arXiv subjects

Noah F. Q. Yuan

Publications and source records attributed to Noah F. Q. Yuan.

At least 19 recordsLinked to original sources

arXiv2026

Superconductivity in non-centrosymmetric rhombohedral NbSe2

Crystal stacking offers a powerful yet underexplored route to engineer symmetry in layered superconductors. Here we report superconductivity in rhombohedral-stacked NbSe2 (3R-NbSe2), a non-centrosymmetric polytype in which global inversion symmetry is removed by stacking alone. Using comprehensive structural, transport, magnetic, and thermodynamic measurements, we establish superconductivity as a bulk property of the 3R phase and find that the in-plane upper critical field exceeds the Pauli paramagnetic limit, indicating the persistence of strong Ising-type spin-orbit coupling. Unlike the thickness-dependent superconductivity in centrosymmetric 2H-NbSe2, the superconducting transition temperature in 3R-NbSe2 shows little dependence on layer number but exhibits an unusually strong sensitivity to disorder. We further observe strongly enhanced nonlinear optical and electrical responses near the superconducting transition, consistent with stacking-induced inversion-symmetry breaking. Our results identify 3R-NbSe2 as a single-phase platform in which stacking engineering reshapes superconductivity and enables nonlinear transport phenomena in layered materials.

cond-mat.supr-con↗

arXiv2025

Large superconducting diode effect driven by edge states in twisted iron-chalcogenide Josephson junctions

The superconducting diode effect (SDE)-the unidirectional, dissipationless flow of supercurrent-is a critical element for future superconducting electronics. Achieving high efficiency under zero magnetic field is a key requirement. The Josephson junction constitutes a versatile SDE platform for exploiting quantum materials that exhibit ferromagnetism, topology, or unconventional superconductivity. However, a single two-dimensional material system that inherently offers these properties and allows for precise interface engineering, such as twisting, remains elusive. Here we report a record-high, field-free diode efficiency of ~30% in twist van der Waals Josephson heterostructures of the sign-change iron-chalcogenide superconductor FeTe0.55Se0.45 and the conventional transition-metal dichalcogenide superconductor 2H-NbSe2. The diode response shows a striking twist-angle dependence: the efficiency peaks at crystallographic alignment and collapses with a small misorientation of ~7 deg. Importantly, the twist-angle evolution of superconducting interference measurements reveals that efficient nonreciprocity arises from asymmetric edge supercurrents, whereas bulk transport suppresses the effect. These findings establish edge states as the driving mechanism of the unconventional SDE, linking it to exotic pairing and topology in multiband iron-based superconductors. Our findings reveal intricate physics involving novel pairing symmetry, magnetism, and topology in the multiband iron-based superconductor, and offer a new route to high-performance superconducting diodes.

cond-mat.supr-con↗

arXiv2025

Fabrication and Characterization of the Moiré surface state on a topological insulator

A Moire superlattice on the topological insulator surface is predicted to exhibit many novel properties but has not been experimentally realized. Here, we developed a two-step growth method to successfully fabricate a topological insulator Sb2Te3 thin film with a Moire superlattice, which is generated by a twist of the topmost layer via molecular beam epitaxy. The established Moire topological surface state is characterized by scanning tunneling microscopy and spectroscopy. By application of a magnetic field, new features in Landau levels arise on the Moire region compared to the pristine surface of Sb2Te3, which makes the system a promising platform for pursuing next-generation electronics. Notably, the growth method, which circumvents contamination and the induced interface defects in the manual fabrication method, can be widely applied to other van der Waals materials for fabricating Moire superlattices.

cond-mat.mtrl-sci↗

arXiv2025

Induced Zeeman effect of moiré surface states in topological insulators

Recently, moiré superlattices have been found on the surface of topological insulators due to the rotational misalignment of topmost layers. In this work, we study the effects of moiré superlattices on the Landau levels of topological surface states. We find that an extra Zeeman term besides the intrinsic one can be induced by the orbital effect of the magnetic field in moiré surface states. As a result, the originally field-independent zeroth Landau level of moiré surface states could be tilted by the out-of-plane magnetic field.

