The muon spin rotation ($\mu \mathrm{SR}$) technique has been applied to determine the behavior of the in-plane magnetic penetration depth (${\lambda }_{ab}$) in the vortex state of the unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ as a means of gaining insight into its still unknown superconducting order parameter. A recent $\mu \mathrm{SR}$ study of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ reported a $T$-linear temperature dependence for ${\lambda }_{ab}$ at low temperatures that was not identified in an earlier $\mu \mathrm{SR}$ study. Here we show that there is no significant difference between the data in the early and recent $\mu \mathrm{SR}$ studies and both are compatible with the limiting low-temperature ${\lambda }_{ab}\propto T^2$ dependence expected from measurements of $\mathrm{\Delta }{\lambda }_{ab}\left(T\right)={\lambda }_{ab}\left(T\right)-{\lambda }_{ab}\left(0\right)$ in the Meissner state by other techniques. However, we argue that at this time there is no valid theoretical model for reliably determining the absolute value of ${\lambda }_{ab}$ in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ from $\mu \mathrm{SR}$ measurements. Instead, we identify the formation of an unusual square vortex lattice that introduces another constraint on candidate superconducting order parameters for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

The quantum anomaly of a global symmetry is known to strongly constrain the allowed low-energy physics in a clean and isolated quantum system. However, the effect of quantum anomalies in disordered systems is much less understood, especially when the global symmetry is only preserved on average by the disorder. In this work, we focus on disordered systems with both average and exact symmetries $A\times K$, where the exact symmetry $K$ is respected in every disorder configuration, and the average $A$ is only preserved on average by the disorder ensemble. When there is a mixed quantum anomaly between the average and exact symmetries, we argue that the mixed state representing the ensemble of disordered ground states cannot be featureless. While disordered mixed states smoothly connected to the anomaly-compatible phases in clean limit are certainly allowed, we also found disordered phases with no clean-limit counterparts, including the glassy states with strong-to-weak symmetry breaking, and average topological orders for certain anomalies. We construct solvable lattice models to demonstrate each of these possibilities. We also provide a field-theoretic argument as a criterion for whether a given average-exact mixed anomaly admits a compatible average topological order.

Despite the numerous reports on the time-honored superconductor ${\mathrm{NiBi}}_3$, theoretical investigations remain limited, particularly regarding the impact of spin-orbit coupling (SOC). Based on first-principles calculations, the electronic topology of ${\mathrm{NiBi}}_3$ is mainly studied herein. Nodal loops which intersect to form a nodal chain structure are enclosed by two valence bands, and they are fully gapped by SOC. The full gap contains topological surface states defined by a ${\mathbb{Z}}_2$ topological invariant, while for the top valence band and bottom conduction band, a three-dimensional massless Dirac point at $\mathrm{\Gamma }$ is produced by the two bands with SOC. In addition, nodal surfaces which prevalently appear in the band structure without SOC are present on the entire Brillouin zone surface, and SOC only partially gaps them, thus leading to nodal lines unaffected by SOC. Type-I and type-II fermions, which are distinguished by the tilt of cones, are shown to coexist in the bulk bands. Furthermore, two Rashba-split bands are found on the (100) surface, whose inner and outer Fermi circles share the same spin polarization direction. Finally, our analysis reveals that the superconductivity of ${\mathrm{NiBi}}_3$ primarily originates from the $p$ orbitals of Bi and $d$ orbitals of Ni coupling with the vibration modes of Bi. The rich topological features identified in this study suggest that ${\mathrm{NiBi}}_3$ holds significant potential for realizations of topological superconductivity and other exotic phenomena.

In many-body systems with U(1) global symmetry, the charge fluctuations in a subregion reveal important insights into entanglement and other global properties. For subregions with sharp corners, bipartite fluctuations have been predicted to exhibit a universal shape dependence on the corner angle in certain quantum phases and transitions, characterized by a “universal angle function” and a “universal coefficient.” However, we demonstrate that this simple formula is insufficient for charge insulators, including composite Fermi liquids. In these systems, the corner contribution may depend on the corner angle, subregion orientation, and other microscopic details. We provide an infinite series representation of the corner term, introducing orientation-resolved universal angle functions with their nonuniversal coefficients. In the small-angle limit or under orientation averaging, the remaining terms' coefficients are fully determined by the many-body quantum metric, which, while not universal, adheres to both a universal topological lower bound and an energetic upper bound. We also clarify the conditions for bound saturation in (anisotropic) Landau levels, leveraging the generalized Kohn theorem and holomorphic properties of many-body wave functions. We find that a broad class of fractional quantum Hall wave functions, including unprojected parton states and composite-fermion Fermi sea wave functions, saturates the bounds.

