arXiv subjects

Joshua Folk

Publications and source records attributed to Joshua Folk.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of a ferro-Josephson effect in twisted graphene

When a spin-polarized current is driven across a magnetic domain wall, the resulting spin-transfer torque may, beyond a critical threshold, set the wall's moments into precession. This precession modulates the Berry curvature experienced by electrons traversing the wall, producing an electromotive force that is topological in nature and proportional to the precession frequency, mapping precisely onto the DC Josephson effect and leading to the name ferro-Josephson effect. We report signatures consistent with this effect in a twisted graphene van der Waals heterostructure, where spin and valley textures are linked by exchange, Hund's coupling, and spin-orbit interactions. Tuned to fillings where the isospin degeneracy is spontaneously broken, the samples develop a sharp peak in the longitudinal resistance within a fraction of a millitesla of $B_\parallel=0$---a peak that disappears as the current is reduced toward zero. In differential resistance the feature resolves into sharp resonances that disperse with $B_\parallel$ on microtesla and picoampere scales. We argue that these arise from the current-driven precession of spin-domain-wall moments, in competition with the in-plane anisotropy set by a minuscule applied field, and that they establish nonlinear transport as a sensitive probe of isospin domain-wall dynamics at energy scales far below $k_BT$.

cond-mat.mes-hall↗

arXiv2026

Metastable magnetic domains and the anomalous $B_\parallel=0$ resistance peak in twisted double bilayer graphene

In graphene moir\'es, valley polarization gives rise to orbital magnetism, manifested as an anomalous Hall effect and resulting in Barkhausen jumps in longitudinal resistance when changing domain configurations modify quasiparticle scattering. Beyond a simple picture of polarized domains, however, spin and valley textures within and between the domains are less well understood, as is the effect of these textures on transport. In the valley-polarized quarter-metal state of twisted double bilayer graphene, a sharp and metastable peak in longitudinal resistance often appears at zero in-plane magnetic field, whose microscopic origin has yet to be identified. Here, we show that this peak depends on the configuration of domains of orbital magnetism, which is itself set by the gate-voltage trajectory used to enter the ordered state and by the magnetic field --- particularly the in-plane component --- present during that trajectory. The sensitivity of the effect to in-plane magnetic field components points to spin, linked to valley polarization through spin-orbit coupling, as the key degree of freedom in both the domain formation and the resistance peak.

cond-mat.mes-hall↗

arXiv2026

Anderson Orthogonality as Measurement Backaction in Coupled Quantum Dots

Measurement perturbs a quantum system by coupling it to external degrees of freedom, but detector backaction depends on the physical mechanism of measurement itself. In solid-state devices, detectors driven far from equilibrium to enable faster measurements produce backaction that can often be understood as classical noise. However, a strong measurement can also induce backaction from quantum many-body correlations in the detector that are intrinsic to the measurement, even without shot noise. Here, we probe this near-equilibrium backaction through the effect of a quantum-dot charge sensor on tunnelling between a second quantum dot and its reservoirs. The measurement realizes the Anderson Orthogonality Catastrophe (AOC): electrons in the detector leads reorganize in response to an abrupt change in local scattering potential, suppressing resonant tunnelling while enabling inelastic processes that exchange energy with the detector. Changing the detector energy level tunes the AOC backaction from negligible to dominant in the tunnelling dynamics. More broadly, these results establish detector-induced many-body correlations as a controllable influence on quantum dynamics.

cond-mat.mes-hall↗

arXiv2026

Anomalous metal and superconducting phases in rhombohedral graphene

Two-dimensional superconductivity is now well established in graphene-based systems, with many such realizations showing evidence for unconventional pairing. Yet in several of the gate-tuned phases that otherwise exhibit clear signatures of superconductivity, the resistance does not vanish as temperature is lowered, instead saturating at a finite value. Here we report a systematic study of this behavior in rhombohedral graphene on a WSe$_2$ substrate, finding regions of gate space with zero-resistance superconductivity alongside others with finite saturation resistance. At zero magnetic field, these regions appear as isolated pockets in gate space that otherwise exhibit strikingly similar phenomenology, including abrupt transitions to the normal state as temperature, perpendicular magnetic field, and current are raised above critical values. A small in-plane field expands and merges these pockets without qualitatively altering their behavior, producing a sharp boundary at millikelvin base temperature between states of zero or finite resistance. The finite-resistance state reproduces key phenomenology associated with the anomalous metal, a state that has been observed in thin-film superconductors for decades but lacks an accepted theoretical explanation. The tunability and reproducibility of ultra-clean rhombohedral graphene place strong constraints on extrinsic explanations and provide a new platform for understanding this behavior.

