arXiv subjects

Qianhui Shi

Publications and source records attributed to Qianhui Shi.

7 recordsLinked to original sources

arXiv2026

Magnetic Field Reorganization of Electronic States in Moir\'e Bilayer Graphene

Magnetic fields are widely used to diagnose quantum phases in two-dimensional systems through quantum oscillations or by tuning spin and valley polarizations, but magnetic fields can also reshape the underlying electronic structure. Here, using a bilayer graphene/hBN moir\'e system, we reveal a rich magnetic field induced evolution of the semiclassical orbit network, encompassing Lifshitz transitions, magnetic breakdown, and scattering between coexisting electron and hole pockets. At magnetic fields of 1 T to 2 T, quantum oscillation frequencies and the Hall density change markedly over a broad carrier density range, signaling magnetic breakdown and magnetic Lifshitz transitions. This evolution is valley contrasting: Berry curvature hot spots near the breakdown junctions enhance magnetic breakdown in the K valley while suppressing it in the K$^\prime$ valley, whereas valley-antisymmetric orbital magnetic moments split the corresponding Lifshitz transitions. The resulting valley-selective trajectories manifest at higher fields as valley-symmetry-breaking Hofstadter gaps. At elevated temperatures and low magnetic fields, scattering between coexisting electron and hole pockets produces nearly density-independent resistance oscillations whose frequency tracks the sum of their Fermi surface areas, persisting after conventional Onsager oscillations are thermally washed out. Our results provide a unified picture of how modest magnetic fields reorganize moir\'e electronic states as the system evolves from semiclassical transport toward the Hofstadter regime.

cond-mat.mes-hall↗

arXiv2026

Contrasting $\Gamma$- and K-Valley Moir\'e Physics in Twisted Monolayer/Bilayer WSe$_2$

Electronic orbital character plays a central role in determining electronic correlations, spin-orbit coupling, dimensionality, and ultimately the quantum phases of condensed-matter systems. Two-dimensional moir\'e materials have emerged as highly tunable platforms for exploring correlated phenomena, but the role of orbital degrees of freedom remains largely unexplored. Here, we identify twisted monolayer/bilayer WSe$_2$ as a platform in which displacement-field tuning enables moir\'e physics to be realized in both the $K$ and $\Gamma$ valleys. The distinct orbital characters of these valleys give rise to contrasting correlated phases at moir\'e filling factors $\nu=1$ and $\nu=1/3$. At $\nu=1$, the $K$-valley state is a weak insulator, consistent with an antiferromagnetic state near a van Hove singularity in the intermediate-coupling regime, similar to that observed in twisted bilayer WSe$_2$. In contrast, the $\Gamma$-valley state exhibits a pronounced Pomeranchuk effect, consistent with proximity to a Mott transition. At $\nu=1/3$, the $K$ valley hosts a robust generalized Wigner crystal, whereas the $\Gamma$-valley state lies near the crystallization boundary and again exhibits a Pomeranchuk effect, with localization enhanced by increasing temperature or magnetic field. Our work highlights the importance of orbital character in defining quantum phases in moir\'e systems, and identify the $\Gamma$ valley as a promising platform for exploring correlated phenomena near quantum phase transitions, where competing phases and enhanced fluctuations may give rise to unconventional phases.

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arXiv2023

Spin-selective magneto-conductivity in WSe$_2$

Material systems that exhibit tunable spin-selective conductivity are key components of spintronic technologies. Here we demonstrate a novel type of spin-selective transport, based on the unusual Landau level (LL) sequence observed in bilayer WSe$_2$ under large applied magnetic fields. We find that the conductivity depends strongly on the relative iso-spin ordering between conducting electrons in a partially filled LL and the localized electrons of lower energy filled LLs, with conductivity observed to be almost completely suppressed when the spin-ratio and field-tuned Coulomb energy exceed a critical threshold. Switching between "on/off" states is achievable through either modulation of the external magnetic or electric fields, with many-body interaction driving a collective switching mechanism. In contrast to magnetoresistive heterostructures, this system achieves electrically tunable spin filtering within a single material, driven by interaction between free and localized spins residing in energy-separated spin/valley polarized bands. Similar spin-selective conductivity may be realizable in multi-flat band systems at zero magnetic field.

