Imaging inter-valley coherent order in magic-angle twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly correlated electronic phases that spontaneously break its underlying symmetries1,2. Here we investigate the correlated phases of MATTG using scanning tunnelling microscopy and identify marked signatures of interaction-driven spatial symmetry breaking. In low-strain samples, over a filling range of about two to three electrons or holes per moiré unit cell, we observe atomic-scale reconstruction of the graphene lattice that accompanies a correlated gap in the tunnelling spectrum. This short-scale restructuring appears as a Kekulé supercell-implying spontaneous inter-valley coherence between electrons-and persists in a wide range of magnetic fields and temperatures that coincide with the development of the gap. Large-scale maps covering several moiré unit cells further reveal a slow evolution of the Kekulé pattern, indicating that atomic-scale reconstruction coexists with translation symmetry breaking at a much longer moiré scale. We use auto-correlation and Fourier analyses to extract the intrinsic periodicity of these phases and find that they are consistent with the theoretically proposed incommensurate Kekulé spiral order3,4. Moreover, we find that the wavelength characterizing moiré-scale modulations monotonically decreases with hole doping away from half-filling of the bands and depends weakly on the magnetic field. Our results provide essential insights into the nature of the correlated phases of MATTG in the presence of strain and indicate that superconductivity can emerge from an inter-valley coherent parent state.

Superconductivity in rhombohedral trilayer graphene.

To access superconductivity via the electric field effect in a clean, two-dimensional device is a central goal of nanoelectronics. Recently, superconductivity has been realized in graphene moiré heterostructures1-4; however, many of these structures are not mechanically stable, and experiments show signatures of strong disorder. Here we report the observation of superconductivity-manifesting as low or vanishing resistivity at sub-kelvin temperatures-in crystalline rhombohedral trilayer graphene5,6, a structurally metastable carbon allotrope. Superconductivity occurs in two distinct gate-tuned regions (SC1 and SC2), and is deep in the clean limit defined by the ratio of mean free path and superconducting coherence length. Mapping of the normal state Fermi surfaces by quantum oscillations reveals that both superconductors emerge from an annular Fermi sea, and are proximal to an isospin-symmetry-breaking transition where the Fermi surface degeneracy changes7. SC1 emerges from a paramagnetic normal state, whereas SC2 emerges from a spin-polarized, valley-unpolarized half-metal17 and violates the Pauli limit for in-plane magnetic fields by at least one order of magnitude8,9. We discuss our results in view of several mechanisms, including conventional phonon-mediated pairing10,11, pairing due to fluctuations of the proximal isospin order12, and intrinsic instabilities of the annular Fermi liquid13,14. Our observation of superconductivity in a clean and structurally simple two-dimensional metal provides a model system to test competing theoretical models of superconductivity without the complication of modelling disorder, while enabling new classes of field-effect controlled electronic devices based on correlated electron phenomena and ballistic electron transport.

Isospin Pomeranchuk effect in twisted bilayer graphene.

In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature1, owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene2. Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition3 at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann's constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness4,5, with implications for the nature of finite-temperature electron transport6-8, as well as for the mechanisms underlying isospin ordering and superconductivity9,10 in twisted bilayer graphene and related systems.

Half- and quarter-metals in rhombohedral trilayer graphene.

Ferromagnetism is most common in transition metal compounds where electrons occupy highly localized d orbitals. However, ferromagnetic order may also arise in low-density two-dimensional electron systems1-5. Here we show that gate-tuned van Hove singularities in rhombohedral trilayer graphene6 drive spontaneous ferromagnetic polarization of the electron system into one or more spin and valley flavours. Using capacitance and transport measurements, we observe a cascade of transitions tuned to the density and electronic displacement field between phases in which quantum oscillations have fourfold, twofold or onefold degeneracy, associated with a spin- and valley-degenerate normal metal, spin-polarized 'half-metal', and spin- and valley-polarized 'quarter-metal', respectively. For electron doping, the salient features of the data are well captured by a phenomenological Stoner model7 that includes valley-anisotropic interactions. For hole filling, we observe a richer phase diagram featuring a delicate interplay of broken symmetries and transitions in the Fermi surface topology. Finally, we introduce a moiré superlattice using a rotationally aligned hexagonal boron nitride substrate5,8. Remarkably, we find that the isospin order is only weakly perturbed, with the moiré potential catalysing the formation of topologically nontrivial gapped states whenever itinerant half- or quarter-metal states occur at half- or quarter-superlattice band filling. Our results show that rhombohedral graphene is an ideal platform for well-controlled tests of many-body theory, and reveal magnetism in moiré materials4,5,9,10 to be fundamentally itinerant in nature.