We present a theoretical study of mapping between Chern bands and generalized Landau levels in twisted bilayer ${\mathrm{MoTe}}_2$ ($t{\mathrm{MoTe}}_2$), where fractional Chern insulators down to zero magnetic fields have been observed. We construct an exact Landau-level representation of moiré bands, where the basis functions, characterized by a uniform quantum geometry, are derived from Landau-level wave functions dressed by spinors aligned or antialigned with the layer pseudospin skyrmion field. We further generalize the dressed zeroth Landau level to a variational wave function with an ideal yet nonuniform quantum geometry and variationally maximize its weight in the first moiré band. The variational wave function has a high overlap with the first band and quantitatively captures the exact-diagonalization spectra of fractional Chern insulators at hole-filling factors ${\nu }_h=2/3$ and $3/5$, providing a clear theoretical mechanism for the formation and properties of the fractionalized states. Our work introduces a variational approach to studying fractional states by mapping Chern bands to Landau levels, with application to other systems beyond $t{\mathrm{MoTe}}_2$ also demonstrated.

First-principles density functional theory (DFT) codes which employ a localized basis offer advantages over those which use plane-wave bases, such as better scaling with system size and better suitability to low-dimensional systems. The trade-off is that care must be taken in order to generate a good localized basis set which is efficient and accurate in a variety of environments. Here we develop and make freely available optimized local basis sets for two common two-dimensional materials, graphene and hexagonal boron nitride, for the siesta DFT code. Each basis set is benchmarked against the abinit plane-wave code, using the same pseudopotentials and exchange-correlation functionals. We find that a significant improvement is obtained by including the $l+2$ polarization orbitals ($4f$) in the basis set, which greatly improves angular flexibility. The optimized basis sets yield much better agreement with plane-wave calculations for key features of the physical system, including total energy, lattice constant, and cohesive energy. The optimized basis sets also result in a speedup of the calculations with respect to the nonoptimized, native choices.

We investigate the signatures of fractionalization in quantum spin liquids by studying different phases of the Kitaev honeycomb model in the presence of an out-of-plane magnetic field through which the model becomes nonintegrable. Using the infinite projected entangled pair states (iPEPS) ansatz, along with analytical calculations and exact diagonalization, we calculate dynamical signatures of fractionalized particles through spin-spin and dimer-dimer correlations. Our analysis demonstrates the ability of these correlations to discern distinct fractionalized quantum sectors, namely Majorana fermions and the emergent $Z_2$ fluxes, in both the chiral spin-liquid (CSL) phase under weak field and the emergent intermediate gapless phase (IGP) under moderate field. Importantly, our calculation reveals the nature of IGP observed at moderate fields, a region of ongoing debate, indicating that this phase is a Majorana metal induced by strong flux fluctuations.

The rare-earth-based geometrically frustrated triangular magnets have attracted considerable attention due to the intricate interplay between strong spin-orbit coupling and the crystal electric field (CEF), which often leads to effective spin-1/2 degrees of freedom and therefore promotes strong quantum fluctuations at low temperatures, thus offering an excellent route to stabilize a quantum spin liquid (QSL) ground state. We have investigated the ground state magnetic properties of a polycrystalline sample of ${\text{SmTa}}_7{\text{O}}_{19}$ which we propose to have a gapless QSL ground state by employing powder x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), DC and AC-magnetic susceptibility, $M$ versus $H$ isotherm, specific heat, and muon spin rotation/relaxation measurements ($\mu \mathrm{SR}$) down to 30 mK. The combined structural and electronic studies reveal the formation of an edge-sharing equilateral triangular lattice of ${\mathrm{Sm}}^{3+}$ ions in $ab$ plane. The DC and AC magnetic susceptibility, and heat capacity measurements reveal that ${\text{SmTa}}_7{\text{O}}_{19}$ does not exhibit any long-range magnetic ordering transition down to 50 mK. The zero-field (ZF)-$\mu \mathrm{SR}$ study strongly refutes the long-range magnetically ordered ground state and/or any partial spin-freezing down to at least 30 mK. The ZF-muon-spin relaxation rate is weakly temperature-dependent between 50 and 20 K, rapidly increases below $\sim 20\mathrm{K}$ and saturates at low temperatures between 2 K and 30 mK, which has been attributed to a characteristic signature of QSL systems. Further, our longitudinal-field (LF)-$\mu \mathrm{SR}$ measurements at 0.1 K reveal a dynamic nature of the magnetic ground state. In addition, our high-field specific heat data suggest a gapless nature of spin excitations in this compound.