cond-mat.mes-hall↗

arXiv2025

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in 2H-NbS2 flakes

Symmetry breaking in a layered superconductor with Ising spin-orbit coupling has offered an opportunity to realize unconventional superconductivity. To be more specific, orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, exhibiting layer-dependent finite-momentum pairing, may emerge in transition metal dichalcogenides materials (TMDC) in the presence of an in-plane magnetic field. Orbital FFLO state can be more robust against the magnetic field than the conventional superconducting state with zero-momentum pairing. This feature renders its potential in field-resilient superconducting functionality. Although, orbital FFLO state has been reported in NbSe2 and MoS2, it is not yet clear if orbital FFLO state can be extended to other TMDC superconductor. Here, we report the observation of orbital FFLO state in 2H-NbS2 flakes and its dependence on the thickness of flake. We conclude that the relatively weak interlayer coupling is instrumental in stabilizing orbital FFLO state at higher temperature with respect to the critical temperature and lower magnetic field with respect to paramagnetic limit in NbS2 in comparison to its NbSe2 counterpart.

cond-mat.supr-con↗

arXiv2025

Coexistence of topological surface states and superconductivity in Dirac semimetal NiTe$_2$

The coexistence of topological bands around the Fermi level ($E_F$) and superconductivity provides a fundamental platform for exploring their interplay. However, few materials inherently display both properties. In this study, we demonstrate the coexistence of topological surface states at the $E_F$ and superconductivity in NiTe$_2$ single crystals, a material hitherto not recognized as superconducting. Quasiparticle interference measurements performed via scanning tunneling microscopy suggest the presence of topological surface states at the $E_F$, which is further corroborated by density functional theory simulations. Experimental evidence for superconductivity is provided via electronic transport measurements and specific heat capacity analyses. Our results suggest that NiTe$_2$ represents a promising platform for investigating the rich interplay between topological states and superconductivity.

cond-mat.supr-con↗

arXiv2025

Orbital Fulde-Ferrell State versus Orbital Larkin-Ovchinnikov State

Orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states are analyzed within mean-field theory, where orbital FF state is layer-polarized with inversion symmetry broken, while orbital LO state is Josephson vortex array with translation symmetry reduced. Phase diagrams of orbital FFLO states are obtained, and properties such as induced orders, superconducting diode effects, Fraunhofer pattern and topological defects are studied for the probe of FF versus LO states.

cond-mat.supr-con↗

arXiv2025

Large moiré superstructure of stacked incommensurate charge density waves

Recent advances in van der Waals heterostructures have opened the new frontier of moiré physics, whereby tuning the interlayer twist angle or adjusting lattice parameter mismatch have led to a plethora of exotic phenomena such as unconventional superconductivity and fractional quantum spin Hall effect. We extend the concept of moiré engineering to materials that host incommensurate orders, where we discovered a long-period, thermally-hysteretic moiré superlattice in a layered charge density wave (CDW) compound, EuTe$_\text{4}$. Using high-momentum-resolution X-ray diffraction performed on ultrathin flakes, we found two coexisting, incommensurate CDWs with slightly mismatched in-plane wavevectors. The interaction between these two CDWs leads to their joint commensuration with the high-symmetry lattice as well as a large moiré superstructure with an in-plane period of 13.6~nm. Due to different out-of-plane orders of the incommensurate CDWs, the moiré superstructure exhibits a clear thermal hysteresis, accounting for the large hysteresis observed in electrical resistivity and numerous metastable states induced by light or electrical pulses. Our findings pave the way for a new development in moiré engineering based on an incommensurate lattice. They further highlight the important role of interlayer ordering in determining the macroscopic properties of these stacked incommensurate structures.