Energy Gap of the Even-Denominator Fractional Quantum Hall State in Bilayer Graphene.

Bernal bilayer graphene hosts even-denominator fractional quantum Hall states thought to be described by a Pfaffian wave function with non-Abelian quasiparticle excitations. Here, we report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and by direct measurement of the chemical potential. We find a transport activation gap of 5.1 K at B=12 T for a half filled N=1 Landau level, consistent with density matrix renormalization group calculations for the Pfaffian state. However, the measured thermodynamic gap of 11.6 K is smaller than theoretical expectations for the clean limit by approximately a factor of 2. We analyze the chemical potential data near fractional filling within a simplified model of a Wigner crystal of fractional quasiparticles with long-wavelength disorder, explaining this discrepancy. Our results quantitatively establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of the even-denominator state.

On-Chip Time-Domain Terahertz Spectroscopy of Superconducting Films below the Diffraction Limit.

Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ∼0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 to 750 GHz. We measure the optical conductivity of a 7.5-µm wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.

Gate-Defined Topological Josephson Junctions in Bernal Bilayer Graphene.

Recent experiments on Bernal bilayer graphene (BLG) deposited on monolayer WSe_{2} revealed robust, ultraclean superconductivity coexisting with sizable induced spin-orbit coupling. Here, we propose BLG/WSe_{2} as a platform to engineer gate-defined planar topological Josephson junctions, where the normal and superconducting regions descend from a common material. More precisely, we show that if superconductivity in BLG/WSe_{2} is gapped and emerges from a parent state with intervalley coherence, then Majorana zero-energy modes can form in the barrier region upon applying weak in-plane magnetic fields. Our results spotlight a potential pathway for "internally engineered" topological superconductivity that minimizes detrimental disorder and orbital-magnetic-field effects.

Cascade of Multielectron Bubble Phases in Monolayer Graphene at High Landau Level Filling.

The phase diagram of an interacting two-dimensional electron system in a high magnetic field is enriched by the varying form of the effective Coulomb interaction, which depends strongly on the Landau level index. While the fractional quantum Hall states that dominate in the lower-energy Landau levels have been explored experimentally in a variety of two-dimensional systems, much less work has been done to explore electron solids owing to their subtle transport signatures and extreme sensitivity to disorder. Here, we use chemical potential measurements to map the phase diagram of electron solid states in N=2, N=3, and N=4 Landau levels in monolayer graphene. Direct comparison between our data and theoretical calculations reveals a cascade of density-tuned phase transitions between electron bubble phases up to two, three, or four electrons per bubble in the N=2, 3, and 4 Landau levels, respectively. Finite-temperature measurements are consistent with melting of the solids for T≈1 K.

Introduction to the Special Issue: Advancing Racial Justice in Clinical Child and Adolescent Psychology.

Relative to White youth, racially and ethnically marginalized youth in the U.S. are less likely to initiate treatment, stay in treatment, and receive adequate care. This special issue attends to racial injustice in clinical child and adolescent psychology. While numerous factors drive these racial disparities, this special issue focuses specifically on opportunities and responsibilities we have as mental health providers, teachers, mentors, researchers, and gatekeepers to make our field more racially just. In this introduction to the special issue, we review barriers and solutions across multiple contexts including structural, institutional, and practice-based. We also discuss challenges and opportunities to diversify our field and increase the representation of racially and ethnically marginalized practitioners and scholars in clinical child and adolescent psychology. We then briefly review the special issue articles and make final recommendations for how to move the field forward.

