Coulomb drag between nanoscale conductors is of both fundamental and practical interest. Here, we theoretically study drag in a double quantum dot (QD) system consisting of a biased drive QD and an unbiased drag QD coupled via a direct interdot Coulomb interaction. We demonstrate that the Coulomb drag is driven by charge fluctuations in the drive QD, and show how the properties of the associated quantum noise allow one to distinguish it from, e.g., shot-noise-driven drag in circuits of weakly interacting quantum conductors. In the strong-interaction regime exhibiting an orbital (“pseudospin”) Kondo effect, the drag is governed by charge fluctuations induced by pseudospin-flip cotunneling processes. The quenching of pseudospin-flip processes by Kondo correlations are found to suppress the drag at low bias and introduce a zero-bias anomaly in the second-order differential transconductance. Finally, we show that the drag is maximized for values of the interdot interaction matching the lead couplings. Our findings are relevant for the understanding of drag in QD systems and provide experimentally testable predictions in different transport regimes.

We show that electronic materials with disallowed rotational symmetries that enforce quasiperiodic order can exhibit quantum oscillations and that these are generically associated with exotic “spiral Fermi surfaces.” These Fermi surfaces are self-intersecting, and characterized by a winding number of their surface tangent—a topological invariant—that is larger than one. We compute the nature of the quantum oscillations in two experimentally relevant settings which give rise to spiral Fermi surfaces: a “nearly-free-electron” quasicrystal, and ${30}^{\circ }$ twisted bilayer graphene.

Thanks to the development in quantum Monte Carlo technique, the compact U(1) lattice gauge theory coupled to fermionic matter at ($2+1$)D is now accessible with large-scale numerical simulations, and the ground state phase diagram as a function of fermion flavor ($N_f$) and the strength of gauge fluctuations is mapped out. Here we focus on the large fermion flavor case ($N_f=8$) to investigate the dynamic properties across the deconfinement-to-confinement phase transition. In the deconfined phase, fermions coupled to the fluctuating gauge field to form U(1) spin liquid with continua in both spin and dimer spectral functions, and in the confined phase fermions are gapped out into valence bond solid phase with translational symmetry breaking and gapped spectra. The dynamical behaviors provide supporting evidence for the existence of the U(1) deconfined phase and could shine light on the nature of the U(1)-to-VBS phase transition which is of the ${\mathrm{QED}}_3$-Gross-Neveu chiral O(2) universality whose properties are still largely unknown.

The observation of quantum oscillations in topological Kondo insulators ${\mathrm{SmB}}_6$ and ${\mathrm{YbB}}_{12}$ is a recent puzzling experimental discovery. Quantum oscillations observed in the resistivity and the magnetization are usually explained by the existence of the Fermi surface. However, Kondo insulators do not have a Fermi surface and thus should not show quantum oscillations. By performing dynamical mean-field calculations for topologically nontrivial Kondo insulators in a magnetic field, we analyze the effect of correlations on the emergence of quantum oscillations in narrow-gap topological Kondo insulators and demonstrate that the interplay between correlations and nonlocal hybridization, ubiquitously occurring in topological Kondo insulators, can lead to observable quantum oscillations without the necessity of a Fermi surface. Particularly, we show that correlations make it easier to observe quantum oscillations in the magnetization and the resistivity of the bulk material. The fundamental mechanism for these quantum oscillations is a combination of correlation effects and Landau levels coming very close to the Fermi energy. We furthermore demonstrate that quantum oscillations in a three-dimensional system can be understood by analyzing the physics on the two-dimensional planes in the momentum space for which the hybridization in direction of the magnetic field vanishes. We believe that this scenario is relevant to understanding the observation of quantum oscillations in the magnetic torque for ${\mathrm{SmB}}_6$ as well as oscillations in the resistivity and the magnetic torque of ${\mathrm{YbB}}_{12}$.

The three-qubit Toffoli gate plays an important role in quantum error correction and complex quantum algorithms such as Shor's factoring algorithm, motivating the search for efficient implementations of this gate. Here we introduce a Toffoli gate suitable for exchange-coupled electron spin qubits in silicon quantum dot arrays. Our protocol is a natural extension of a previously demonstrated resonantly driven cnot gate for silicon spin qubits. It is based on a single exchange pulse combined with a resonant microwave drive, with an operation time on the order of 100 ns and fidelity exceeding 99%. We analyze the impact of calibration errors and $1/f$ noise on the gate fidelity and compare the gate performance to Toffoli gates synthesized from two-qubit gates. Our approach is readily generalized to other controlled three-qubit gates such as the Deutsch and Fredkin gates.

