The half-Heusler alloy RuMnGa having valence electron count (VEC) 18 has recently been theoretically proposed to exhibit compensated ferrimagnetic (CFiM) character instead of the expected nonmagnetic ground state. On the other hand, a preliminary experimental study proposed ferromagnetic (FM) ordering. As no half-Heusler system with VEC 18 is known to exhibit magnetic ordering, we have investigated the details of crystal structure and magnetic properties of RuMnGa using a combination of experimental tools, viz., x-ray and neutron diffraction techniques, dc and ac susceptibility, isothermal magnetization, heat capacity, resistivity and neutron depolarisation measurements. Rietveld refinements of x-ray and neutron diffraction data suggest single phase nature of the compound with elemental composition ${\mathrm{RuMn}}_{0.86}{\mathrm{Ga}}_{1.14}$. We have shown that the system exhibits FM-type ordering owing to the inherent presence of this minor off-stoichiometry, showing very low magnetic moment. The system also exhibits reentrant canonical spin-glass behavior, which is rarely observed in half-Heusler alloys. The temperature coefficient of resistivity changes its sign from negative to positive and further to negative as the temperature decreases.

Considering the vast compositional space of Heusler alloys, first-principles-based calculations are ideally suitable for predicting the ground state structure and tailoring the magnetic properties of these alloys. We perform density-functional-theory-based calculations for step-by-step identification of the most stable phase of ${\mathrm{Fe}}_2\mathrm{MnSn}$, taking into account all the different structural phases exhibited for this alloy (viz., cubic $\mathrm{L}{2}_1$, cubic XA, tetragonal $\mathrm{L}{2}_1$, tetragonal XA, and hexagonal ${\mathrm{D}0}_{19}$), followed by the calculations of magnetic properties. We identify the magnetic ground state of each phase and then the most stable structural phase by taking into account electronic and geometric relaxation, spin polarization, and vibrational free energy contributions. The ferromagnetic configuration of all the phases is found to be energetically the most favorable magnetic state, while the ferromagnetic hexagonal phase is identified to be the stable structural phase of ${\mathrm{Fe}}_2\mathrm{MnSn}$, with a sizable magnetization of $6.45{\mu }_B/\mathrm{f}.\mathrm{u}.$ Furthermore, the exchange interactions in the hexagonal phase are calculated using the Liechtenstein approach, and this phase shows a high Curie temperature of 729 K attributed to the strong Fe-Fe exchange coupling. The stable hexagonal phase reveals an in-plane magnetic anisotropy of $-1.24 \mathrm{MJ}/{\mathrm{m}}^3$. The large magnetization and high Curie temperature of this phase can make this material suitable for desired magnetic applications. Computational investigations such as this one, in addition to being a cost effective pathway, may provide many valuable insights for the experimental realization and application of a given alloy.

Intrinsic topological superconducting materials are exotic and vital to develop the next-generation topological superconducting devices, topological quantum calculations, and quantum information technologies. Here, we predict the topological and nodal superconductivity of NiAs-type $M\mathrm{S}$ $(M=\mathrm{Nb}\text{and}\mathrm{Ta})$ transition-metal sulfides. We reveal their higher-order topology nature with an index of $Z_4=2$. These materials have a higher $T_{\mathrm{c}}$ than the Nb or Ta metal superconductors due to their flat band and strong electron-phonon coupling nature. Electron doping and lighter isotopes can effectively enhance the $T_{\mathrm{c}}$. Our findings show that the $M\mathrm{S}$ $(M=\mathrm{Nb}\text{and}\mathrm{Ta})$ systems can be platforms to study exotic physics in the higher-order topological superconductors, and provide a theoretical support to utilize them as the topological superconducting devices in the field of advanced topological quantum calculations and information technologies.