Developing Empirical Latent Profiles of Impulsive Aggression and Mood in Youths across Three Outpatient Samples.

OBJECTIVE: Aggression with impulsivity and reactivity (AIR) may distinguish a subset of youth from those with attention problems, rule-breaking behavior, or mood disorders, potentially with differential treatment response. Yet, DSM-5 and ICD-10 do not include an AIR diagnosis. Thus, we empirically grouped youths into profiles based on AIR, manic, depressive, rule-breaking, and self-harm behaviors; examined which profiles replicated across three samples; and characterized profile sets on demographic and clinical features. METHOD: After harmonizing data from three samples (n = 679, n = 392, n = 634), Latent Profile Analysis (LPA) assigned youth to profiles based on caregiver-reported measures of AIR, manic, depressive, rule-breaking, and self-harm behaviors. Profiles from each sample were grouped into sets based on profile similarity. Analyses tested differences in diagnoses, sex, and race, age, functioning, and mood severity. RESULTS: Eight-profile solutions fit best. Seven profiles replicated across samples: high AIR and self-harm, lower depressive and manic scores; high AIR, manic symptoms, and self-harm; high depression symptoms; three smaller sets with high manic and depressive symptoms and moderate AIR; and two high rates of bipolar diagnoses and family bipolar history. Two sets were high on both AIR and mood symptoms, were the most impaired, and had the highest comorbidity. CONCLUSIONS: Analyses support an empirical definition of AIR, separate from mood disorders. Profile sets distinguished by level of AIR and mood symptoms differed in demographic and diagnostic characteristics as well as functioning. Importantly, a set emerged with high AIR but low mood indicators and with high rates of ADHD and ODD, but not mood disorder.

Enhancing Quality of Care Through Evidence-Based Practice: Training and Supervision Experiences.

OBJECTIVE: Evidence-based practice (EBP) is the preferred approach to treatment in mental health settings because it involves the integration of the best available research, clinical expertise, and patient values to optimize patient outcomes. Training on empirically supported treatments (ESTs) in mental health settings is an important component of EBP, and supervision of therapists' implementation of ESTs is critical for therapists to develop and maintain a strong EBP skill set. This study aimed to evaluate training and supervision histories of therapists in outpatient and inpatient psychiatric care settings as an essential first step in improving patient outcomes. METHODS: Electronic surveys were completed by 69 therapists, most of whom had a master's degree, within a psychiatry and behavioral sciences department at an academic institution. Participating therapists were recruited from several outpatient and inpatient mental health settings serving children, adolescents, and adults. RESULTS: Although most therapists reported completing some form of EST-related coursework, a majority did not receive any supervision related to implementation of ESTs (51% for cognitive-behavioral therapy cases, 76% for dialectical behavior therapy cases, and 52% for other EST cases) during graduate and postgraduate training. CONCLUSIONS: Although research from the past decade has supported the need for improvements in training on ESTs, and especially in supervision, problems related to limited exposure to training and supervision among therapists still exist. These findings have implications for how mental health centers can evaluate staff members' EST training and supervision experiences, training needs, and associated training targets to improve the quality of routine care.

Imaging the Breakdown of Ohmic Transport in Graphene.

Ohm's law describes the proportionality of the current density and electric field. In solid-state conductors, Ohm's law emerges due to electron scattering processes that relax the electrical current. Here, we use nitrogen-vacancy center magnetometry to directly image the local breakdown of Ohm's law in a narrow constriction fabricated in a high mobility graphene monolayer. Ohmic flow is visible at room temperature as current concentration on the constriction edges, with flow profiles entirely determined by sample geometry. However, as the temperature is lowered below 200 K, the current concentrates near the constriction center. The change in the flow pattern is consistent with a crossover from diffusive to viscous electron transport dominated by electron-electron scattering processes that do not relax current.

Quantum Oscillations in Two-Dimensional Insulators Induced by Graphite Gates.

