2{\mathrm{O}}_{25}$, are also shown to be the hosts of type-III charge-two WPPs.