To describe all possible injection locking configurations of spin torque nano-oscillators (STNO), previous analytical descriptions are extended by introducing the most general form of the driving force: $\mathcal{F}\propto {\varepsilon }_1\cos \psi +i{\varepsilon }_2\sin \psi$. We provide the expressions of the corresponding forcing functions $\mathcal{F}$ for the six basic conservative and dissipative forcing torques ($\propto \mathit{m}\times {\mathit{u}}_{\mathit{i}}$ and $\propto \mathit{m}\times \mathit{m}\times {\mathit{u}}_{\mathit{i}}$ with $i=x,y,z$), and demonstrate at the example of a uniform in-plane magnetized STNO, that the general case ($|{\varepsilon }_1|\ne |{\varepsilon }_2|$) as well as special cases, can occur depending on (i) the nature of the forcing torque, (ii) the direction ${\mathit{u}}_{\mathit{i}}$ with respect to the equilibrium direction, (iii) the harmonic order $n$ and (iv) the operation point (dc current). These relations provide a straightforward means to analyze more complex forcing torques such as a rotating field or the superposition of damping-like and field-like spin transfer torques. Besides the phase properties we also address in detail the power properties of the injection locked state for which two new parameters are introduced: the locking power range $\mathrm{\Delta }\mathrm{\Pi }$ and the power angle ${\psi }_p$. They can provide important complementary information on the driving forces from experiment. The general description presented here is not limited to STNOs and is valid for any nonisochronous auto-oscillator driven by an elliptical forcing of conservative or dissipative nature.

We present a theoretical formalism to address the dynamics of textured, noncollinear antiferromagnets subject to spin current injection. We derive sine-Gordon type equations of motion for the antiferromagnets, which are applicable to technologically important antiferromagnets such as ${\mathrm{Mn}}_3\mathrm{Ir}$ and ${\mathrm{Mn}}_3\mathrm{Sn}$, and enables an analytical approach to domain wall dynamics in those materials. We obtain the expression for domain wall velocity, which is estimated to reach $\sim 1$ km/s in ${\mathrm{Mn}}_3\mathrm{Ir}$ by exploiting spin Hall effect with electric current density $\sim {10}^{11}\mathrm{A}/{\mathrm{m}}^2$.

In this paper, we study nonanalyticities or cusps near a topological phase transition driven by changes of global topologies rather than by the formation of conventional (local) order. The particular phase transitions between topologically ordered superfluids and nontopological superfluids studied in this paper are always accompanied by the emergence or disappearance of Majorana boundary modes, while at the critical point these boundary modes are liberated and evolve into the bulk. Proliferation of massless Majorana fermions in the bulk at the transition leaves unique thermodynamic signatures. For a very broad class of isotropic superfluids or superconductors (with or without time-reversal symmetry), the resultant transitions in $d$ spatial dimensions will be of $(d+1)\mathrm{th}$ order. The $(d+1)\mathrm{th}$-order derivatives of the grand potential are either discontinuous in even dimensions or logarithmically divergent in odd dimensions. Bulk quantities such as compressibility can develop a cusp along with the change of topology of the superfluids. We compute quantitatively the cusp structures and further investigate the effect of interactions induced by massive fluctuation fields. These cusps are robust as direct manifestations of Majorana fermions and these interactions, and can be potentially considered as an alternative approach for detecting these puzzling particles.

Since very few type-I superconductors are known and most are elemental superconductors, there are very few experimental platforms where the interaction between type-I superconductivity and topologically nontrivial band structure can be probed. The rhenium aluminide ${\mathrm{Al}}_6\mathrm{Re}$ has recently been identified as a type-I superconductor with a transition of 0.74 K and a critical field of $\sim 50$ Oe. Here, we report its magnetotransport behavior including de Haas–van Alphen and Shubnikov–de Haas oscillations. Angular dependence of the magnetoresistance reveals a highly anisotropic Fermi surface with dominant hole character. From the strong oscillatory component $\mathrm{\Delta }{R}_{xx}$ in high magnetic fields up to 33 T, the Landau index infinite-field intercept in the case of a single oscillation frequency, and the phase factor $\phi$ where multiple frequencies coexist, are both $\sim 1/4$. This intermediate value is suggestive of possible nontrivial band topology but does not allow strong conclusions.

The iron-based superconductors represent a promising platform for high-temperature superconductivity, but the interactions underpinning their pairing present a puzzle. The ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ family is unique among these materials for having magnetic order which onsets within the superconducting state, just below the superconducting transition. Superconductivity and magnetic order are normally antagonistic and often vie for the same unpaired electrons, but in this family the magnetism arises from largely localized Eu moments and they coexist, with the competition between these evenly matched opponents leading to reentrant superconducting behavior. To help elucidate the physics in this family and the interactions between the magnetic order and superconductivity, we investigate the $H-T$ phase diagram near optimal Rh doping through specific heat, resistivity, and magnetization measurements, and study the electronic structure by angular-resolved photoemission spectroscopy. The competition between the Eu and FeAs layers may offer a route to directly access the electronic structure under effective magnetic fields via angle-resolved photoemission spectroscopy, which is ordinarily a strictly zero-field technique.