cond-mat.mes-hall↗

arXiv2026

Entropic signatures of the single-impurity Kondo state

The Kondo singlet---a many-body state formed by entanglement between a localized spin and the Fermi sea---has been studied extensively through its transport signatures in quantum dots. Here we report a thermodynamic measurement of the entropy suppression associated with the formation of the Kondo singlet, using temperature-dependent charge sensing and a Maxwell relation to track the suppression of spin entropy as the first electron is added to a strongly-coupled GaAs quantum dot. Plotting $dN/dT$ against the simultaneously measured occupation $N$ reveals an asymmetric lineshape with its peak shifted to $N>1/2$---a hallmark of Kondo screening---that weakens with increasing temperature and is qualitatively reproduced by numerical renormalization group (NRG) calculations, with a small but persistent offset to lower occupation relative to the theory. An independent measurement of conductance versus occupation on the same device provides a test of these quantities through the mixed-valence crossover and matches NRG within experimental uncertainty.

cond-mat.mes-hall↗

arXiv2026

Remote entropy measurement in coupled quantum dots

Recent experiments have demonstrated that measurements of the entropy change associated with the addition of electrons to semiconductor- and graphene-based quantum dots accurately quantify the spin and orbital degeneracy of the states into which they are added. However, measuring more exotic entropies requires probing the entropy change of an entire system in response to an added particle. Here, we demonstrate that Maxwell relation-based measurements probe not only the entropy change associated with the added electron but also that of the surrounding system as it responds to that electron. Using a pair of capacitively coupled GaAs quantum dots, we show that charge measurements on one dot reveal entropy changes associated with the entire two-dot system, both at weak dot--reservoir coupling where microstate counting applies and at stronger coupling where numerical renormalization group calculations are required.

cond-mat.mes-hall↗

arXiv2025

Pervasive spin-triplet superconductivity in rhombohedral graphene

Magnetic fields typically suppress superconductivity once the Zeeman energy exceeds the pairing gap, unless mechanisms such as unconventional pairing, strong spin-orbit coupling, or intrinsic magnetism intervene. Several graphene platforms realize such mitigating routes, exhibiting superconductivity resilient to magnetic fields. Here we report superconductivity in rhombohedral heptalayer graphene that is both induced and stabilized by in-plane magnetic field ($B_{\parallel}$), with critical fields far beyond the Pauli paramagnetic limit. The superconductivity spans a wide gate range and emerges from a sharp zero-field resistive ridge that tracks approximately constant conduction band filling. The presence of zero-field superconductivity and the evolution of the critical temperature with $B_{\parallel}$ are highly gate sensitive. We also observe a weak superconducting diode effect in several distinct regimes within the superconducting phase, including nearby to an integer quantum anomalous Hall state generated by a boron nitride moir\'e superlattice, indicating a potential coexistence of valley imbalance and superconductivity. These results establish several intriguing new properties of spin-triplet, field-induced superconductivity in a thick rhombohedral graphene stack.

cond-mat.mes-hall↗

arXiv2025

Superconductivity from dual-surface carriers in rhombohedral graphene

Intrinsic rhombohedral graphene hosts an unusual low-energy electronic wavefunction, predominantly localized at its outer crystal faces with negligible presence in the bulk. Increasing the number of graphene layers amplifies the density of states near charge neutrality, greatly enhancing the susceptibility to symmetry-breaking phases. Here, we report superconductivity in rhombohedral graphene arising from an unusual charge-delocalized semimetallic normal state, characterized by coexisting valence- and conduction-band Fermi pockets split to opposite crystal surfaces. In octalayer graphene, the superconductivity appears in five apparently distinct pockets for each sign of an external electric displacement field ($D$). In a moir\'e superlattice sample where heptalayer graphene is aligned on one side to hexagonal boron nitride, two pockets of superconductivity emerge from a single sharp resistive feature. At higher $D$ the same resistive feature additionally induces an $h/e^{2}$-quantized anomalous Hall state at dopings near one electron per moir\'e unit cell. Our findings reveal a novel superconducting regime in multilayer graphene and create opportunities for coupling to nearby topological states.

cond-mat.mes-hall↗

arXiv2025

Direct signatures of Anderson orthogonality catastrophe in nonequilibrium quantum dots