We consider a planar heterostructure consisting of a normal metal in proximity to an antiferromagnetic insulator, with an interlayer exchange coupling between the metal and the insulator. The coupling to the two sublattices of the antiferromagnetic insulator is allowed to be asymmetric. An effective electron-electron interaction in the normal metal, mediated by antiferromagnetic magnons in the insulator, is derived to second order in the interlayer exchange coupling. Particular emphasis is placed on including analytically derived expressions for the effective interactions including Umklapp processes in the solutions to the superconducting gap equation. The gap equation is first solved at the critical temperature as an eigenvalue problem by linearizing the gap equation. The eigenvectors yield information on the symmetry of the superconducting gap at the onset of superconductivity, and we derive a phase diagram for the order parameter in this case. In the various regimes of the phase diagram, we find $p$-wave, $f$-wave, and $d$-wave superconductivity, with $p$-wave superconductivity in the dominant part of the phase diagram. Umklapp processes, that come into play with increasing size of the Fermi surface, yield $f$- and $d$-wave symmetries as the preferred symmetries when band filling approaches half filling. To investigate the stability of this order parameter symmetry as the temperature is lowered, we also consider the nonlinear gap equation at zero temperature. We conclude that the phase diagram and the symmetries of the superconducting order parameter essentially are left intact as the temperature is lowered to zero temperature.

Despite extensive existing studies, a complete understanding of the role of disorder in affecting the physical properties of two-dimensional Dirac fermionic systems remains a standing challenge, largely due to obstacles encountered in treating multiple scattering events for such inherently strong scattering systems. Using graphene as an example and a nonperturbative numerical technique, here we reveal that the low-energy quasiparticle properties are considerably modified by multiple scattering processes even in the presence of weak scalar potentials. We extract unified power-law energy dependencies of the self-energy with fractional exponents from the weak scattering limit to the strong scattering limit from our numerical analysis, leading to sharp reductions of the quasiparticle residues near the Dirac point, eventually vanishing at the Dirac point. The central findings stay valid when the Anderson-type impurities are replaced by correlated Gaussian- or Yukawa-type disorder with varying correlation lengths. The improved understanding gained here also enables us to provide better interpretations of the experimental observations surrounding the temperature and carrier density dependencies of the conductivity in ultrahigh mobility graphene samples. The approach demonstrated here is expected to find broad applicability in understanding the role of various other types of impurities in two-dimensional Dirac systems.

In recent experimental and theoretical studies of graphene, disorder scattering processes have been suggested to play an important role in its electronic and transport properties. In a preceding paper, it has been shown that the nonperturbative momentum-space Lanczos method is able to accurately describe all the multiple impurity scattering events and account for the quasiparticle and transport properties of disordered monolayer graphene. In the present study, we expand the range of applicability of this recursive method by numerically investigating the quasiparticle and transport properties of Bernal-stacked bilayer graphene in the presence of scalar Anderson disorder. The results are further compared with the findings of the same system using a self-consistent Born approximation, as well as the central findings in the preceding paper for monolayer graphene. It is found that in both systems, proper inclusions of all the scattering events are needed in order to reliably capture the role of disorder via multiple impurity scattering. Furthermore, we reveal the dependences of the transport properties of disordered bilayer graphene on the carrier density and temperature, and explore the role of interlayer scattering at varying strengths. Our findings may help to provide some new angles into the quasiparticle and transport properties of disordered bilayer graphene.

Two-dimensional ferromagnetic semiconductors are gaining increasing interest because of their promising applications in spintronics. However, the applications are greatly hindered by the weak ferromagnetic couplings and low Curie temperatures. Therefore, rationally designing two-dimensional ferromagnetic semiconductors with high Curie temperatures and enhancing their Curie temperatures are highly desirable, which inevitably requires a fundamental understanding of modulating superexchange interactions. Here, we propose two-dimensional intrinsic ferromagnetic semiconductors $\mathrm{Mo}\mathit{XY}$ $(X\hspace{0.17em}=\hspace{0.17em}\mathrm{S},\mathrm{Se};Y\hspace{0.17em}=\hspace{0.17em}\mathrm{Br},\mathrm{I})$. They are predicted to have unique quasi-1D transport behavior, high Curie temperatures (ranging from 290 K to 322 K), and large magnetic anisotropy energy. Based on the Kanamori's mechanism, we propose a general path-resolved indicator of the superexchange interaction strength to unravel how ferromagnetic superexchange interactions are modulated by two heavily used strategies, namely, ligand substitution and strain engineering. Our paper could provide a fundamental understanding of modulating superexchange interactions in two-dimensional ferromagnetic semiconductors.

