We study superconductivity in a two-dimensional, disordered marginal Fermi liquid. At the semiclassical level, the transition temperature $T_c$ is strongly suppressed because marginal Fermi liquid effects destroy well-defined quasiparticles. However, we show that interference between quantum-critical collective modes must be included and these enhance $T_c$, violating Anderson's theorem. Our results suggest that phase coherence in a disordered, quantum-critical system can survive and manifest in the form of collective excitations, despite the absence of coherent quasiparticles.

The discovery of superconductivity in $\mathrm{Ca}{\mathrm{C}}_6$ with a critical temperature ($T_c$) of 11.5 K reignited much interest in exploring high-temperature superconductivity in graphite intercalation compounds (GICs). Here we identify a GIC $\mathrm{Na}{\mathrm{C}}_4$, discovered by ab initio evolutionary structure search, as a superconductor with a computed $T_c$ of 41.2 K at 5 GPa. This value is eight times higher than that of the synthesized GIC $\mathrm{Na}{\mathrm{C}}_2$ and possesses the highest $T_c$ among available GICs. The remarkable superconductivity of GIC $\mathrm{Na}{\mathrm{C}}_4$ mainly arises from the coupling of π electrons in graphene with the low-frequency vibrations involving both Na and C atoms. These findings suggest that Na GICs may hold great promise as high-$T_c$ superconductors.

Recently, Li et al. [C. Li et al., Phys. Rev. B 107, 224106 (2023)] investigated the mechanical behavior of cubic HfC and TaC under simple shear and pure shear using first-principles calculations. Unfortunately, the paper contains some serious and fundamental flaws in the field of continuum mechanics and nanomechanics. The results presented appear to be qualitatively and quantitatively incorrect; they would be correct if we were in the small/linear deformation/strain regime, which we are not. A correct description therefore requires a finite/nonlinear deformation/strain apparatus. The solution for simple shear, even from density functional theory calculations, must follow Frenkel's sinusoidal solution.

In our recent study [Phys. Rev. B 107, 224106 (2023)], we investigated the effects of shear methods on the shear strengths and deformation modes of two typical transition metal carbides using first-principles calculations and preliminarily unified the shear stresses obtained in different ways based on the Mohr-Coulomb relation. Mázdziarz raised a comment on our study and claimed that our work contains some serious and fundamental flaws in the field of continuum mechanics and nanomechanics. Unfortunately, the author seemed to misunderstand our work. Therefore, we respond to the preceding Comment [Phys. Rev. B 108, 216101 (2023)] on our article to clarify the confusion and misunderstanding.

Transition metal dichalcogenides are famed as functional materials harboring a rich variety of charge density wave (CDW) distortions. Recent experiments indicate the degeneration of CDW phases in transition metal dichalcogenides on various substrates. Here we show by comprehensive first-principles calculations that the CDW phases at the interface of $1T\text{−}{\mathrm{TiSe}}_2/2H\text{−}{\mathrm{NbSe}}_2$ heterostructures are tunable. In the heterostructures, the CDW phase is inhibited in ${\mathrm{TiSe}}_2$ layers while retained in ${\mathrm{NbSe}}_2$ layers. The collapse of CDW in ${\mathrm{TiSe}}_2$ is caused by the interface work function difference and the induced charge transfer, and confined to the first two ${\mathrm{TiSe}}_2$ layers closest to the interface. This interface effect is maintained even when an external electric field is applied to balance the difference in work function, but biaxial tensile strain can effectively restore the CDW phase in the ${\mathrm{TiSe}}_2$ layer. As for ${\mathrm{NbSe}}_2$, since the half-filled Nb-d orbitals are the only electronic states around the Fermi level, substrate effect can hardly affect the CDW in it. Our study paves the way for manipulating phase transitions of transition metal dichalcogenides via van der Waals interface engineering.