We propose schemes for unambiguous direct observation of Anderson orthogonality catastrophe (AOC) effects in a quantum dot coupled to a charge detector, allowing to estimate the AOC exponent $\alpha$. We show that certain easy-to-measure observables have a robust dependence on $\alpha$ in the non-equilibrium regimes of source-drain voltage bias or thermal imbalance. Our results are obtained using a rate equation formalism in which the AOC effects on tunnel rates are incorporated in an exact manner, and directly support recent experimental results.

cond-mat.mes-hall↗

arXiv2025

Roadmap on Quantum Thermodynamics

The last two decades has seen quantum thermodynamics become a well established field of research in its own right. In that time, it has demonstrated a remarkably broad applicability, ranging from providing foundational advances in the understanding of how thermodynamic principles apply at the nano-scale and in the presence of quantum coherence, to providing a guiding framework for the development of efficient quantum devices. Exquisite levels of control have allowed state-of-the-art experimental platforms to explore energetics and thermodynamics at the smallest scales which has in turn helped to drive theoretical advances. This Roadmap provides an overview of the recent developments across many of the field's sub-disciplines, assessing the key challenges and future prospects, providing a guide for its near term progress.

quant-ph↗

arXiv2025

Single-gate tracking behavior in flat-band multilayer graphene devices

A central feature of many van der Waals (vdW) materials is the ability to precisely control their charge doping, $n$, and electric displacement field, $D$, using top and bottom gates. For devices composed of only a few layers, it is commonly assumed that $D$ causes the layer-by-layer potential to drop linearly across the structure. Here, we show that this assumption fails for a broad class of crystalline and moir\'e vdW structures based on Bernal- or rhombohedral-stacked multilayer graphene. We find that the electronic properties at the Fermi level are largely dictated by special layer-polarized states arising at Bernal-stacked crystal faces, which typically coexist in the same band with layer-delocalized states. We uncover a novel mechanism by which the layer-delocalized states completely screen the layer-polarized states from the bias applied to the remote gate. This screening mechanism leads to an unusual scenario where voltages on either gate dope the band as expected, yet the band dispersion and associated electronic properties remain primarily (and sometimes exclusively) governed by the gate closer to the layer-polarized states. Our results reveal a novel electronic mechanism underlying the atypical single-gate-controlled transport characteristics observed across many flat-band graphitic structures, and provide key theoretical insights essential for accurately modeling these systems.

cond-mat.mes-hall↗

arXiv2025

Back-action effects in charge detection

Charge detection offers a powerful probe of mesoscopic structures based on quantum dots, but it also invariably results in measurement back-action (MBA). If strong, MBA can be detrimental to the physical properties being probed. In this work, we focus on the effects of MBA on an Anderson impurity model in which the impurity is coupled electrostatically to a detector. Introducing a novel non-perturbative method, we explore the interplay of coherent dynamics, strong correlations and non-equilibrium conditions. The effects of MBA can be seen most clearly in the temperature derivative of occupation. In the non-equilibrium case, we identify this as arising due to an energy flow from the detector to the impurity.

cond-mat.mes-hall↗

arXiv2024

Interplay of electronic crystals with integer and fractional Chern insulators in moir\'e pentalayer graphene

The rapid development of moir\'e quantum matter has recently led to the remarkable discovery of the fractional quantum anomalous Hall effect, and sparked predictions of other novel correlation-driven topological states. Here, we investigate the interplay of electronic crystals with integer and fractional Chern insulators in a moir\'e lattice of rhomobohedral pentalayer graphene (RPG) aligned with hexagonal boron nitride. At a doping of one electron per moir\'e unit cell, we see a correlated insulator with a Chern number that can be tuned between $C=0$ and $+1$ by an electric displacement field, accompanied by an array of other such insulators formed at fractional band fillings, $\nu$. Collectively, these states likely correspond to trivial and topological electronic crystals, some of which spontaneously break the discrete translational symmetry of the moir\'e lattice. Upon applying a modest magnetic field, a narrow region forms around $\nu=2/3$ in which transport measurements imply the emergence of a fractional Chern insulator, along with hints of weaker states at other fractional $\nu$. In the same sample, we also see a unique sequence of incipient Chern insulators arising over a broad range of incommensurate band filling near two holes per moir\'e unit cell. Our results establish moir\'e RPG as a fertile platform for studying the competition and potential intertwining of electronic crystallization and topological charge fractionalization.

