Electron spins in silicon quantum dots are excellent qubits due to their long coherence times, scalability, and compatibility with advanced semiconductor technology. Although high gate fidelities can be achieved with spin qubits, charge noise in the semiconductor environment still hinders further improvements. Despite the importance of charge noise, key questions about the specific nature of the fluctuators that cause charge noise remain unanswered. Here, we probe individual two-level fluctuators (TLFs) in Si/SiGe quantum dots through simple quantum-dot transport measurements and analyses based on the Allan variance and factorial hidden Markov modeling. We find that the TLF switching times depend sensitively on gate voltages, decrease with temperature, and depend on the current through a nearby quantum dot. A model for the data of the primary TLF we study indicates that it may be a bistable charge dipole near the plunger gate electrode, heated by a current through the sensor dot, and experiencing state transitions driven not by direct electron-phonon coupling but through some other mechanism such as coupling to electrons passing through the sensor dot.

We theoretically study the nonlinear optical response of ABC trilayer graphene with inversion symmetry broken by the application of a displacement field perpendicular to the trilayer. We show that rhombohedral trilayer graphene exhibits a large bulk photovoltaic effect arising from a DC shift-current response. The conductivity of the trilayer contains features similar to AB bilayer graphene as well as features distinct from AB bilayer graphene. The additional features of ABC trilayer graphene relative to AB bilayer graphene arise from the quantum geometric features of the electronic band structure and can be tuned by varying the displacement field. We focus on a regime of displacement field where certain band gaps close and reopen away from the charge neutrality point, leading to drastic changes in the quantum geometric structure in momentum space, a feature characteristic of the trilayer graphene band structure. These features manifest as a sign change in shift-current conductivity in a certain frequency window and can thus serve as a probe of quantum geometry.

We study the effect of electron-atomic phonon coupling on the location of the Fermi Liquid to Wigner Crystal transition in the two-dimensional electron gas realized in various material platforms. Based on dimensional estimates of the relevant parameters, we conclude that (as conventionally assumed) atomic phonons are negligible in traditional semiconductor quantum well systems, but likely play a significant role in various recently synthesized atomically thin two-dimensional materials.

A Kondo model with symplectic symmetry was recently put forward as the effective low-energy theory of a superconducting-island device coupled to multiple leads. This model, which possesses non-Fermi liquid physics and effective anyons, was argued to belong to the class of topological Kondo effects. Here, we clarify the extent of stability of its exotic fixed point using perturbative and numerical renormalization group in conjunction with bosonization and conformal field theory. In contrast to previous claims, we show that asymmetry in the coupling to the leads destabilizes the non-Fermi liquid. Other destabilizing perturbations include asymmetry in the superconducting pairing or internal energy of the individual quantum dots in the island. Nevertheless, these perturbations all generate the same relevant operators. Thus, only a small number of couplings need to be tuned individually, and these can be selected according to experimental convenience. Our results highlight a common misconception that anisotropy in single-channel Kondo couplings is always irrelevant. As demonstrated, relevant terms will emerge whenever the group generators do not span the full space of impurity operators. This calls for a more detailed inspection of models that exhibit this property, such as large-spin impurities and $\mathrm{SO}\left(M\right)$ Kondo models.

We explore Bernal bilayer graphene in the presence of long-range Coulomb interactions, short-range Hund's coupling, and proximity-induced Ising spin-orbit coupling using self-consistent Hartree-Fock simulations. We show that the interplay between these three ingredients produces an intricate phase diagram comprising a multitude of symmetry-broken metallic states tunable via doping and applied displacement field. In particular, we find intervalley coherent and spin-canted ground states that may hold the key to understanding spin-orbit-enabled superconductivity observed in this platform. We also investigate various phase transitions where a continuous $\mathrm{U}\left(1\right)$ symmetry is broken to ascertain the possible role of critical fluctuations on pairing.

Nuclear magnetic resonance (NMR) is particularly relevant for studies of internuclear spin coupling at zero and ultralow fields (ZULFs), where spin-spin interactions dominate over Zeeman ones. Here, we report on ZULF NMR in CdTe. In this semiconductor all magnetic isotopes have spin $I=1/2$, so that internuclear interactions are never overshadowed by quadrupole effects. Our experiments rely on warm-up spectroscopy, a technique that combines optical pumping, additional cooling via adiabatic demagnetization, and detection of the oscillating-magnetic-field-induced warm-up of the nuclear spin system via the Hanle effect. We show that NMR spectra exhibit a rich fine structure, consistent with the low abundance of magnetic isotopes in CdTe, their zero quadrupole moments, and direct and indirect interactions between them. A model assuming that the oscillating magnetic field power is absorbed by nuclear spin clusters composed of up to four magnetic isotopes allows us to reproduce the shape of a major part of the measured spectra.