cond-mat.mes-hall↗

arXiv2023

Evidence for a Superfluid-to-solid Transition of Bilayer Excitons

One of the most spectacular properties associated with Bose-Einstein condensation (BEC) is superfluidity in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide ranging Bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid - termed a supersolid. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained elusive. Here we report observation of a superfluid to insulator transition in the layer-imbalanced regime of bilayer magneto-excitons. Mapping the transport behavior of the bilayer condensate as a function of density and temperature, suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid upon increasing the temperature, which could indicate that the low temperature solid is also a quantum coherent phase.

cond-mat.mes-hall↗

arXiv2021

Bilayer WSe$_2$ as a natural platform for interlayer exciton condensates in the strong coupling limit

Exciton condensates (EC) are macroscopic coherent states arising from condensation of electron-hole pairs. Bilayer heterostructures, consisting of two-dimensional electron and hole layers separated by a tunnel barrier, provide a versatile platform to realize and study EC. The tunnel barrier suppresses recombination yielding long-lived excitons. However, this separation also reduces interlayer Coulomb interactions, limiting the exciton binding strength. Here, we report the observation of EC in naturally occurring 2H-stacked bilayer WSe$_2$. In this system, the intrinsic spin-valley structure suppresses interlayer tunneling even when the separation is reduced to the atomic limit, providing access to a previously unattainable regime of strong interlayer coupling. Using capacitance spectroscopy, we investigate magneto-EC, formed when partially filled Landau levels (LL) couple between the layers. We find that the strong-coupling EC show dramatically different behaviour compared with previous reports, including an unanticipated variation of the EC robustness with the orbital number, and find evidence for a transition between two types of low-energy charged excitations. Our results provide a demonstration of tuning EC properties by varying the constituent single-particle wavefunctions.

cond-mat.mes-hall↗

arXiv2020

Diffusivity Reveals Three Distinct Phases of Interlayer Excitons in MoSe2/WSe2 Heterobilayers

Charge separated interlayer excitons in transition metal dichalcogenide (TMDC) heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe2/WSe2 heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré-potential to the modestly mobile exciton gas as exciton density increases to ne/h ~ 1011 cm-2 and from the exciton gas to the highly mobile charge separated electron/hole plasma for ne/h > 1012 cm-2. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

cond-mat.mes-hall↗

arXiv2020

Odd- and even-denominator fractional quantum Hall states in monolayer WSe$_2$

Monolayer (ML) semiconducting transition-metal dichalcogenides (TMDs) represent a unique class of two-dimensional (2D) electron systems. Their atomically thin structure -- just like graphene -- facilitates gate-tunability, while the sizable band gap and strong spin-orbit coupling hold promise for properties beyond graphene. Measurements under large magnetic fields have revealed an unusual LL structure, distinct from other 2D electron systems. However, owing to limited sample quality and poor electrical contact, probing the lowest Landau levels (LLs) has been challenging, and observation of electron correlations within the fractionally filled LLs regime has not been possible. Here, through bulk electronic compressibility measurements, we investigate the LL structure of ML WSe$_2$ in the extreme quantum limit, and observe fractional quantum Hall (FQH) states in the lowest three LLs. The odd-denominator FQH sequences demonstrate a systematic evolution with the LL orbital index, which has not been observed in any other system but is consistent with generic theoretical expectations. In addition, we observe an even-denominator state in the second LL that is expected to host non-Abelian statistics. Our results suggest that the 2D semiconductors can provide an experimental platform that closely resembles idealized theoretical models in the quantum Hall regime.

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