Sensing applications that utilize nanomechanical resonators require careful control of nonlinear effects in their eigenmodes to ensure robust measurement. While the effect of intra- and intermodal nonlinearities on the resonant frequencies of doubly clamped elastic beams have been widely studied using theory and experiment, commensurate studies on cantilevered beams are limited in comparison. Here, we present such a detailed study that includes an explicit and simple formula for the flexural resonant frequencies of slender cantilevered beams that accounts for intra- and intermodal nonlinearities. Using this general theory, numerical results for the modal nonlinear coefficients are tabulated for the first 20 flexural eigenmodes of cantilevered beams possessing uniform cross sections. The accuracy of this theory, and the effect of cantilever aspect ratio (length/width) on these nonlinear coefficients, is explored using high-accuracy laser Doppler vibrometry experiments. We anticipate that these results will find utility in single- and multimode applications, where the effect of finite oscillation amplitude on the cantilever resonant frequencies can significantly impact measurement design and interpretation.

We have probed the contribution of interfacial spin-orbit scattering to spin transport in ${\mathrm{Mn}}_3\mathrm{Sn}\text{/}\mathrm{Pt}$ bilayers in the temperature range from 5 K to 300 K. Generally, the spin transport of ${\mathrm{Mn}}_3\mathrm{Sn}\text{/}\mathrm{Pt}$ bilayers depends on both magnetic order and giant interfacial spin-orbit scattering. The magnetic order transition characteristics of ${\mathrm{Mn}}_3\mathrm{Sn}$ have been first confirmed by the anomalous Hall effect measurement, and the presence of giant interfacial spin-orbit scattering has dramatically modified the extrinsic scattering mechanisms. Robust and quantitative descriptions for the interfacial spin-orbit scattering contribution have been given. Key parameters for spin transport have been extracted through spin Hall magnetoresistance and weak localization measurements to uncover further the contributions of giant interfacial spin-orbit scattering. These results illustrate the unique role the interfacial properties play in defining spin transport in noncollinear antiferromagnet/heavy metal bilayer systems, which has opened novel strategies to design spintronic devices.

This paper aims to enhance the spin polarization of the Heusler alloy ${\mathrm{Co}}_2\mathrm{FeGa}$ by replacing Co with Cr. It is found that, as the substitution content $x$ increases from $0\to 1\to 2$ in ${\mathrm{Co}}_{2-x}{\mathrm{Cr}}_x\mathrm{FeGa}$ alloys, the Fermi level in the spin-up direction is tuned from the low density of states (DOS) of hybridized orbital $t_{2g}$ to the small pseudogap between $e_g$ and $t_{1u}$ and finally to the area of high DOS formed by the antibonding $t_{1u}$ and $e_u$ hybridized orbitals. Meanwhile, the Fermi level of the spin-down direction remains in the pseudogap. Hence a transition from a ferromagnetic metal ($x=0$) to a ferrimagnetic (FIM) spin-gapless semiconductor (SGS; $x=1$) and further to a FIM near half-metal (HM) ($x=2$) was realized with the spin polarization enhanced from 63% to 95%. Furthermore, the disorder between Cr(B) and Ga does not destroy the HM property of ${\mathrm{Cr}}_2\mathrm{FeGa}$. Theoretically, the intrinsic anomalous Hall conductivity induced by the Berry curvature is found to increase with doping Cr in ordered ${\mathrm{Co}}_{2-x}{\mathrm{Cr}}_x\mathrm{FeGa}$. Experimentally, the saturation magnetic moments of ${\mathrm{Co}}_2\mathrm{FeGa}$ and CoCrFeGa were measured to be $5.21{\mu }_{\mathrm{B}}/\text{f.u.}$ and $2.25{\mu }_{\mathrm{B}}/\text{f.u.}$ at 5 K, respectively, which is consistent with first-principles calculations. However, in ${\mathrm{Cr}}_2\mathrm{FeGa}$, there is an obvious deviation between the experimental value of $0.36{\mu }_{\mathrm{B}}/\text{f.u.}$ at 5 K and the calculated value of $0.96{\mu }_{\mathrm{B}}/\text{f.u.}$, which is attributed to the swap disorder in the sample. The longitudinal resistivity shows ${\mathrm{Co}}_2\mathrm{FeGa}$ a metallic conductor behavior, whereas both CoCrFeGa and ${\mathrm{Cr}}_2\mathrm{FeGa}$ exhibit semiconductorlike behavior from 5 to 350 K. In the compound CoCrFeGa, the small anomalous Hall conductivity (${\sigma }_{xy}^A$) of 52.7 S/cm at 5 K is in line with the property of SGS. Both intrinsic and extrinsic factors contribute to the anomalous Hall effect in CoCrFeGa, with the intrinsic contribution being larger than that of the extrinsic part. For ${\mathrm{Cr}}_2\mathrm{FeGa}$, a maximum of the value of ${\sigma }_{xy}^A$ 14.1 S/cm is observed at 5 K, and this value goes to zero above 150 K due to Curie transition.