Magneto-conductance in thin wires often exhibits complicated patterns due to the quantum interference of conduction electrons. These patterns reflect microscopic structures in the wires, such as defects or potential distributions. In this study, we propose an inverse design method to automatically generate a microscopic structure that exhibits desired magneto-conductance patterns. We numerically demonstrate that our method accurately generates defect positions in wires and can be effectively applied to various complicated patterns. We also discuss techniques for designing structures that facilitate experimental investigation.

Topological spin textures with an exceptional stability due to the topological protection have great potential for spintronics applications. Understanding the reversal mechanisms and stability is important for designing practical devices. Here, we prepared a system, $\mathrm{MgO}/{\mathrm{Mn}}_2\mathrm{CoAl}$/Pd ultrathin films, in which trivial uniform magnetic domains and nontrivial labyrinth domains and magnetic skyrmions can be generated. The reversal behaviors for those spin textures were investigated by magnetic domain imaging and magneto-transport measurements under the influence of a magnetic field. With an external magnetic field, the trivial uniform domains can be reversed, while the nontrivial labyrinth domains and magnetic skyrmions are persistent. We also investigated the stability after switching off the field. The uniform domains stop their reversal, and the topologically nontrivial spin textures transform to a demagnetized state. These reversal behaviors and stability of the domains can be explained by referring to changes in the competition of interactions with applied magnetic field. Our results demonstrate that a large enough perpendicular magnetic anisotropy is required to stabilize topological spin textures for practical spintronics applications.

A multimode framework that can be utilized in the analysis and design of metasurfaces with sub- and superwavelength periodicity involving dielectric, plasmonic, or 2D materials is presented. The numerical tool is based on the concept of quasinormal modes (QNMs), which are used to efficiently reconstruct the structure's electromagnetic response. It is general and can be applied to any non-Hermitian periodic resonant system irrespective of pitch value, incidence angle, frequency regime, material composition, and type of supported resonance, including Fabry-Pérot modes, whispering-gallery modes, localized plasmonic resonances and surface states, lattice resonances, dielectric Mie modes, etc. The framework is employed to study two contemporary metasurface structures involving 2D materials: (i) a graphene-based plasmonic metasurface that supports tightly confined surface plasmons which form Fabry-Pérot resonances and (ii) a dielectric metasurface that supports Mie-like resonances and a bound state in the continuum (BIC), enhanced with a transition-metal dichalcogenide bilayer. The ability of supporting sharp resonances through quasidark or qBIC modes as well as the tuning of their resonant frequency via electrical gating are investigated. In each case, the QNMs are retrieved using a modified finite-element scheme that can handle periodic systems with dispersion. They are then fed to the modal framework to specify the spectral response for the zeroth and higher diffraction orders. The results are validated by comparing against time-harmonic finite-element simulations, showing excellent agreement.

Impurities and random disorder are known to affect topological superconducting phases and their Majorana zero modes (MZMs). In particular, it is a common assumption that disorder negatively influences the braiding dynamics of MZMs. Recently, it was shown, however, that random disorder can also stabilize or even increase topological phases. Here, we investigate quantitatively how a single impurity can lead to braiding errors. We show that in most scenarios, the impurity increases the dynamical hybridization of the MZMs, reducing the braiding performance. In addition, we show how random disorder, i.e., impurities on all lattice sites but with different strengths, affects braiding. As for the static case, we observe a window of opportunity where random disorder decreases the average energy of a braid, and thus improves braiding outcomes. This window of opportunity is, however, limited due to an increase of diabatic effects in the presence of disorder. Nevertheless and contrary to physical intuition, disorder can, in certain situations, be beneficial and improve braiding outcomes.

We report on high-pressure magnetotransport study of itinerant chiral magnet FeGe with near-room-temperature ($T_c=278$ K) helical/skyrmion spin texture. The pressure-induced reduction of exchange splitting leads to the Fermi surface reconstruction upon quantum phase transition (QPT) at $P \sim 19$ GPa as well as the suqsequent transition into the degenerate semiconductor state. Furthermore, we discover unconventional spontaneous anomalous Hall effect in the short-range-order chiral-spin ground state above QPT, signifying macroscopic time-reversal symmetry breaking. Our results highlight the high-$T_c$ itinerant magnets with broken inversion symmetry as promising candidates for ground state formation near QPT.