We demonstrate a mechanism for magnetoresistance oscillations in insulating states of two-dimensional (2D) materials arising from the interaction of the 2D layer and proximal graphite gates. We study a series of devices based on different 2D systems, including mono- and bilayer T_{d}-WTe_{2}, MoTe_{2}/WSe_{2} moiré heterobilayers, and Bernal-stacked bilayer graphene, which all share a similar graphite-gated geometry. We find that the 2D systems, when tuned near an insulating state, generically exhibit magnetoresistance oscillations corresponding to a high-density Fermi surface, in contravention of naïve band theory. Simultaneous measurement of the resistivity of the graphite gates shows that the oscillations of the sample layer are precisely correlated with those of the graphite gates. Further supporting this connection, the oscillations are quenched when the graphite gate is replaced by a low-mobility metal, TaSe_{2}. The observed phenomenon arises from the oscillatory behavior of graphite density of states, which modulates the device capacitance and, as a consequence, the carrier density in the sample layer even when a constant electrochemical potential is maintained between the sample and the gate electrode. Oscillations are most pronounced near insulating states where the resistivity is strongly density dependent. Our study suggests a unified mechanism for quantum oscillations in graphite-gated 2D insulators based on electrostatic sample-gate coupling.

Experimental Determination of the Energy per Particle in Partially Filled Landau Levels.

We describe an experimental technique to measure the chemical potential µ in atomically thin layered materials with high sensitivity and in the static limit. We apply the technique to a high quality graphene monolayer to map out the evolution of µ with carrier density throughout the N=0 and N=1 Landau levels at high magnetic field. By integrating µ over filling factor ν, we obtain the ground state energy per particle, which can be directly compared to numerical calculations. In the N=0 Landau level, our data show exceptional agreement with numerical calculations over the whole Landau level without adjustable parameters as long as the screening of the Coulomb interaction by the filled Landau levels is accounted for. In the N=1 Landau level, a comparison between experimental and numerical data suggests the importance of valley anisotropic interactions and reveals a possible presence of valley-textured electron solids near odd filling.

Do the Transmissible Liability Index (TLI) and Adolescent Cannabis Use Predict Paranoid and Schizotypal Symptoms at Young Adulthood?

Background: Adolescent cannabis use is an established risk factor for the development of psychosis, but the premorbid vulnerability factors and specificity versus generality of the psychotic symptom domains affected in cannabis-psychosis relationships remain incompletely understood. To improve our understanding of these relationships, we used longitudinal data to examine the individual and interactive effects of preadolescent transmissible liability to substance use disorders (SUD), measured via the transmissible liability index (TLI), and adolescent cannabis use on the development of two distinct psychotic symptom domains, paranoid and schizotypal personality traits in young adulthood. Methods: We performed secondary analysis of data from the Center for Education and Drug Abuse (CEDAR) study, which longitudinally assessed offspring of men with (N = 211) and without (N = 237) lifetime history of SUD at ages 10-12, and across adolescence as they transitioned to young adulthood. TLI scores were calculated at age 10-12, self-reported cannabis use was assessed at age 16, and paranoid and schizotypal symptoms were assessed at age 19. Results: Cannabis use at age 16 and family history of SUD were significantly associated with paranoid and schizotypal symptoms at age 19, but TLI scores were not. The interactive effect of TLI x cannabis use was also not significant. Paranoid and schizotypal symptoms showed different dose-dependent sensitivities to cannabis exposure at age 16. Conclusions: These findings indicate that adolescent cannabis use and family history of SUD differentially contribute to the development of paranoid and schizotypal personality traits through mechanisms that do not include behavioral disinhibition.

Stressful life events and transitions in problematic alcohol use involvement among US adults.