We analyze superconductivity in a multiorbital fermionic system near the onset of nematic order, using doped FeSe as an example. We associate nematicity with a spontaneous polarization between $d_{xz}$ and $d_{yz}$ orbitals (a Pomeranchuk-type order) and analyze the pairing mediated by soft nematic fluctuations. Such a pairing gives rise to a highly anisotropic gap function whose structure strongly varies with temperature and leads to strongly non-BCS behavior in thermodynamics, spectroscopy, and transport. We compute the specific heat and its directional variation with a magnetic field, magnetic susceptibility, density of states, tunneling conductance, Raman intensity, superfluid stiffness, and penetration depth, both without and with impurity scattering. For the latter, we also compute optical conductivity and the $T_c$ variation. We find good agreement with existing data for ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ and ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and suggest new experiments.

We measured the microwave response of a HgTe quantum well Josephson junction embedded into an RF SQUID loop, which is inductively coupled to a superconducting resonator. The side-contacted devices studied here operate in bulk transport mode, with a separation between the superconducting contacts smaller than both the estimated carrier mean free path and superconducting coherence length. We extract the current-phase relation and the phase-dependent microwave dissipation, that, at low temperature, primarily is related to photon-induced transitions between the Andreev bound states. We study the effects of gate voltage and temperature on our devices and compare the measurements with a tight-binding model based on the Bogoliubov-de Gennes equations. A combined analysis of both microwave admittance components allows us to confirm the presence of a small gap in the Andreev bound state spectrum at phase $\pi$, indicating high interface transparency, matching our observations in dc measurements of a similar device. Our paper demonstrates the versatility of microwave measurements as a tool for Josephson junction characterization and highlights the importance of the interface properties for side-contacted Josephson devices.

The composite fermion Fermi liquid (CFL) state at $\nu =1/2$ filling of a Landau level is a paradigmatic non-Fermi liquid borne out purely by Coulomb interactions. But in what ways is this exotic state of matter different from a Fermi liquid? The CFL entanglement entropy was indeed found to exhibit a significant enhancement compared to free electrons [J. Shao et al., Phys. Rev. Lett. 114, 206402 (2015)], which was subsequently ruled out as a finite-size effect by the study of a lattice CFL analog [R. V. Mishmash and O. I. Motrunich, Phys. Rev. B 94, 081110(R) (2016)]. Moreover, the enhancement was not observed in a quasi-one-dimensional limit of the Coulomb ground state at $\nu =1/2$ [S. D. Geraedts et al., Science 352, 197 (2016)]. Here, we revisit the problem of entanglement scaling in the CFL state realized in a two-dimensional electron gas. Using Monte Carlo evaluation of the second Rényi entropy $S_2$ for the CFL variational wave function, we show that the entanglement enhancement is present not only at $\nu =1/2$, but also at $\nu =1/4$, as well as in bosonic CFL states at $\nu =1$ and $\nu =1/3$ fillings. In all cases, we find the scaling of $S_2$ with subsystem size to be enhanced compared to the noninteracting case, and insensitive to the choice of geometry and projection to the lowest Landau level. We also demonstrate that for CFL states, the variance of the particle number in a subsystem obeys area-law scaling with a universal subleading corner contribution, in stark contrast with free fermions. Our results establish the enhanced entanglement scaling and suppressed charge fluctuations as fingerprints of non-Fermi-liquid correlations in CFL states.

