arXiv subjects

Geliang Yu

Publications and source records attributed to Geliang Yu.

18 recordsLinked to original sources

arXiv2026

Tailoring pure valley-Zeeman spin-orbit coupling in WSe$_2$-encapsulated monolayer graphene

Engineering proximity effects in twisted van der Waals heterostructures offers a powerful platform for designing electronic properties. While theoretical predictions of quantum interference in transition metal dichalcogenide-encapsulated graphene can selectively control the spin-orbit coupling component, experimental realizations have remained elusive. Here, we report pure valley-Zeeman spin-orbit coupling in monolayer graphene, achieved by encapsulation between two parallel twisted WSe$_2$ monolayers. We observed a symmetry-enforced reordering of Landau levels, which is driven by the competition between the fixed valley-Zeeman energy and the magnetic-field-dependent cyclotron energy. This reordering is characterized by a transition from symmetry-broken states in the quantum Hall effect to a restored fourfold degeneracy with integer or half-integer quantum Hall sequences. We also demonstrate the ability to completely quench the proximity spin-orbit coupling by tuning the encapsulated geometry.

cond-mat.mes-hall↗

arXiv2026

Quantum Hall Effect at 0.002T

Graphene enables precise carrier-density control via gating, making it an ideal platform for studying electronic interactions. However, sample inhomogeneities often limit access to the low-density regimes where these interactions dominate. Enhancing carrier mobility is therefore crucial for exploring fundamental properties and developing device applications. Here, we demonstrate a significant reduction in external inhomogeneity using a double-layer graphene architecture separated by an ultra-thin hexagonal boron nitride layer. Mutual screening between the layers reduces scattering from random Coulomb potentials, resulting in a quantum mobility exceeding. Shubnikov de-Haas oscillations emerge at magnetic fields below 1 mT, while integer quantum Hall features are observed at 0.002T. Furthermore, we identify a fractional quantum Hall plateau at a filling factor of at 2T. These results demonstrate the platform's suitability for investigating strongly correlated electronic phases in graphene-based heterostructures.

cond-mat.mes-hall↗

arXiv2025

Chemical vapor deposition growth of continuous monolayer antiferromagnetic CrOCl films

The discovery of two-dimensional magnetic materials has provided an ideal platform for exploring physical phenomena in the two-dimensional limit. However, intrinsic two-dimensional antiferromagnetic materials have been rarely reported, limiting systematic studies of their electronic properties. The discovery of novel intrinsic two-dimensional antiferromagnets and the development of robust synthesis strategies, therefore, remain significant challenges. Here, we report the chemical vapor deposition synthesis of CrOCl monolayer films and nanosheets that exhibit excellent air stability. The CrOCl morphology is tunable, ranging from two-dimensional nanosheets to three-dimensional flower-like structures, with lateral sizes ranging from several microns to continuous monolayer films. Structural characterization confirms the materials composition and high crystalline quality. Furthermore, magnetic measurements, supported by theoretical calculations, reveal a N\'eel temperature for CrOCl of ~14 K. This work provides a reliable route for preparing two-dimensional antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2025

The interplay of ferroelectricity and magneto-transport in non-magnetic moir\'{e} superlattices

The coupling of ferroelectricity and magnetic order provides rich tunability for engineering material properties and demonstrates great potential for uncovering novel quantum phenomena and multifunctional devices. Here, we report interfacial ferroelectricity in moir\'{e} superlattices constructed from graphene and hexagonal boron nitride. We observe ferroelectric polarization in an across-layer moir\'{e} superlattice with an intercalated layer, demonstrating a remnant polarization comparable to its non-intercalated counterpart. Remarkably, we reveal a magnetic-field enhancement of ferroelectric polarization that persists up to room temperature, showcasing an unconventional amplification of ferroelectricity in materials lacking magnetic elements. This phenomenon, consistent across devices with varying layer configurations, arises purely from electronic rather than ionic contributions. Furthermore, the ferroelectric polarization in turn modulates quantum transport characteristics, suppressing Shubnikov-de Haas oscillations and altering quantum Hall states in polarized phases. This interplay between ferroelectricity and magneto-transport in non-magnetic materials is crucial for exploring magnetoelectric effects and advancing two-dimensional memory and logic applications.