We present a detailed theoretical description of quantum coherent electron transport in voltage-biased crosslike Andreev interferometers. Making use of the charge conjugation symmetry encoded in the quasiclassical formalism, we elucidate a crucial role played by geometric and electron-hole asymmetries in these structures. We argue that a nonvanishing Aharonov-Bohm-like contribution to the current $I_S$ flowing in the superconducting contour may develop only in geometrically asymmetric interferometers making their behavior qualitatively different from that of symmetric devices. The current $I_N$ in the normal contour—along with $I_S$—is found to be sensitive to phase-coherent effects thereby also acquiring a $2\pi$-periodic dependence on the Josephson phase. In asymmetric structures this current develops an odd-in-phase contribution originating from electron-hole asymmetry. We demonstrate that both phase-dependent currents $I_S$ and $I_N$ can be controlled and manipulated by tuning the applied voltage, temperature, and system topology, thus rendering Andreev interferometers particularly important for future applications in modern electronics.

$\mathrm{A}{\mathrm{u}}_2\mathrm{Pb}$ is a candidate of natural topological superconductors that has attracted much attention in the last few years. Combining ab initio calculations with machine-learning accelerated crystal structure searches, we have found two ground states of $\mathrm{A}{\mathrm{u}}_2\mathrm{Pb}$: $Pca{2}_1$ phase at ambient pressure and $I\overline{4}2d$ phase at high pressure. The $Pca{2}_1$ phase is energetically more favorable than the known Pbcn structure and can be a candidate for x-ray diffraction refinement; our calculations suggest that high-pressure $I\overline{4}2d$ is a conventional superconductor. By the high-pressure electric resistance measurements, we observed evidence of a $T_c$ enhancement. $T_c$ reaches a maximum value of around 4 K at 5 GPa, then decreases with further compression. The superconductivity can remain unchanged after pressure releasing, in line with our theoretical predictions. These results show that $\mathrm{A}{\mathrm{u}}_2\mathrm{Pb}$ exhibits abundant behaviors under varied pressure and temperature, which can help to understand how to adjust its electronic properties by pressure.

An external magnetic field can induce a transition in $\alpha \text{−}{\mathrm{RuCl}}_3$ from an ordered zigzag state to a disordered state that is possibly related to the Kitaev quantum spin liquid. Here, we present field-dependent inelastic neutron scattering and magnetocaloric effect measurements implying the existence of an additional transition out of the quantum spin-liquid phase at an upper field limit $B_u$. The neutron scattering shows three distinct regimes of magnetic response. In the low-field ordered state the response shows magnon peaks; the intermediate-field regime shows only continuum scattering, and above $B_u$ the response shows sharp magnon peaks at the lower bound of a strong continuum. Measurable dispersion of magnon modes along the $(0,0,L)$ direction implies non-negligible interplane interactions. Combining the magnetocaloric effect measurements with other data, a $T\text{−}B$ phase diagram is constructed. The results constrain the range where one might expect to observe quantum spin-liquid behavior in $\alpha \text{−}{\mathrm{RuCl}}_3$.

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the ${}^{31}\mathrm{P}$-NMR relaxation rate on ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2(x=0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x=0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature $T_{\mathrm{c}}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and $T_{\mathrm{c}}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the $T_{\mathrm{c}}$ dome as a function of Weiss temperature $\theta$, which measures the distance to the QCP. Moreover, the $T_{\mathrm{c}}\text{−}\theta$ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$.

We study an extended Aubry-André-Harper model with simultaneous modulation of hopping on-site potential and $p$-wave superconducting pairing. For the case of commensurate modulation of $\beta =1/2$ it is shown that the model hosts four different types of topological states: Adiabatic cycles can be defined which pump particles two types of Majorana fermions or Cooper pairs. In the incommensurate case we calculate the phase diagram of the model in several regions. We characterize the phases by calculating the mean inverse participation ratio and perform multifractal analysis. In addition we characterize whether the phases found are topologically trivial or not. We find an interesting critical extended phase when incommensurate hopping modulation is present. The rise between the inverse participation ratio in regions separating localized and extended states is gradual rather than sharp. When in addition the on-site potential modulation is incommensurate we find several sharp rises and falls in the inverse participation ratio. In these two cases all different phases exhibit topological edge states. For the commensurate case we calculate the evolution of the Hofstadter butterfly and the band Chern numbers upon variation of the pairing parameter for zero and finite on-site potential. For zero on-site potential the butterflies are triangularlike near zero pairing when gap closure occurs they are squarelike and hexagonal-like for larger pairing but with the Chern numbers switched compared to the triangular case. For the finite case gaps at quarter and three-quarters filling close and lead to a switch in Chern numbers.

We demonstrate a possibility of using geometry to deterministically control nonlocal correlation of waves undergoing mesoscopic transport through a disordered waveguide. In case of nondissipative medium, we find an explicit relationship between correlation and the shape of the system. Inverting this relationship, we realize inverse design: we obtain specific waveguide shape that leads to a predetermined nonlocal correlation. The proposed technique offers an approach to coherent control of wave propagation in random media that is complementary to wave-front shaping.

We study the field for the penetration of a first vortex, $H_{\mathrm{P}}$, for vortex matter nucleated in micron-sized samples with edges aligned along the nodal and antinodal directions of the $d$-wave superconducting order parameter of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8-\delta }$. Here we present evidence that the $H_{\mathrm{P}}$ for vortex matter nucleated in mesoscopic samples with edges parallel to the nodal direction is larger than for the antinodal case, $\sim 72$% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the Bean-Livingston barrier for vortex penetration.