A spin Hamiltonian that characterizes interatomic interactions between spin moments is highly valuable in predicting and comprehending the magnetic properties of materials. Here, we explore a method for explicitly calculating interatomic exchange interactions in noncollinear configurations of magnetic materials considering only a bilinear spin Hamiltonian in a local scenario. Based on density-functional theory calculations of dimers adsorbed on metallic surfaces, and with a focus on the Dzyaloshinskii-Moriya interaction (DMI) which is essential for stabilizing chiral noncollinear magnetic states, we discuss the interpretation of the DMI when decomposed into microscopic electron and spin densities and currents. We clarify the distinct origins of spin currents induced in the system and their connection to the DMI. In addition, we reveal how noncollinearity affects the usual DMI, which is solely induced by spin-orbit coupling, and DMI-like interactions brought about by noncollinearity. We explain how the dependence of the DMI on the magnetic configuration establishes a connection between high-order magnetic interactions, enabling the transition from a local to a global spin Hamiltonian.

We study possible patterns for spontaneous symmetry breaking in a Dirac fermion model, which is applicable to twisted bilayer graphene at charge neutrality. We show how a chiral SU(4) symmetry emerges and construct the corresponding low-energy model that includes a Fierz-complete set of symmetry-allowed four-fermion interactions. We employ an unbiased renormalization group treatment to identify the critical points that describe transitions into different ordered phases. The resulting phase diagram depends on the number of fermion flavors and we show that the coupling between ordering channels prevents many of the possible mean-field orders from being accessible at relevant, small flavor numbers. We argue that, as a consequence, twisted bilayer graphene is governed by a quantum Hall state or an SU(4) manifold of insulating spin-valley orders with emergent Lorentz symmetry that contains intervalley coherent, spin Hall, and valley Hall states. We study how SU(4)-breaking perturbations affect the accessibility and can additionally stabilize symmetry-broken (semi)metallic states.

We investigate the response to modular transformations and the fractional statistics of Abelian multicomponent fractional quantum Hall (FQH) states. In particular, we analytically derive the modular matrices encoding the statistics of anyonic excitations for general Halperin states using the conformal field theories (CFTs). We validate our theory by several microscopic examples, including the spin-singlet state using the anyon condensation picture and the Halperin (221) state in a topological flat-band lattice model using numerical calculations. Our results, uncovering that the modular matrices and associated fractional statistics are solely determined by the $K$ matrix, further strengthens the correspondence between the two-dimensional (2D) CFTs and $(2+1)\mathrm{D}$ topological orders for multicomponent FQH states.

Perovskite oxides are known to exhibit many magnetic, electronic, and structural phases as function of doping and temperature. These materials are theoretically frequently investigated by the $\mathrm{DFT}+U$ method, typically in their ground state structure at $T=0$. We show that by combining machine learning force fields (MLFFs) and $\mathrm{DFT}+U$ based molecular dynamics, it becomes possible to investigate the crystal structure of complex oxides as function of temperature and $U$. Here, we apply this method to the magnetic transition metal compounds ${\mathrm{LaMnO}}_3$ and ${\mathrm{SrRuO}}_3$. We show that the structural phase transition from orthorhombic to cubic in ${\mathrm{LaMnO}}_3$, which is accompanied by the suppression of a Jahn-Teller distortion, can be simulated with an appropriate choice of $U$. For ${\mathrm{SrRuO}}_3$, we show that the sequence of orthorhombic to tetragonal to cubic crystal phase transitions can be described with great accuracy. We propose that the $U$ values that correctly capture the temperature-dependent structures of these complex oxides can be identified by comparison of the MLFF simulated and experimentally determined structures.

