arXiv subjects

Sandeep Joy

Publications and source records attributed to Sandeep Joy.

10 recordsLinked to original sources

arXiv2026

Competing Orders Driven by Wigner Crystal Phase in Rhombohedral Graphene

Rhombohedral graphene systems provide a unique platform where strong electronic interactions and nontrivial band topology coexist at low carrier densities and high displacement fields, giving rise to a rich landscape of emergent electronic phases. Here, we report that the highly insulating state on the low-density side of chiral superconductivity in rhombohedral pentalayer graphene (R5G) corresponds to a Wigner crystal (WC) phase. In addition, a hole-doped metallic Wigner crystal (h-mWC) phase emerges near the WC boundary. Under an out-of-plane magnetic field, the system hosts competing magnetic-field-stabilized superconductivity (fSC) and unconventional reentrant quantum Hall (RIQH) states. These emergent phases are closely connected to the underlying WC and mWC states and evolve continuously across phase boundaries. Our results establish that WC phase plays an important role in the phase diagram of rhombohedral multilayer graphene and highlight its connection to a rich landscape of emergent phases.

cond-mat.mes-hall↗

arXiv2025

Is disorder a friend or a foe to melting of Wigner-Mott insulators?

Wigner crystals are extremely fragile, which is shown to result from very strong geometric frustration germane to long-range Coulomb interactions. Physically, this is manifested by a very small characteristic energy scale for shear density fluctuations, which are gapless excitations in a translationally invariant system. The presence of disorder, however, breaks translational invariance, thus suppressing gapless excitations and pushing them to higher density. We illustrate this general principle by explicit microscopic model calculations, showing that this mechanism very effectively stabilizes disordered Wigner lattices to much higher temperatures and densities than in the clean limit. On the other hand, we argue that in two dimensions disorder significantly ``smears" the melting transition, producing spatial coexistence of solid-like and liquid-like regions -- just as recently observed in STM experiments. Our results paint a new physical picture for melting of Wigner-Mott solids in two dimensions, corresponding to a Mott-Hubbard model with spatially varying local electronic bandwidth.

cond-mat.str-el↗

arXiv2025

Chiral Wigner crystal phases induced by Berry curvature

We consider the impact of Berry phase on the Wigner crystal (WC) state of a two-dimensional electron system. We consider first a model of Bernal bilayer graphene with a perpendicular displacement field, and we show that Berry curvature leads to a new kind of WC state in which the electrons acquire a spontaneous orbital angular momentum when the displacement field exceeds a critical value. We determine the phase boundary of the WC state in terms of electron density and displacement field at low temperature. We then derive the general effective Hamiltonian that governs the ordering of the physical electron spin. We show that this Hamiltonian includes a chiral term that can drive the system into chiral spin-density wave or spin liquid phases. The phenomena we discuss are relevant for the valley-polarized Wigner crystal phases observed in multilayer graphene.

cond-mat.str-el↗

arXiv2025

Tunable interplay of orbital and spin magnetization in trigonal tellurium

Orbital effects, despite their fundamental significance and potential to engender novel physical phenomena and enable new applications, have long been underexplored compared to their spin counterparts. Recently, surging interest in the orbital degree of freedom has led to the discovery of a plethora of orbital-related effects, underscoring the need for a deeper understanding of their roles in quantum materials. Here, we report systematic experimental evidence consistent with orbital magnetization and spontaneous rotational symmetry breaking in trigonal Tellurium, an elemental semiconductor with a unique helical crystal structure that serves as a natural platform for investigating orbital effects. Detailed angular dependent linear and nonlinear magnetotransport measurements, supported by symmetry-guided Boltzmann transport analysis, support the interpretation of coexistence of current-induced spin polarization and orbital magnetization. With the goal of disentangling the interplay between spin and orbital degrees of freedom through electrostatic gating, this work establishes a general framework for understanding orbital magnetization in chiral crystals and beyond, paving the way for its utilization in orbitronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Disorder-induced liquid-solid phase coexistence in 2D electron systems