cond-mat.str-el↗

arXiv2024

Evidence for the novel type of orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual superconducting state, defies high magnetic fields beyond the Pauli paramagnetic limit. It exhibits a spatial modulation of the superconducting order parameter in real space and is exceptionally rare. Recently, an even more exotic variant - the orbital FFLO state - was predicted and identified in the transition metal dichalcogenide superconductor 2H-NbSe2. This state emerges in thin samples with thicknesses below ~40 nm, at the boundary between two and three dimensions. The complex interplay between Ising spin orbit coupling and the Pauli paramagnetic effect can lead to a stabilization of the FFLO state in a relatively large range of the magnetic phase diagram, even well below the Pauli limit. In this study, we present experimental evidence of the formation of this orbital FFLO state in bulk 2H-NbSe2 samples. This evidence was obtained using high-resolution DC magnetization and magnetic torque experiments in magnetic fields applied strictly parallel to the NbSe2 basal plane. Both quantities display a crossover to a discontinuous first-order superconducting transition at the normal state boundary in magnetic fields of 4 T and above. This is usually seen as a sign that Pauli paramagnetic pair breaking effects affect the superconducting state. The magnetic torque reveals a small step-like reversible anomaly, indicating a magnetic field-induced thermodynamic phase transition within the superconducting state. This anomaly bears many similarities to the FFLO transitions in other FFLO superconductors, suggesting the potential existence of an orbital FFLO state in bulk 2H-NbSe2 samples. Additionally, we observe a pronounced in-plane 6-fold symmetry of the upper critical field in the field range above this phase transition, which has previously been interpreted as a hallmark of the orbital FFLO state in thin 2H-NbSe2.

cond-mat.supr-con↗

arXiv2024

Direct observation of quantum vortex fractionalization in multiband superconductors

Magnetic field is expelled from a superconductor, unless it forms quantum vortices, consisting of a core singularity with current circulating around it. The London quantization condition implies that there is one core singularity per quantum of magnetic flux in single-component superconductors, while in multiband materials fractional vortices are possible. Here, we report the first observation of quantum vortex core fractionalization on the potassium terminated surface of multiband superconductor KFe2As2 by scanning tunneling microscopy. We observe splitting of an integer-flux vortex into several fractional vortices, leading to disparity between numbers of flux quanta and vortex cores. Our findings demonstrate that fractionalized core singularities are possible in a multiband superconductor, opening avenue for new experimental platforms with quasiparticles with fractional statistics.

cond-mat.supr-con↗

arXiv2024

Fractional vortices and Ising superconductivity in multiband superconductors

Inspired by the recent experiments in monolayer iron-based superconductors, we theoretically investigate properties of a two-dimensional multiband superconductor, focusing on two aspects. First, for vortex bound states, the spatial anisotropy and positions of electron density peaks are associated with interband couplings. Second, even with inversion symmetry, there allows a Ising-type spin-orbit coupling, leading to the enhanced in-plane upper critical field.

cond-mat.supr-con↗

arXiv2024

Topological metal and high-order Dirac point in cubic Rashba model

We investigate the properties of the two-dimensional model with Rashba-type spin-orbit coupling cubic in electron momentum. In the normal phase, edge states emerge on open boundaries. In the superconducting phase, edge states could evolve into gapped fermionic edge states. Applications to realistic materials of interface superconductors are also discussed.

cond-mat.supr-con↗

arXiv2023

Edelstein effect and supercurrent diode effect

We self-consistently calculate the supercurrent diode effect from microscopic models of quasi one- and two-dimensional clean superconductors with spin-orbit coupling under external Zeeman fields, and show that the Edelstein effect is responsible for the supercurrent diode effect. In turn, the supercurrent diode effect may serve as a direct measurement of the Edelstein effect as its application.