${\mathrm{Mn}}_{1-x}{\mathrm{Cd}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ spinel series is engineered to host a controlled and wide-range tuning of magnetic frustration through a nonmagnetic ion (Cd) doping in ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ spinel oxide. While ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ exhibits weak geometric magnetic frustration (GMF), the doping brings in an interplay between various competing magnetic-exchange interactions involving the Jahn-Teller inactive ${\mathrm{Cr}}^{3+}$($3d{}^3$) spins that lie on a frustrated pyrochlore lattice in cubic spinel oxide. Magnetic frustration is observed to be the maximum for the intermediate values of $\mathit{x}$ which are associated with large spin-relaxation times and an evolution in the nature of glassy spin dynamics. The dominant antiferromagnetic coupling between the Mn sites and Cr sites (which induces the intra-Cr-lattice spins to be nearly ferromagnetically coupled in $x=0)$ becomes weaker due to magnetic dilution of the Mn site by diamagnetic ${\mathrm{Cd}}^{2+}$ ions. As a result, the net interactions between ${\mathrm{Cr}}^{3+}$ ions in the pyrochlore lattice become antiferromagnetic beyond $x\gtrsim 0.6$. This causes the associated GMF to become very large for $x\gtrsim 0.6$. As a result, while the structure remains cubic till the lowest temperature for $0\le x\le 0.60$, the appearance of strong GMF induces spin-driven magnetostructural transitions from a high-temperature cubic (paramagnetic) to a low-temperature tetragonal (and antiferromagnetic) phase. The obtained results demonstrate a direct link between GMF and spin-lattice coupling and provide a road map to design compounds with engineered magnetostructural transitions associated with large spin-lattice coupling.

Spinful triplet Cooper pairs can be generated from their singlet counterparts available in a conventional superconductor (S) using two or more noncollinear magnetic moments, typically contributed by different magnets in a multilayered heterostructure. Here, we theoretically demonstrate that a S interfaced with a canted antiferromagnet (AFM) harbors spinful triplet Cooper pairs capitalizing on the intrinsic noncollinearity between the two AFM sublattice magnetizations. As the AFM canting can be controlled by an applied field, our paper proposes a simple bilayer structure that admits controllable generation of spin-triplet Cooper pairs. Employing the Bogoliubov–de Gennes framework, we delineate the spatial dependence of the spin-triplet correlations. We further evaluate the superconducting critical temperature as a function of the AFM canting, which provides one experimental observable associated with the emergence of these triplet correlations.

An underestimation of the fundamental band gap values by the density functional theory within the local density approximation and associated approaches is a well-known challenge of ab initio electronic structure computations. Motivated by recent optical experiments [D. Santos-Cottin et al., arXiv:2301.08014], we have revisited first-principles results obtained earlier for ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ and extended the computational studies to the whole class of systems ${\mathrm{EuCd}}_2{X}_2$ ($X=\mathrm{P}$, As, Sb, Bi), to ${\mathrm{EuIn}}_2{X}_2$ ($X=\mathrm{P}$, As, Sb), and to nonmagnetic ${\mathrm{AEIn}}_2{\mathrm{As}}_2$ ($AE=\mathrm{Ca}$, Sr, Ba) employing a hybrid functional method. We find that our approach provides the magnitude of the energy gap for ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ in agreement with the experimental value. Actually, our results indicate that ${\mathrm{EuSn}}_2{\mathrm{As}}_2, {\mathrm{BaIn}}_2{\mathrm{As}}_2, {\mathrm{EuCd}}_2{\mathrm{Bi}}_2$ and ${\mathrm{EuCd}}_2\mathrm{SbBi}$ are robust topological insulators, while all other compounds are topologically trivial semiconductors. The trivial band gaps of ${\mathrm{EuCd}}_2{\mathrm{P}}_2, {\mathrm{EuCd}}_2{\mathrm{As}}_2$, and ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ are in the range of 1.38–1.48 eV, 0.72–0.79 eV, and 0.46–0.49 eV, respectively. The topologically trivial Eu-based systems are antiferromagnetic semiconductors with a strong red shift of the energy gap in a magnetic field caused by the exchange coupling of the band states to spins localized on the $4f$-shell of Eu ions. Additionally, the ${\mathrm{EuIn}}_2{\mathrm{X}}_2$ ($X=\mathrm{P}$, As) compounds show altermagnetic exchange-induced band spin-splitting, particularly noticeable in the case of states derived from $5d$-Eu orbitals.