cond-mat.mtrl-sci↗

arXiv2025

Quantum melting of generalized electron crystal in twisted bilayer MoSe2

Electrons can form an ordered solid crystal phase ascribed to the interplay between Coulomb repulsion and kinetic energy. Tuning these energy scales can drive a phase transition from electron solid to liquid, i.e. melting of Wigner crystal. Generalized Wigner crystals (GWCs) pinned to moire superlattices have been reported by optical and scanning-probe-based methods. Using transport measurements to investigate GWCs is vital to a complete characterization, however, still poses a significant challenge due to difficulties in making reliable electrical contacts. Here, we report the electrical transport detection of GWCs at fractional fillings nu = 2/5, 1/2, 3/5, 2/3, 8/9, 10/9, and 4/3 in twisted bilayer MoSe2. We further observe that these GWCs undergo continuous quantum melting transitions to liquid phases by tuning doping density, magnetic and displacement fields, manifested by quantum critical scaling behaviors. Our findings establish twisted bilayer MoSe2 as a novel system to study strongly correlated states of matter and their quantum phase transitions.

cond-mat.str-el↗

arXiv2025

Anomalous Meets Topological Hall Effect in Cr2Ge2Te6 Heterostructures

Introducing topologically protected skyrmions in graphene holds significant importance for developing high-speed, low-energy spintronic devices. Here, we present a centrosymmetric ferromagnetic graphene/trilayer Cr2Ge2Te6/graphene heterostructure, demonstrating the anomalous and topological Hall effect due to the magnetic proximity effect. Through gate voltage control, we effectively tune the emergence and size of skyrmions. Micromagnetic simulations reveal the formation of skyrmions and antiskyrmions, which respond differently to external magnetic fields, leading to oscillations in the topological Hall signal. Our findings provide a novel pathway for the formation and manipulation of skyrmions in centrosymmetric two-dimensional magnetic systems, offering significant insights for developing topological spintronics.

cond-mat.mes-hall↗

arXiv2024

Strongly coupled magneto-exciton condensates in large-angle twisted double bilayer graphene

Excitons, the bosonic quasiparticle emerging from Coulomb interaction between electrons and holes, will undergo a Bose-Einstein condensation(BEC) and transition into a superfluid state with global phase coherence at low temperatures. An important platform to study such excitonic physics is built on double-layer quantum wells or recent two-dimensional material heterostructures, where two parallel planes of electrons and holes are separated by a thin insulating layer. Lowering this separation distance ($d$) enhances the interlayer Coulomb interaction thereby strengthens the exciton binding energy. However, an exceedingly small $d$ will lead to the undesired interlayer tunneling, which results the annihilation of excitons. Here, we report the observation of a sequences of robust exciton condensates(ECs) in double bilayer graphenes twisted to $\sim 10^\circ$ with no insulating mid-layer. The large momentum mismatch between the two graphene layers well suppress the interlayer tunneling, allowing us to reach the separation lower limit $\sim$ 0.334 nm and investigate ECs in the extreme coupling regime. Carrying out transport measurements on the bulk and edge of the devices, we find incompressible states corresponding to ECs when both layers are half-filled in the $N=0$ and $N=1$ Landau levels (LLs). The comparison between these ECs and theoretical calculations suggest that the low-energy charged excitation of ECs can be meron-antimeron or particle-hole pair, which relies on both LL index and carrier type. Our results establish large-angle twisted bilayers as an experimental platform with extreme coupling strength for studying quantum bosonic phase and its low-energy excitations.

cond-mat.mes-hall↗

arXiv2024

Blood Works for Graphene Production

Blood, a ubiquitous and fundamental carbohydrate material composed of plasma, red blood cells, white blood cells, and platelets, has been playing an important role in biology, life science, history, and religious study, while graphene has garnered significant attention due to its exceptional properties and extensive range of potential applications. Achieving environmentally friendly, cost-effective growth using hybrid precursors and obtaining high-quality graphene through a straightforward CVD process has been traditionally considered mutually exclusive. This study demonstrates that we can produce high-quality graphene domains with controlled thickness through a one-step growth process at atmospheric pressure using blood as a precursor. Raman spectroscopy confirms the uniformity of the blood-grown graphene films, and observing the half-integer quantum Hall effect in the measured devices highlights its outstanding electronic properties. This unprecedented approach opens possibilities for blood application, facilitating an unconventional route in graphene growth applications.

cond-mat.mes-hall↗

arXiv2024

Gate control of 2D magnetism in tri- and four-layers $\rm CrI_3$/graphene heterostructures

We conduct experimental studies on the electrical transport properties of monolayer graphene directly covered by a few layers of $\rm CrI_3$. We do not observe the expected magnetic exchange coupling in the graphene but instead discover proximity effects featuring gate and magnetic field tunability. The tunability of gate voltage is manifested in the alignment of the lowest conduction band of $\rm CrI_3$ and the Fermi level of graphene, which can be controlled by the gate voltage. The coexistence of the normal and atypical quantum Hall effects in our device also corresponds to gate-control modulation doping. The lowest conduction band depends on the magnetic states of the $\rm CrI_3$ and can be altered by the magnetic field, which corresponds to the resistance loops during back-and-forth sweeps of the magnetic field. Our results serve as a reference for exploiting the magnetic proximity effects in graphene.