cond-mat.supr-con↗

arXiv2023

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor

The critical field behavior of a layered Ising superconductor with finite number of layers is studied. Under in-plane magnetic fields, the finite-momentum superconductivity dubbed as the orbital Fulde- Ferrell-Larkin-Ovchinnikov state is found in the regime of low field and high temperature. Our theory agrees well with the experimental results in Nature 619, 46 (2023).

cond-mat.supr-con↗

arXiv2023

Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor

The conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state relies on the Zeeman effect of an external magnetic field to break time-reversal symmetry, forming a state of finite-momentum Cooper pairing. In superconductors with broken inversion symmetries, the Rashba or Ising-type spin-orbit coupling (SOC) can interact with either the Zeeman or the orbital effect of magnetic fields, extending the range of possible FFLO states, though evidence for these more exotic forms of FFLO pairing has been lacking. Here we report the discovery of an unconventional FFLO state induced by coupling the Ising SOC and the orbital effect in multilayer 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite momentum cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of normal metal, uniform Ising superconducting phase, and a six-fold orbital FFLO state. This study highlights an alternative route to finite-momentum superconductivity and provides a universal mechanism to prepare orbital FFLO states in similar materials with broken inversion symmetries.

cond-mat.supr-con↗

arXiv2023

Surface supercurrent diode effect

We propose a new type of supercurrent diode effect on the surface of a superconductor with surface states under in-plane magnetic fields. Surface supercurrent diode effect can lead to a perfect supercurrent diode in a considerably wide range of fields. For comparison, the conventional supercurrent diode effect due to the spin-orbit coupling in a two-dimensional superconductor cannot be perfect in usual cases. Candidates such as the (001) surface of iron-based superconductors BaFe$_{2-x}$Co$_x$As$_2$ are discussed. Calculations are performed under the Ginzburg-Landau formalism.

cond-mat.supr-con↗

arXiv2023

Anomalous open orbits in Hofstadter spectrum of Chern insulator

The nontrivial band topology can influence the Hofstadter spectrum. We investigate the Hofstadter spectrum for various models of Chern insulators under a rational flux $\frac{ϕ_{0}}{q}$, here $ϕ_{0}=\frac{h}{e}$ and $q$ being an integer. We find two major features. First, the number of splitting subbands is $|q-C|$ with Chern number $C$. Second, the anomalous open-orbit subbands with Chern numbers $q-1$ and $-q-1$ emerge, which are beyond the parameter window $(-q/2,q/2)$ of the Diophantine equation studied by Thouless-Kohmoto-Nightingale-den Nijs [Phys. Rev. Lett. \textbf{49}, 405 (1982)]. These two findings are explained by semiclassical dynamics. We propose that the number of splitting subbands can be utilized to determine Chern number in cold atom systems, and the open-orbit subbands can provide routes to study exotic features beyond the Landau level physics.

cond-mat.mes-hall↗

arXiv2022

Critical field measure for topological superconductivity

A promising direction for harnessing the laws of quantum mechanics to perform quantum computation is the topological quantum computation, for which topological superconductivity is one of the physical platforms. Intensive theoretical studies have been carried out and followed by tremendous experimental efforts in the realization of topological superconductivity, though debates remain mainly because of the inadequacy of the convincing detection techniques. Here we report a theoretical finding in a superconductor with surface or edge states, where the critical field is found to obey the unique power-law temperature dependence $B_{c3}\sim (T_c-T)^γ$ with $T_c$ being the onset critical temperature of superconductivity and the fractional exponent $γ= 2/3$, differing from the conventional values of $γ=1/2$ and $1$. The topological surface superconductivity is hence expected in the three-dimensional (3D) system and the topological edge superconductivity in the two-dimensional (2D) system. The application of this measure to the intrinsic topological superconductors FeTe$_{1-x}$Se$_{x}$ ($x\sim 0.5$) supports the validity of such an accessible, convenient, and reliable transport technique for the identification of topological superconductivity. The new form of theory on topological superconductivity together with the developed identification technique is expected to guide the search of topological superconductors in future.

cond-mat.supr-con↗