Recently emerging complex photonic structures exhibiting giant optical nonlinearity through strong light-matter coupling require new theoretical approaches to accurately capture the interplay of the photonic and interacting-matter degrees of freedom. Extending the finite-difference time-domain method, we develop an algorithm for solving the nonlinear Maxwell-Bloch equations. This allows first-principles modeling of exciton-polariton systems for arbitrarily complex photonic structures with photon-exciton coupling and frequency dependent nonlinear response included without approximations or phenomenological parameters. We first validate the algorithm by reproducing the bistable hysteresis cycle of polaritons in the nonlinear regime. We then give a key example of its utility by simulating polariton dynamics in integrated photonic circuitry composed of spatially localized bistable nodes connected by high speed waveguides. We propose a polariton circuit element inspired by saltatory conduction in biological neurons. This design supports faster polariton signal propagation than previous designs, however, requires a full account of the nonlinear field distributions in both propagation and growth directions to be calculated.

It is well recognized that the low-energy physics of many Kitaev materials is governed by two dominant energy scales, the Ising-type Kitaev coupling $K$ and the symmetric off-diagonal $\mathrm{\Gamma }$ coupling. An understanding of the interplay between these two scales is therefore the natural starting point toward a quantitative description that includes subdominant perturbations that are inevitably present in real materials. This study focuses on the classical $K\text{−}\mathrm{\Gamma }$ model on the honeycomb lattice, with a specific emphasis on the region $K<0$ and $\mathrm{\Gamma }>0$, which is the most relevant for the available materials and which remains enigmatic in both quantum and classical limits, despite much effort. We employ large-scale Monte Carlo simulations on specially designed finite-size clusters and unravel the presence of a complex multisublattice magnetic order in a wide region of the phase diagram, whose structure is characterized in detail. We show that this order can be quantified in terms of a coarse-grained scalar-chirality order, featuring a counterrotating modulation on the two spin sublattices. We also provide a comparison to previous studies and discuss the impact of quantum fluctuations on the phase diagram.

We study interaction-mediated magnetism on the surface of ABC-multilayer graphene driven by its zero-energy topological flat bands. Using the random-phase approximation, we treat onsite Hubbard repulsion and find multiple competing magnetic states, due to both intra- and intervalley scattering, with the latter causing an enlarged magnetic unit cell. At half-filling and when the Hubbard repulsion is weak, we observe two different ferromagnetic orders. Once the Hubbard repulsion becomes more realistic, new ferrimagnetic orders arise with distinct incommensurate intra- or intervalley scattering vectors depending on interaction strength and doping, leading to a multitude of competing magnetic states.

We investigate quantum algorithms derived from tensor networks to simulate the static and dynamic properties of quantum many-body systems. Using a sequentially prepared quantum circuit representation of a matrix product state (MPS) that we call a quantum tensor network (QTN), we demonstrate algorithms to prepare ground and excited states on a quantum computer and apply them to molecular nanomagnets (MNMs) as a paradigmatic example. In this setting, we develop two approaches for extracting the spectral correlation functions measured in neutron-scattering experiments: (a) a generalization of the SWAP test for computing wave function overlaps and, (b) a generalization of the notion of matrix product operators to the QTN setting which generates a linear combination of unitaries. The latter method is discussed in detail for translationally invariant spin-half systems, where it is shown to reduce the qubit resource requirements compared with the SWAP method and may be generalized to other systems. We demonstrate the versatility of our approaches by simulating spin-1/2 and spin-3/2 MNMs, with the latter being an experimentally relevant model of a ${\mathrm{Cr}}_8^{3+}$ ring. Our approach has qubit requirements that are independent of the number of constituents of the many-body system and scale only logarithmically with the bond dimension of the MPS representation, making them appealing for implementation on near-term quantum hardware with mid-circuit measurement and reset.

We study the Hubbard and Heisenberg models on hyperbolic lattices with open boundary conditions by means of mean-field approximations, spin-wave theory, and quantum Monte Carlo (QMC) simulations. For the Hubbard model we use the auxiliary-field approach and for Heisenberg systems the stochastic series expansion algorithm and concentrate on bipartite lattices where the QMC simulations are free of the negative sign problem. The hyperbolic lattices have an extensive number of sites on the boundary, such that one has to distinguish between bulk and total density of states. The considered lattices are characterized by a Dirac-like total density of states, Schläfli indices $\{p,q\}=\{10,3\}$ and $\{8,3\}$, as well as by flat bands, $\{8,8\}$. The Dirac total density of states cuts off the logarithmic divergence of the staggered spin susceptibility and allows for a finite $U$ metal-to-insulator transition. This transition has the same mean-field exponents as for the Gross-Neveu transition in Euclidean space. We argue that this transition is induced by the open-boundary conditions and that it will be absent in the periodic case. In the presence of flat bands we observe the onset of magnetic ordering at any finite $U$. This conclusion holds even though the bulk density of states is constant at the Fermi energy for the three considered lattices. The magnetic state at intermediate coupling can be described as a global antiferromagnet. It breaks the $C_p$ rotational and time-reversal symmetries but remains invariant under combined $C_p\mathcal{T}$ transformations. The state is characterized by macroscopic ferromagnetic moments that globally cancel. We observe that fluctuations on the boundary of the system are greatly enhanced: While spin-wave calculations predict the breakdown of antiferromagnetism on the boundary but not in the bulk, QMC simulations show a marked reduction of the staggered moment on the edge of the system.