We report on single-crystal growth and physical properties of the quantum critical Kondo-lattice system ${\mathrm{YbNi}}_4{\left({\mathrm{P}}_{1-x}{\mathrm{As}}_x\right)}_2$ with $0\le x\le 1$ which hosts a ferromagnetic quantum critical point at $x\approx 0.1$. We performed measurements of the magnetization, electrical resistivity, thermopower, heat capacity, and resonant x-ray emission spectroscopy. Arsenic substitution leads to a homogeneous increase of the unit-cell volume, with well-defined As concentrations in large parts of the single crystals. All data consistently show that with increasing $x$ the Kondo temperature increases, while the magnetic anisotropy observed at low $x$ fully vanishes toward $x=1$. Consequently, at low temperatures, the system shows a crossover from pronounced non-Fermi liquid behavior for $x\le 0.2$ to a Fermi liquid behavior for $x>0.2$ with weak correlations. There is a continuous change in Yb valence from nearly trivalent at low $x$ to a slightly lower value for $x=0.6$, which correlates with the Kondo temperature. Interestingly, specific heat measurements at very low temperatures show that $C/T$ strongly increases toward lower $T$ for $x=0.13$ and $x=0.2$ with a very similar power law. This suggests that in ${\mathrm{YbNi}}_4{\left({\mathrm{P}}_{1-x}{\mathrm{As}}_x\right)}_2$ a quantum critical line rather than a quantum critical point might exist.

A few-electron quantum dot is shown to exhibit a millionfold magnetoresistance if the source and drain electron reservoirs have spin accumulations $\mathrm{\Delta }{\mu }_S$ and $\mathrm{\Delta }{\mu }_D$ that are induced and controlled by two ferromagnetic contacts. If $\mathrm{\Delta }{\mu }_S$ and $\mathrm{\Delta }{\mu }_D$ have the same sign, a single-electron spin is trapped on the quantum dot and the current vanishes due to Pauli spin blockade between the single quantum dot and the spin-polarized reservoirs. Charge can flow freely when $\mathrm{\Delta }{\mu }_S$ and $\mathrm{\Delta }{\mu }_D$ are antiparallel. Transport calculations for sequential tunneling show that when the thermal energy is small compared to the singlet-triplet splitting energy, the current on/off ratio is above ${10}^6$ for Si quantum dots. The device can operate as a spin transistor with huge magnetoresistance, but can also readily be switched to operate as a spin qubit by reconfiguring the source-drain bias and the gate voltage.

Recent reports of colossal negative magnetoresistance (CMR) in a few magnetic semimetals and semiconductors have attracted attention because these materials are devoid of the conventional mechanisms of CMR such as mixed valence, double-exchange interaction, and Jahn-Teller distortion. New mechanisms have thus been proposed, including topological band structure, ferromagnetic clusters, orbital currents, and charge ordering. The CMR in these compounds has been reported in two forms: either a resistivity peak or a resistivity upturn suppressed by a magnetic field. Here we reveal both types of CMR in a single antiferromagnetic semiconductor ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{As}}_6$. Using the transport and thermodynamic measurements, we demonstrate that the peak-type CMR is likely due to the percolation of magnetic polarons with increasing magnetic field, while the upturn-type CMR is proposed to result from the melting of a charge order under the magnetic field. We argue that similar mechanisms operate in other compounds, offering a unifying framework to understand CMR in seemingly different materials.

The spatial distribution of spurious dots in SiMOS single-electron transistors (SETs), fabricated on an industrial 300 mm process line, has been statistically analyzed. To have a deeper understanding of the origin of these spurious dots, we analyzed SETs with three different oxide thicknesses: 8, 12, and 20 nm. By combining spurious dot triangulation cryo measurement with simulations of strain, gate bias, and location of the electron wave function, we demonstrate that most spurious dots are formed through the combined effects of strain and gate bias, leading to variations in the conduction band energy. Despite the similar thermal expansion coefficients of polycrystalline silicon gates and single-crystalline silicon substrates, strain remains a crucial factor in spurious dot formation. This learning can be used to optimize the device design and the oxide thickness, to reduce the density of spurious dot while keeping quantum dot tunability.