We study the quantum dynamics of a many-body system subject to coherent evolution and coupled to a non-Markovian bath. We propose a technique to unravel the non-Markovian dynamics in terms of quantum jumps, a connection that was so far only understood for single-body systems. We develop a systematic method to calculate the probability of a quantum trajectory and formulate it in a diagrammatic structure. We find that non-Markovianity renormalizes the probability of realizing a quantum trajectory and that memory effects can be interpreted as a perturbation on top of the Markovian dynamics. We show that the diagrammatic structure is akin to that of a Dyson equation and that the probability of the trajectories can be calculated analytically. We then apply our results to study the measurement-induced entanglement transition in random unitary circuits. We find that non-Markovianity does not significantly shift the transition but stabilizes the volume law phase of the entanglement by shielding it from transient strong dissipation.

Recurrent neural networks (RNNs), originally developed for natural language processing, hold great promise for accurately describing strongly correlated quantum many-body systems. Here, we employ two-dimensional RNNs to investigate two prototypical quantum many-body Hamiltonians exhibiting topological order. Specifically, we demonstrate that RNN wave functions can effectively capture the topological order of the toric code and a Bose-Hubbard spin liquid on the kagome lattice by estimating their topological entanglement entropies. We also find that RNNs favor coherent superpositions of minimally entangled states over minimally entangled states themselves. Overall, our findings demonstrate that RNN wave functions constitute a powerful tool to study phases of matter beyond Landau's symmetry-breaking paradigm.

Utilizing a low-temperature scanning-tunneling microscope, we construct Fe kagome-honeycomb lattices on Ag(111) and investigate their lattice parameter dependent electronic properties. The probed spectra exhibit the characteristic lattice peaks, which gradually merge and form a flat-band peak with decreasing the ratio of the high-order and first nearest-neighbor (NN) hopping, in line with the tight-binding calculations. The one-to-one correspondence unambiguously demonstrates a nontrivial band compression effect by minimizing the high-order NN hopping. Our result provides an efficient approach to realizing flat bands and engineering the electronic properties with artificial structures.

We consider a superlattice formed by tunnel-connected identical holes, periodically placed in a two-dimensional topological insulator. We study tunneling transport through helical edges of these holes and demonstrate that the band structure of such helical crystal can be controlled by both gate electrodes and external magnetic filed. For integer and half-integer values of dimensionless magnetic flux through the holes, the spectrum possesses Dirac points whose positions and velocities can be tuned by gates. The deviation of magnetic flux from these special values by $\delta \varphi$ makes the Dirac cones massive, with the gap value $\mathrm{\Delta }\propto \left|\delta \varphi \right|$. At certain gate-dependent values of $\delta \varphi$ different Dirac points converge to a double Dirac point and then disappear with further increase of $\delta \varphi .$ Interaction between carriers may lead to strong renormalization of parameters $\alpha$ and $\beta$ controlling total tunnel coupling between holes and spin flip tunneling processes, respectively. We plot the renormalization flow in the plane $(\alpha ,\beta )$ and demonstrate multicritical behavior of the crystal—there is a multicritical fully unstable fixed point separating three different phases: independent rings, independent shoulders, and perfect spin-flip channels. We also find that defects in the crystal may lead to a formation of topologically protected qubits, which are not destroyed by temperature and can be also manipulated both by gates and by magnetic field. The possibility of purely electrical high-temperature control of the qubits opens a wide avenue for applications in the area of quantum computing.

We compute the effect of a $d$-wave magnetization (altermagnetism) on the spectrum of bound states (Andreev levels) in a junction between two $s$-wave superconductors (gap ${\mathrm{\Delta }}_0$ and phase difference $\varphi$). Compared to a nonmagnetic junction, the $\varphi$ dependence of the spectrum is shifted by an offset $\pm \delta \varphi$, dependent on the spin direction, so that the Andreev levels become spin polarized. In a planar junction, oriented along the crystalline axis of $d_{xy}$-wave symmetry, the excitation energies are determined by the normal-state transmission probability $T$ according to $E={\mathrm{\Delta }}_0\sqrt{1-T{sin}^2\frac{1}{2}(\varphi \pm \delta \varphi )\phantom{{}^0}}$. We calculate the corresponding Josephson energy and supercurrent, recovering the 0-$\pi$ transition of related studies.