cond-mat.mes-hall↗

arXiv2024

Ferroelectricity in twisted double bilayer graphene

Two-dimensional ferroelectrics can maintain vertical polarization up to room temperature, and are, therefore, promising for next-generation nonvolatile memories. Although natural two-dimensional ferroelectrics are few, moir\'{e} superlattices provide us with a generalized method to construct ferroelectrics from non-ferroelectric parent materials. We report a realization of ferroelectric hysteresis in a AB-BA stacked twisted double bilayer graphene (TDBG) system. The ferroelectric polarization is prominent at zero external displacement field and reduces upon increasing displacement fields. TDBG in the AB-BA configuration possesses a superlattice of non-centrosymmetric domains, exhibiting alternatively switchable polarities even without the assistance of any boron nitride layers; however, in the AB-AB stacking case, the development of polarized domains necessitates the presence of a second superlattice induced by the adjacent boron nitride layer. Therefore, twisted multilayer graphene systems offer us a fascinating field to explore two-dimensional ferroelectricity.

cond-mat.mes-hall↗

arXiv2024

Tailoring coercive fields and the Curie temperature via proximity coupling in WSe$_2$/Fe$_3$GeTe$_2$ van der Waals heterostructures

Hybrid structures consisting of two-dimensional (2D) magnets and semiconductors have exhibited extensive functionalities in spintronics and opto-spintronics. In this work, we have fabricated WSe$_2$/Fe$_3$GeTe$_2$ van der Waals (vdW) heterostructures and investigated the proximity effects on 2D magnetism. Through reflective magnetic circular dichroism (RMCD), we have observed a temperature-dependent modulation of magnetic order in the heterostructure. For temperatures above $40$ K, WSe$_2$-covered Fe$_3$GeTe$_2$ exhibits a larger coercive field than that observed in bare Fe$_3$GeTe$_2$, accompanied by a noticeable enhancement of the Curie temperature by $21$ K. This strengthening suggests an increase in magnetic anisotropy in the interfacial Fe$_3$GeTe$_2$ layer, which can be attributed to the spin-orbit coupling (SOC) proximity effect induced by the adjacent WSe$_2$ layers. However, at much lower temperatures ($T<20$ K), a non-monotonic modification of the coercive field is observed, showing both reduction and enhancement, which depends on the thickness of the WSe$_2$ and Fe$_3$GeTe$_2$ layers. Moreover, an unconventional two-step magnetization process emerges in the heterostructure, indicating the short-range nature of SOC proximity effects. Our findings revealing proximity effects on 2D magnetism may shed light on the design of future spintronic and memory devices based on 2D magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2021

Weak Antilocalization Effect up to ~ 120 K in the van der Waals Crystal Fe5-xGeTe2 with Near Room Temperature Ferromagnetism

The weak antilocalization (WAL) effect is known as a quantum correction to the classical conductivity, which never appeared in two-dimensional magnets. In this work, we reported the observation of a WAL effect in the van der Waals ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even reach as high as ~ 120 K. The WAL effect could be well described by the Hikami-Larkin-Nagaoka and Maekawa-Fukuyama theories in the presence of strong spin-orbit coupling (SOC). Moreover, A crossover from a peak to dip behavior around 60 K in both the magnetoresistance and magnetoconductance was observed, which could be ascribed to a rare example of temperature driven Lifshitz transition as indicated by the angle-resolved photoemission spectroscopy measurements and first principles calculations. The reflective magnetic circular dichroism measurements indicate a possible spin reorientation that kills the WAL effect above 120 K. Our findings present a rare example of WAL effect in two-dimensional ferromagnet and also a magnetotransport fingerprint of the strong SOC in Fe5-xGeTe2. The results would be instructive for understanding the interaction Hamiltonian for such high Tc itinerant ferromagnetism as well as be helpful for the design of next-generation room temperature spintronic or twistronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Tunable ferroelectricity in hBN intercalated twisted double-layer graphene

Van der Waals (vdW) assembly of two-dimensional materials has been long recognized as a powerful tool to create unique systems with properties that cannot be found in natural compounds. However, among the variety of vdW heterostructures and their various properties, only a few have revealed metallic and ferroelectric behaviour signatures. Here we show ferroelectric semimetal made of double-gated double-layer graphene separated by an atomically thin crystal of hexagonal boron nitride, which demonstrating high room temperature mobility of the order of 10 m$^2$V$^{-1}$s$^{-1}$ and exhibits robust ambipolar switching in response to the external electric field. The observed hysteresis is tunable, reversible and persists above room temperature. Our fabrication method expands the family of ferroelectric vdW compounds and offers a route for developing novel phase-changing devices.