Recent imaging experiments show a surprisingly robust regime of liquid-solid phase coexistence in a 2D electron system near the quantum melting/freezing transition, with the two phases mixed in mesoscopic domains. Strikingly, the experiments find no noticeable difference in electron density between the liquid and solid domains, which is at odds with both microemulsion scenarios and scenarios in which phase coexistence is driven by fluctuations of a long-ranged disorder potential. Here, we show that such phase coexistence without density difference can be induced by random fluctuations of a short-ranged disorder potential. We further show that disorder tends to stabilize the Wigner Crystal phase to higher densities, which is also consistent with the experiments.

cond-mat.str-el↗

arXiv2023

Wigner crystallization in Bernal bilayer graphene

In Bernal bilayer graphene (BBG), a perpendicular displacement field flattens the bottom of the conduction band and thereby facilitates the formation of strongly correlated electron states at low electron density. Here, we focus on the Wigner crystal (WC) state, which appears in a certain regime of sufficiently large displacement field, low electron density, and low temperature. We first consider a model of BBG without trigonal warping, and we show theoretically that Berry curvature leads to a new kind of WC state in which the electrons acquire a spontaneous orbital magnetization when the displacement field exceeds a critical value. We then consider the effects of trigonal warping in BBG, and we show that they lead to an unusual ``doubly re-entrant" behavior of the WC phase as a function of density. The rotational symmetry breaking associated with trigonal warping leads to a nontrivial ``minivalley order" in the WC state, which changes abruptly at a critical value of displacement field. In both cases, we estimate the phase boundary of the WC state in terms of density, displacement field, and temperature. This paper is complementary to our recent work [Joy et al., Phys. Rev. Lett. 135, 256502 (2025)].

cond-mat.str-el↗

arXiv2023

Cascade of multi-electron 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$\approx$1K.

cond-mat.mes-hall↗

arXiv2023

Upper bound on the window of density occupied by microemulsion phases in two-dimensional electron systems

In two-dimensional electronic systems, direct first-order phase transitions are prohibited as a consequence of the long-range Coulomb interaction, which implies a stiff energetic penalty for macroscopic phase separation. A prominent proposal is that any direct first-order transition is instead replaced by a sequence of ``microemulsion" phases, in which the two phases are mixed in patterns of mesoscopic domains. In this note, we comment on the range $\Delta n$ of average electron density that such microemulsion phases may occupy. We point out that, even without knowing the value of a phenomenological parameter associated with surface tension between the two phases, one can place a fairly strong upper bound on the value of $\Delta n$. We make numerical estimates for $\Delta n$ in the case of the Fermi liquid to Wigner crystal transition and find $\Delta n$ to be on the order of $10^7$\,cm$^{-2}$. This value is much smaller than the width of the phase transition observed in experiments, suggesting that disorder is a more likely explanation for the apparent broadening of the transition.

cond-mat.str-el↗

arXiv2021

Wigner crystallization at large fine structure constant

We consider the fate of the Wigner crystal state in a two dimensional system of massive Dirac electrons as the effective fine structure constant $\alpha$ is increased. In a Dirac system, larger $\alpha$ naively corresponds to stronger electron-electron interactions, but it also implies a stronger interband dielectric response that effectively renormalizes the electron charge. We calculate the critical density and critical temperature associated with quantum and thermal melting of the Wigner crystal state using two independent approaches. We show that at $\alpha \gg 1$, the Wigner crystal state is best understood in terms of logarithmically-interacting electrons, and that both the critical density and the melting temperature approach a universal, $\alpha$-independent value. We discuss our results in the context of recent experiments in twisted bilayer graphene near the magic angle.

cond-mat.str-el↗

arXiv2020

Transparent mirror effect in twist-angle-disordered bilayer graphene

When light is incident on a medium with spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a "transparent mirror." We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto one dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.

cond-mat.mes-hall↗