We conducted electron paramagnetic resonance (EPR) and cyclotron resonance studies on hydrogen-related donors (HDs) in silicon, whose origins have remained unclear for 50 years. In floating-zone silicon substrates irradiated with 2-MeV protons, followed by optimized annealing at $400{}^{\circ }\mathrm{C}$ and $500{}^{\circ }\mathrm{C}$, we identified three original EPR spectral signals originating from HDs, labeled the MT1, MT2, and MT3 centers. The three centers exhibited a tetragonal symmetry around the $\langle 100\rangle$ axis and were classified into two types of HDs: a HD with a lower thermal stability and a normal single-donor nature/positive-$U$ nature (MT1), and a HD with a higher thermal stability and a double-donor nature/negative-$U$ nature (MT2 and MT3). Most importantly, the MT2 center revealed ${}^{29}\mathrm{Si}$ hyperfine interactions closely resembling the B3 EPR center with a tetragonal symmetry, which has been identified as a tetrainterstitial cluster, $I_4$. Contrarily, we could not resolve any ${}^1\mathrm{H}$ hyperfine interactions for the three MT centers. According to the experimental results, an atomistic model was proposed for the HDs based on the $I_4$ center weakly coupled with hydrogen atom(s). We deduce that different numbers of incorporated hydrogen atoms and/or different types of hydrogen bonding to $I_4$ generated the two types of HDs. These findings are the first step in understanding how hydrogen works as a donor in Si and offer important insights into the use of proton-irradiation doping processes in device applications.

In the theory of composites, hierarchical laminate microstructures are known to often show optimal behavior. In this paper, their performance in the context of the Hall effect is evaluated. Using numerical calculations, it is shown that—despite the fact that they are the result of a simple layering process—they can mimic the behavior of chain-mail-inspired composites, which exhibit a sign inversion of the effective Hall coefficient. To obtain such a hierarchical laminate, a two-step strategy is used: In the first step, a rank-three laminate with an effective $S$ tensor that has a sign-inverted trace is introduced. In the second step, the final isotropic hierarchical laminate is obtained from the rank-three laminate using an idea of Schulgasser's. As measured by the conductivity contrast required for the inversion as well as by the modulus of the obtained sign-inverted Hall coefficient, the identified hierarchical laminate performs better than the previously studied chain-mail-inspired composites.

We develop a mean-field theory of a novel Kondo effect emerging in systems without a Fermi surface, which instead emerges under strong magnetic fields. We determine the magnitude of the Kondo condensate, which is a particle pairing composed of conducting Dirac fermions and localized impurities. We focus on the competition between the Kondo effect and the energy gap formation that stems from the pairing among the Dirac fermions leading to the dynamical chiral symmetry breaking. We find that this competition induces a quantum critical point. We also investigate finite-temperature effects. This system at vanishing fermion density can be studied with Monte Carlo lattice simulations, which do not suffer from the sign problem.