cond-mat.mes-hall↗

arXiv2020

Reversible Engineering of Topological Insulator Surface State Conductivity through Optical Excitation

Despite the broadband response, limited optical absorption at a particular wavelength hinders the development of optoelectronics based on Dirac fermions. Heterostructures of graphene and various semiconductors have been explored for this purpose, while non-ideal interfaces often limit the performance. The topological insulator is a natural hybrid system, with the surface states hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk state strongly absorbing light. In this work, we show a large photocurrent response from a field effect transistor device based on intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S. The photocurrent response is non-volatile and sensitively depends on the initial Fermi energy of the surface state, and it can be erased by controlling the gate voltage. Our observations can be explained with a remote photo-doping mechanism, in which the light excites the defects in the bulk and frees the localized carriers to the surface state. This photodoping modulates the surface state conductivity without compromising the mobility, and it also significantly modify the quantum Hall effect of the surface state. Our work thus illustrates a route to reversibly manipulate the surface states through optical excitation, shedding light into utilizing topological surface states for quantum optoelectronics.

cond-mat.mes-hall↗

arXiv2019

Observation of ballistic avalanche phenomena in nanoscale vertical InSe/BP heterostructures

Initiating impact ionization of avalanche breakdown essentially requires applying a high electric field in a long active region, hampering carrier-multiplication with high gain, low bias and superior noise performance. Here we report the observation of ballistic avalanche phenomena in sub-MFP scaled vertical indium selenide (InSe)/black phosphorus (BP) heterostructures. The heterojunction is engineered to avalanche photodetectors (APD) and impact ionization transistors, demonstrating ultra-sensitive mid-IR light detection (4 {\mu}m wavelength) and ultra-steep subthreshold swing, respectively. These devices show an extremely low avalanche threshold (<1 volt), excellent low noise figures and distinctive density spectral shape. Further transport measurement evidences the breakdown originals from a ballistic avalanche phenomenon, where the sub-MFP BP channel enables both electrons and holes to impact-ionize the lattice and abruptly amplify the current without scattering from the obstacles in a deterministic nature. Our results shed light on the development of advanced photodetectors and efficiently facilitating carriers on the nanoscale.

cond-mat.mtrl-sci↗

arXiv2018

Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2

It is well known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations, few-layer NbSe2 - an archetypal example of ultrathin superconductors - has been found to remain superconducting down to monolayer thickness. Here we report measurements of both the superconducting energy gap and critical temperature in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N < 5, as does their ctitical temperature. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to Tc. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of the gap and Tc expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of the gap and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors.

cond-mat.supr-con↗

arXiv2017

Stepwise quantized surface states and delayed Landau level hybridization in Co cluster-decorated BiSbTeSe2 topological insulator devices

In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top and bottom surfaces. However, due to the no-go theorem, the two TSSs always appear as a pair and are expected to quantize synchronously. Quantized transport of a separate Dirac channel is still desirable, but has never been observed in graphene even after intense investigation over a period of 13 years, with the potential aim of half-QHE. By depositing Co atomic clusters, we achieved stepwise quantization of the top and bottom surfaces in BiSbTeSe2 (BSTS) TI devices. Renormalization group flow diagrams13, 22 (RGFDs) reveal two sets of converging points (CVPs) in the (Gxy, Gxx) space, where the top surface travels along an anomalous quantization trajectory while the bottom surface retains 1/2 e2/h. This results from delayed Landau-level (LL) hybridization (DLLH) due to coupling between Co clusters and TSS Fermions.

cond-mat.mes-hall↗

arXiv2015

Nonlocal Response and Anamorphosis: the Case of Few-Layer Black Phosphorus

Few-layer black phosphorus was recently rediscovered as a narrow-bandgap atomically thin semiconductor and has already attracted unprecedented attention due to its interesting properties. One feature of this material that sets it apart from other atomically thin crystals is its structural in-plane anisotropy which manifests in strongly anisotropic transport characteristics. However, traditional angle-resolved conductance measurements present a challenge for nanoscale systems such as black phosphorus, calling for new approaches in precision studies of transport anisotropy. Here we show that the nonlocal response, being exponentially sensitive to the anisotropy value, provides a powerful tool for determining the anisotropy. This is established by combining measurements of the orientation-dependent nonlocal resistance response with the analysis based on the anamorphosis relations. We demonstrate that the nonlocal response can differ by orders of magnitude for different crystallographic directions even when the anisotropy is at most order-one, allowing us to extract accurate anisotropy values.

cond-mat.mes-hall↗