cond-mat.mes-hall↗

arXiv2024

Topological electronic crystals in twisted bilayer-trilayer graphene

In a dilute two-dimensional electron gas, Coulomb interactions can stabilize the formation of a Wigner crystal. Although Wigner crystals are topologically trivial, it has been predicted that electrons in a partially-filled band can break continuous translational symmetry and time-reversal symmetry spontaneously to form a form of topological electron crystal known as an anomalous Hall crystal. Here, we report the observation of a generalized version of the anomalous Hall crystal in twisted bilayer-trilayer graphene, whose formation is driven by the moire potential. The crystal forms at a band filling factor of one electron per four moir\'e unit cells ($\nu=1/4$) and quadruples the unit-cell area, coinciding with an integer quantum anomalous Hall effect. The Chern number of the state is exceptionally tunable, and can be switched reversibly between $+1$ and $-1$ by electric and magnetic fields. Several other topological electronic crystals arise in a modest magnetic field, originating from $\nu=1/3$, $1/2$, $2/3$, and $3/2$. The quantum geometry of the folded bands is likely very different from that of the original parent band, enabling possible future discoveries of correlation-driven topological phenomena

cond-mat.mes-hall↗

arXiv2024

Topological flat bands in a family of multilayer graphene moir\'e lattices

Moir\'e materials host a wealth of intertwined correlated and topological states of matter, all arising from flat electronic bands with nontrivial quantum geometry. A prominent example is the family of alternating-twist magic-angle graphene stacks, which exhibit symmetry-broken states at rational fillings of the moir\'e band and superconductivity close to half filling. Here, we introduce a second family of twisted graphene multilayers made up of twisted sheets of $M$- and $N$-layer Bernal-stacked graphene flakes. Calculations indicate that applying an electric displacement field isolates a flat and topological moir\'e conduction band that is primarily localized to a single graphene sheet below the moir\'e interface. Phenomenologically, the result is a striking similarity in the hierarchies of symmetry-broken phases across this family of twisted graphene multilayers. Our results show that this family of structures offers promising new opportunities for the discovery of exotic new correlated and topological phenomena, enabled by using the layer number to fine tune the flat moir\'e band and its screening environment.

cond-mat.mes-hall↗

arXiv2022

Superconductivity in Twisted Double Bilayer Graphene Stabilized by WSe$_2$

Superconductivity has been previously observed in magic-angle twisted stacks of monolayer graphene but conspicuously not in twisted stacks of bilayer graphene, although both systems host topological flat bands and symmetry-broken states. Here, we report the discovery of superconductivity in twisted double bilayer graphene (TDBG) in proximity to WSe$_2$. Samples with twist angles 1.24$^\circ$ and 1.37$^\circ$ superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized states near van Hove singularities and next to regions with broken isospin symmetry, showing the correlation between a high density of states and the emergence of superconductivity in TDBG while revealing a possible role for isospin fluctuations in the pairing.

cond-mat.supr-con↗

arXiv2022

Strongly correlated zero-bias anomaly in double quantum dot measurements

Experiments in doped transition metal oxides often show suppression in the single-particle density of states at the Fermi level, but disorder-induced zero-bias anomalies in strongly correlated systems remain poorly understood. Numerical studies of the Anderson-Hubbard model have identified a zero-bias anomaly that is unique to strongly correlated materials, with a width proportional to the intersite hopping amplitude t [S. Chiesa, P. B. Chakraborty, W. E. Pickett, and R. T. Scalettar, Phys. Rev. Lett. 101, 086401 (2008)]. In ensembles of two-site systems, a zero-bias anomaly with the same parameter dependence also occurs, suggesting a similar physical origin [R. Wortis and W. A. Atkinson, Phys. Rev. B 82, 073107 (2010)]. We describe how this kinetic-energy-driven zero-bias anomaly in ensembles of two-site systems may be seen in a mesoscopic realization based on double quantum dots. Moreover, the double-quantum-dot measurements provide access not only to the ensemble-average density of states but also to the details of the transitions which give rise to the zero-bias anomaly.

cond-mat.str-el↗

arXiv2022

Entropy measurement of a strongly coupled quantum dot

The spin 1/2 entropy of electrons trapped in a quantum dot has previously been measured with great accuracy, but the protocol used for that measurement is valid only within a restrictive set of conditions. Here, we demonstrate a novel entropy measurement protocol that is universal for arbitrary mesoscopic circuits and apply this new approach to measure the entropy of a quantum dot hybridized with a reservoir, where Kondo correlations dominate spin physics. The experimental results match closely to numerical renormalization group (NRG) calculations for small and intermediate coupling. For the largest couplings investigated in this work, NRG predicts a suppression of spin entropy at the charge transition due to the formation of a Kondo singlet, but that suppression is not observed in the experiment.

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