We show that anisotropic quantum corrections can dynamically give rise to curved momentum-spacetimes for quasiparticles in metals. In the ($2+1$)-dimensional antiferromagnetic quantum critical metal, a curved momentum-spacetime arises as the critical spin fluctuations generate red shift that dilates frequency of electron unevenly on the Fermi surface. As the disparity of the momentum-dependent red shift is controlled by the shape of the Fermi surface, the momentum-spacetime geometry that emerges at low energies depends on the bare nesting angle of the Fermi surface. With increasing nesting angle, the region in which electron motion is slowed down by critical spin fluctuations shrinks. On the other hand, the increasing nesting angle makes the red shift stronger near the hot spots due to the weakened screening of the interaction. These competing effects result in a nonmonotonic dependence of the cyclotron frequency of electron on the nesting angle of the Fermi surface. The red shift that becomes more singular at the hot spots with increasing nesting angle creates a possibility of realizing a momentum-space black hole horizon beyond a critical nesting angle: The electron motion becomes “perpetually” slowed down as it approaches a hot spot in the same way that the motion of a free falling object freezes near the event horizon of a black hole with respect to an asymptotic observer. However, the analogous horizon in momentum space does not lead to a vanishing cyclotron frequency because the metric singularity at the hot spots is cut off by thermal effects present above the nonzero superconducting transition temperature.

We present a comprehensive study of generalized Wigner crystals across various filling factors for system sizes up to 162 holes employing Hartree-Fock theory and explicitly correlated wave function approaches. While we find broad agreement with the behavior observed in experiments and classical Monte Carlo simulations, we highlight the fact that the Hartree-Fock energy landscape appears to be remarkably complex, exhibiting many competing states, both ordered and disordered, separated by energies of a fraction of $\approx 1 \mathrm{meV}/\mathrm{hole}$. We demonstrate which of the located states are metastable by performing a stability analysis at the Hartree-Fock level. Correlated wave function methods furthermore reveal small correlation energies that are nevertheless large enough to tip the balance of state ordering found within Hartree-Fock theory.

Amid the growing interest in non-Hermitian quantum systems, noninteracting models have received the most attention. Here, through the stochastic series expansion quantum Monte Carlo method, we investigate non-Hermitian physics in interacting quantum systems, e.g., various non-Hermitian quantum spin chains. While calculations yield consistent numerical results under open boundary conditions, non-Hermitian quantum systems under periodic boundary conditions observe an unusual concentration of imaginary-time worldlines over nontrivial winding and require enhanced ergodicity between winding-number sectors for proper convergence. Such nontrivial worldline winding is an emergent physical phenomenon that also exists in other non-Hermitian models and analytical approaches. Alongside the non-Hermitian skin effect and point-gap spectroscopy, it largely extends the identification and analysis of non-Hermitian topological phenomena to quantum systems with interactions, finite temperatures, biorthogonal basis, and periodic boundary conditions in a controlled fashion. Finally, we study the direct physical implications of such nontrivial worldline winding, which bring additional, potentially quasi-long-range, contributions to the entanglement entropy.

The recent discovery of superconductivity in infinite-layer nickelates has drawn considerable attention; however, a consensus on the fundamental building blocks and common ingredients is necessary to understand and describe their ground states and emergent properties is lacking. A series of experimental and theoretical studies have suggested that an effective two-band Hubbard model with Ni $3d_{x^2-y^2}$ and rare-earth ($R$) $5d$ character may describe the low-energy physics. Here, we study the ground state properties of this two-band model on four-leg cylinders using the density-matrix renormalization group (DMRG) technique to better grasp whether such a simple model can embody the essential physics. A key difference compared to single-band Hubbard materials is that the system is self-doped: even at overall half-filling, the $R$ band acts as an electron reservoir, hole doping the Ni layer, and fundamentally altering the physics expected from an undoped antiferromagnet. On the four-leg cylinder, the ground state is consistent with a Luttinger liquid, with antiphase modulations of the charge density in the Ni and $R$ layers having corresponding wave vectors that lock together. Light hole doping away from half-filling releases the locking between the Ni and the $R$ charge modulations, as the electron density in the $R$ band decreases and eventually becomes exhausted at a hole doping concentration that depends sensitively on the effective splitting between the Ni and the $R$ orbitals. The ground state of the doped system is consistent with a Luther-Emery liquid, possessing quasi-long-range superconducting correlations in the Ni layer, similar to the single-band Hubbard model. Our results are consistent with experimental observations and may help to reveal the microscopic mechanism for pairing and other emergent properties not only in the infinite-layer nickelates but also other unconventional superconductors.