Utilizing the exceptional characteristics of two-dimensional (2D) materials for solid-state electronic devices presents an appealing strategy that could potentially address the need to prolong Moore's law. Evidently, the prevailing fraction of technically viable materials, which have already been successfully scaled up for industrial production, belongs to the category of non–van der Waals (n-vdW) materials. In recent years, research on n-vdW 2D materials has garnered significant growth owing to their potential for diverse applications and the development of synthesis techniques. In this paper, we stabilize 1-atom-thick $\mathrm{Sc}X$ ($X=\mathrm{P}$, As, Sb) monolayers drawn from their n-vdW bulk counterpart in the wurtzite phase by applying a minimal tensile strain of 1–2%. The resulting high flexibility, owing to the extremely small in-plane elastic constants (6–43 N/m) and Young's modulus (6–20 N/m), suits them ideally for extensive strain engineering on a large scale. Complex mixing of acoustic and optic phonon modes for higher strains ensures a large shear-piezoelectric coefficient of up to $d_{16}=-228.08$, −469.87, and −397.52 pm/V for ScP, ScAs, and ScSb respectively. This coefficient notably surpasses that in amino acids, making it the highest reported to date, and is accompanied by high in-plane piezoelectric coefficients, $|d_{21}|$ and $|d_{22}|>100$ pm/V and highly strain-tunable shear piezoelectric coefficient $d_{15}$ ranging from −90 to 210 pm/V. The monolayers exhibit rich band structures, including flat bands at the top-most valence band and a large spin splitting of $\sim 100\mathrm{meV}$, making them ideal for applications in LED and laser devices and opening exciting avenues for exploration in spintronics. In this paper, we present an in-depth analysis of band flattening caused by tensile strain and demonstrate the strong integrability of ScP monolayer with Si substrate. The ScP monolayer retains its flat band feature when implanted on silicon, which promises significant advancements in various practical applications.

We compute transport and thermodynamic properties of a two-band spin-fermion model describing itinerant fermions in two dimensions interacting via $Z_2$ antiferromagnetic quantum critical fluctuations by means of a sign-problem-free quantum Monte Carlo approach. We show that the phase diagram of this model indeed contains a $d$-wave superconducting phase at low enough temperatures. However, a crucial question that arises is whether a non-Fermi-liquid metallic regime exists above $T_c$, exhibiting hallmark strange-metal transport phenomenology. Interestingly, we find that this version of the model describes a non-Fermi-liquid metallic regime that displays an approximately $T$-linear resistivity above $T_c$ for a strong fermion-boson interaction. Using the Nernst-Einstein relation, our QMC results also show that this strange metal phase exhibits a crossover from being characterized by a charge compressibility given approximately by ${\chi }_c\sim 1/T$ at high temperatures to being described by a charge diffusivity consistent with the scaling $D_c\sim 1/T$ at low temperatures. Therefore, our paper adds support to the view that the $Z_2$ antiferromagnetic spin-fermion model at strong coupling can be considered a minimal model that describes both unconventional superconductivity and strange metallicity, which are fundamentally interconnected in many important strongly correlated quantum materials.

Driven by recent developments in time-resolved photoemission spectroscopy, we extend the successful method of photoemission orbital tomography (POT) to excitons. Our theory retains the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. Analyzing results from three organic molecules, we classify generic exciton structures and give a simple interpretation in terms of natural transition orbitals. We validate our findings by directly simulating pump-probe experiments with time-dependent density functional theory.

Exciton-polaritons are bosoniclike elementary excitations in semiconductors, which have been recently shown to display large occupancy of topologically protected polariton bound states in the continuum in suitably engineered photonic lattices [V. Ardizzone et al., Nature (London) 605, 447 (2022)], compatible with the definition of polariton condensation. However, a full theoretical description of such condensation mechanism that is based on a driven-dissipative framework like the nonequilibrium Gross-Pitaevskii equation (NEGPE) is still missing. Here, we report on a general multimode theory inspired to the standard NEGPE, showing that it allows to fully interpret the recent experimental findings in patterned photonic lattices, including emission characteristics and condensation thresholds. Beyond that, it is shown that the polariton condensation in these systems is actually the result of an interplay between negative mass confinement of polariton eigenstates (e.g., due to the photonic gap originated from the periodic pattern in plane) and polariton losses. We are then able to show that polariton condensation can also occur in gap-confined bright modes, i.e., coupling of quantum well excitons to a dark photonic mode is not necessarily required to achieve a macroscopic occupation with low population threshold.