Spatially resolved heating and cooling in nanostructures is nowadays measured with various nanoscale thermometry techniques, including scanning thermometry. Yet, the most commonly used theory of nanoscale heating and thermoelectricity, Landauer scattering theory, is not appropriate to model such measurements. Hence, we analyze a minimal model of spatially resolved heat transfer between electrons and phonons in simple thermoelectric nanostructures. This combines Landauer scattering formalism with a Boltzmann equation for transport, revealing the nonlocality of Joule heating and Peltier cooling induced by a scatterer in a nanowire. The corresponding heating or cooling of the phonons is caused by the voltage drop at the scatterer, but is often maximal at a certain distance from the scatterer. This distance is of the order of the electron-phonon scattering length. Scanning thermal microscopy, such as SQUID-on-tip thermometers, should detect this nonlocality as phonon hot spots and cold spots, spatially separated from the scatterer. We provide physical arguments explaining the thermoelectric response of the combined system of wire and scatterer and, in particular, why the resulting heating and cooling is sometimes the opposite to that predicted by the standard Landauer scattering theory.

We report the synthesis and properties of ${\left(\mathrm{H},\mathrm{Li}\right)}_6{\mathrm{Ru}}_2{\mathrm{O}}_6$, which is shown to be a $J_{\mathrm{eff}}=\frac{1}{2}$ system made out of ${\mathrm{Ru}}^{3+}$ moments in a honeycomb geometry. Bulk magnetization, heat capacity, nuclear magnetic resonance (NMR), and muon spin relaxation rule out the presence of static moments or any spin glass phase down to 84 mK. All techniques suggest a crossover to a liquidlike state below about 40 K. The ${}^7\mathrm{Li}$ NMR shift data suggest a nonzero $T$-independent spin susceptibility at low $T$. In zero field, $C_m/T$ shows $T^{-0.9}$ divergence which is consistent with vacancy-induced effects on low-energy excitations of the pristine Kitaev spin liquid. With field, power-law variations in the ${}^7\mathrm{Li}$ NMR spin-lattice relaxation rate $1/T_1$ and magnetic heat capacity $C_m$ show quantitatively different scaling behaviors as compared to those in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$. A two-step entropy release in heat capacity is also observed putatively from $Z_2$ flux (low-$T$ step) and itinerant Majorana fermions (high-$T$ step). Based on these findings, we propose that ${\left(\mathrm{H},\mathrm{Li}\right)}_6{\mathrm{Ru}}_2{\mathrm{O}}_6$ realizes a Kitaev spin liquid with no evidence of inherent magnetic ordering in zero field unlike $\alpha \text{−}{\mathrm{RuCl}}_3$ where an 8-T field is required to suppress magnetic order.

We experimentally determine the bounds of the magnetic-field-induced superconducting and magnetic phases near the crystalline $b$ axis of uranium ditelluride (${\mathrm{UTe}}_2$). By measuring the magnetoresistance as a function of rotation angle and field strength in magnetic fields as large as 41.5 T, we have studied these boundaries in three dimensions of magnetic field direction. The phase boundaries in all cases obey crystallographic symmetries and no additional symmetries, evidence against any symmetry-breaking quadrupolar or higher magnetic order. We find that the upper critical field of the zero-field superconducting state is well-described by an anisotropic mass model. In contrast, the angular boundaries of the $b$-axis-oriented field-reentrant superconducting phase are nearly constant as a function of field up to the metamagnetic transition, with anisotropy between the $ab$ and $bc$ planes that is comparable to the angular anisotropy of the metamagnetic transition itself. We discuss the relationship between the observed superconducting boundaries and the underlying $\mathbf{d}$ vector that represents the spin-triplet order parameter. Additionally, we report an unexplained normal-state feature in resistance and track its evolution as a function of field strength and angle.

Exploiting the ability of optical second harmonic generation (SHG) to visualize antiferromagnetic domains in ${\mathrm{Cr}}_2{\mathrm{O}}_3$, here we explore the potential of SHG to probe the THz-induced ultrafast spin dynamics in this antiferromagnet. Our experiments clearly show that the observed THz-induced SHG transients do not reflect the dynamics of the antiferromagnetic Néel vector. Moreover, depending on the helicity of light at the fundamental frequency, the transients may have different form, corresponding either to linear or quadratic dependence of the SHG intensity on the THz electric field.