OBJECTIVE: We investigated the impact of stressful life events (SLEs) for males and females on transitions in problematic alcohol involvement, both progression and recovery, over a 3-year interval. METHOD: Participants of both Wave 1 (2001-2002) and Wave 2 (2004-2005) of the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) were stratified by sex (14,233 males and 19,550 females). Latent transition analysis estimated the impact of experiencing ≥3 SLE in the year preceding the Wave 1 interview on the probability of transitioning between three empirically-derived stages of alcohol involvement (patterns of alcohol use disorder [AUD] symptoms), across waves. Propensity score methods adjusted for confounding. RESULTS: For males, three or more SLEs were associated with progression from the moderate to the severe problem stage (odds ratio [OR] = 2.23, 95% confidence interval [CI] = 1.17, 4.26). Among those in the severe problem stage, SLEs negatively impacted recovery regardless of sex. Employment/Financial SLEs were associated with a higher odds of transition from the moderate to the no problem stage (OR = 1.60, 95% CI = 1.03, 2.46) and lower odds of transitions from the severe to the moderate problem stage (OR = 0.40, 95% CI = 0.16, 0.99) among males, and from the severe to the no problem stage (OR = 0.26, 95% CI = 0.07, 0.88) among females. CONCLUSION: Stressful life events appear to affect transitions in alcohol involvement over time among those who already have alcohol problems, rather than impacting a transition among those without AUD problems.

Predictors of Hospitalization in a Cohort of Children with Elevated Symptoms of Mania.

Describe hospitalization rates in children with elevated symptoms of mania and determine predictors of psychiatric hospitalizations during the 96 month follow-up. Eligible 6-12.9 year olds and their parents visiting 9 outpatient mental health clinics were invited to be screened with the Parent General Behavior Inventory 10-item Mania Scale. Of 605 children with elevated symptoms of mania eligible for follow-up, 538 (88.9%) had ≥ 1 of 16 possible follow-up interviews and are examined herein. Multivariate Cox regression indicated only four factors predicted hospitalizations: parental mental health problems (HR 1.80; 95% CI 1.21, 2.69); hospitalization prior to study entry (HR 3.03; 95% CI 1.80, 4.43); continuous outpatient mental health service use (HR 3.73; 95% CI 2.40, 5.50); and low parental assessment of how well treatment matched child's needs (HR 3.97; 95% CI 2.50, 6.31). Parental perspectives on mental health services should be gathered routinely, as they can signal treatment failures.

Linear Magnetoelectric Phase in Ultrathin MnPS_{3} Probed by Optical Second Harmonic Generation.

The transition metal thiophosphates MPS_{3} (M=Mn, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. MnPS_{3} is particularly interesting because its Néel ordered state breaks both spatial-inversion and time-reversal symmetries, allowing for a linear magnetoelectric phase that is rare among van der Waals materials. However, it is unknown whether this unique magnetic structure of bulk MnPS_{3} remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magnetoelectric type Néel order in MnPS_{3} persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on SiO_{2} substrates that is absent in bulk materials, which is likely related to substrate induced strain.

Developing and Validating a Definition of Impulsive/Reactive Aggression in Youth.

The goal of this study is to develop a rational data-driven definition of impulsive/reactive aggression and establish distinctions between impulsive/reactive aggression and other common childhood problems. This is a secondary analysis of data from Assessing Bipolar: A Community Academic Blend (ABACAB; N = 636, ages 5-18), Stanley Medical Research Institute N = 392, ages 5-17), and the Longitudinal Assessment of Manic Symptoms (LAMS; N = 679, ages 6-12) studies, which recruited youths seeking outpatient mental health services in academic medical centers and community clinics. Following Jensen et al.'s (2007) procedure, 3 judges independently rated items from several widely used scales in terms of assessing impulsive/reactive aggression. Principal components analyses (PCA) modeled structure of the selected items supplemented by items related to mood symptoms, rule-breaking behavior, and hyperactivity/impulsivity to better define the boundaries between impulsive/reactive aggression and other common childhood symptoms. In the rational item selection process, there was good agreement among the 3 experts who rated items as characterizing impulsive/reactive aggression or not. PCA favored 5 dimension solutions in all 3 samples. Across all samples, PCA resulted in rule-breaking behavior, aggression-impulsive/reactive (AIR), mania, and depression dimensions; there was an additional hyperactive/impulsive dimension in the LAMS sample and a self-harm dimension in ABACAB and Stanley samples. The dimensions demonstrated good internal consistency; criterion validity coefficients also showed consistency across samples. This study is a step toward developing an empirically derived nosology of impulsive aggression/AIR. Findings support the validity of the AIR construct, which can be distinguished from manic and depressive symptoms as well as rule-breaking behavior.