In two-dimensional (2D) layered materials, the layer acts as a crucial degree of freedom, enabling the emergence of exotic physical phenomena through its coupling with charge and spin. Here, using first-principles calculations and group theory analysis, we reveal that layer Nernst and layer thermal Hall effects are present in various 2D $A$-type layered antiferromagnets, including the metal $\mathrm{H}{\mathrm{f}}_2\mathrm{S}$, the normal insulator $\mathrm{Cr}{\mathrm{I}}_3$, and the topological insulator $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$. The application of an out-of-plane electric field removes the key magnetic group symmetries and breaks the spin degeneracy of the electronic structures in these 2D antiferromagnets. This spin splitting induces an imbalanced distribution of Berry curvature between the top and bottom layers. By quantitatively calculating the layer-resolved Nernst and thermal Hall conductivities, we confirm the existence of nonzero transverse charge and heat currents in response to a longitudinal temperature gradient field. The calculated conductivities are comparable to, or might even exceed, those of previously well-known altermagnets and noncollinear antiferromagnets that exhibit anomalous Nernst and thermal Hall effects. Our findings provide deeper insights into the thermal transport properties of 2D layered antiferromagnets and pave the way for future developments in antiferromagnetic spintronics, spin caloritronics, and layertronics.

Magnetically frustrated triangular lattice materials are predicted to host exotic quantum states in the presence of conduction electrons. Yet, materials wherein the balance between short-range frustrated exchange interactions and long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions can be fine tuned remain rare. Here, we present a comprehensive investigation of the triangular-lattice antiferromagnet ${\mathrm{CeLi}}_3{\mathrm{Bi}}_2$. Heat capacity, electrical resistivity, and nuclear magnetic resonance (NMR) indicate that ${\mathrm{CeLi}}_3{\mathrm{Bi}}_2$ is a low-carrier-density semimetal, in agreement with our band structure calculations. Further, NMR spin-lattice relaxation results reveal competing RKKY interactions and weak Kondo screening. Thermodynamic, magnetization, and neutron diffraction data are well described by an isolated ground-state Kramers doublet that orders antiferromagnetically at $T_N=1.28$ K in a stripe structure, a strong indicator of magnetic frustration. Our combined findings place ${\mathrm{CeLi}}_3{\mathrm{Bi}}_2$ as a promising platform for tuning the interplay between geometric frustration and RKKY physics.

Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes-like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer graphene and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound, suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density.

Centrosymmetric ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ exhibits a variety of multi-$Q$ magnetic states as a function of temperature and applied magnetic field, including a square skyrmion-lattice phase. The material's behavior is strongly dependent on the direction of the applied field, with different phase diagrams resulting for fields applied parallel or perpendicular to the crystallographic $c$ axis. Here, we present the results of muon-spin relaxation (${\mu }^{+}\mathrm{SR}$) measurements on single crystals of ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$. Our analysis is based on the computation of muon stopping sites and consideration of quantum zero-point motion effects of muons, allowing direct comparison with the underlying spin textures in the material. The muon site is confirmed experimentally, using angle-dependent measurements of the muon Knight shift. Using transverse-field ${\mu }^{+}\mathrm{SR}$ with fields applied along either the [001] or [100] crystallographic directions, we distinguish between the magnetic phases in this system via their distinct muon response, providing additional evidence for the skyrmion and meron-lattice phases, while also suggesting the existence of RKKY-driven muon hyperfine coupling. Zero-field ${\mu }^{+}\mathrm{SR}$ provides clear evidence for a transition between two distinct magnetically ordered phases at 39 K.

A quantitative method based on harmonic Hall voltage analysis for measuring gate voltage-driven changes in magnetic anisotropy at ferromagnet/oxide (FM/Ox) interfaces has been developed and experimentally validated. The method involves applying an AC voltage simultaneously across the Hall bar and oxide gate using the same voltage source, measuring the first and second harmonic Hall voltage responses, and performing subsequent data analysis. Measurements conducted at high modulation frequencies primarily probe electrostatic effects while minimizing the influence of slower magnetoionic dynamics. Based on the devised method, the voltage-controlled magnetic anisotropy (VCMA) efficiency for the Co/AlOx interface is found to be approximately $-40$ fJ/(V m), which is in agreement with previous reports. Additionally, this approach enables the measurement of voltage-induced changes in anomalous Hall resistance. The results are interpreted using a spin-dependent screening model, which describes the experimental data remarkably well. The developed methodology applies to a wide range of materials for characterizing and optimizing VCMA and related phenomena in spintronic devices.