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Topological insulators have been studied intensively over the last decades. Among these materials, three-dimensional (3D) zirconium pentatelluride (ZrTe$_5$) stands out as one of the most intriguing for both theoretical and experimental studies because of its diverse range of distinct topological phases. In this work, we employ density functional theory to study the electronic structure and quantum oscillations exhibited by various topological phases of 3D bulk ZrTe$_5$. We have discovered that by analyzing combined patterns in band structures, Fermi surfaces, and Shubnikov-de Haas (SdH) oscillations we can determine the corresponding topological phase without relying on the conventional calculation of topological invariants or boundary state contributions. This approach facilitates the identification of topological phases in ZrTe$_5$ directly from experimental quantum oscillation measurements. Using this method, we have analyzed the entire process of topological phase transition, revealing changes in the topology of the Fermi pockets and validating the shapes deduced from the experimental data for the topological phases.

The superconducting transition metal dichalcogenides 2H-NbSe$_2$ and 2H-NbS$_2$ are intensively studied on account of their unique electronic properties such as Ising superconductivity, found in multi- and monolayers, with upper critical fields beyond the Pauli limit. Even in bulk crystals, there are reports of multiband superconductivity and exotic states, such as the Fulde-Ferrell-Larkin-Ovchinnikov phase. In this work, we investigate the superconducting properties of 2H-NbS$_2$ through a detailed high-field mapping of the phase diagram by means of magnetotransport and magnetostriction experiments. We compare the phase diagram between bulk crystals and a 6~nm thick flake of 2H-NbS$_2$ and find a drastically enhanced Maki parameter in the flake, signifying a change of the relevant pair breaking mechanism from orbital to paramagnetic pair breaking, which we attribute to an effect of enhanced spin-orbit coupling., Comment: 17 pages, 3 figures

Two-dimensional electron systems (2DES) are promising for investigating correlated quantum phenomena. In particular, 2D oxides provide a platform that can host various quantum phases such as quantized Hall effect, superconductivity, or magnetism. The realization of such quantum phases in 2D oxides heavily relies on dedicated heterostructure growths. Here we show the integer quantum Hall effect achieved in chemical vapor deposition grown Bi2O2Se - a representative member of a more accessible oxide family. A single or few sub-band 2DES can be prepared in thin films of Bi2O2Se, where the film thickness acts as the sole design parameter and the sub-band occupation is determined by the electric field effect. This new oxide platform exhibits characteristic advantages in structural flexibility due to its layered nature, making it suitable for scalable growth. The unique small mass distinguishes Bi2O2Se from other high-mobility oxides, providing a new platform for exploring quantum Hall physics in 2D oxides.

The switching and optical properties of phase-change thin films are actively investigated for future smart optical devices. The possibility of having more than one stable state, the large optical contrast between phases, and the fast and reversible switching are some attractive properties driving the research interest. Optical devices based on phase change alloys are considered the frontier contenders for tunable photonics. The combination of vivid structural color formation, with partial amorphization/crystallization of phase change alloys, and the associated optical tunability could be integrated into an energy-efficient reflective display device with high pixel density. This work demonstrates a contrast formation due to relative height differences from isolated pixelated structures. A reflective heterostructure device consisting of a low-loss Sb2Se3 alloy on a gold substrate was produced. With a focused ion beam, a pixelated metasurface structure was produced. Moreover, the ability to create local height differences using an ion beam was employed to create a structural color combination mimicking traditional LED like RGB pixels. We believe our approach in creating metapixels on phase change thin film surfaces could open up research interest in phase change alloys and moving away from semi/static plasmonic systems into truly dynamic display devices.

The diverse and intriguing phenomena observed in twisted bilayer systems, such as graphene and transition metal dichalcogenides, prompted new questions about the emergent effects that they may host. However, the practical challenge of realizing these structures on a scale large enough for spectroscopic investigation, remains a significant hurdle, resulting in a scarcity of direct measurements of the electronic band structure of twisted transition metal dichalcogenide bilayers. Here we present a systematic nanoscale angle-resolved photoemission spectroscopy investigation of bulk, single layer, and twisted bilayer WS2 with a small twist angle of 4.4{\deg}. The experimental results are compared with theoretical calculations based on density functional theory along the high-symmetry directions {\Gamma}-K and {\Gamma}-M. Surprisingly, the electronic band structure measurements suggest a structural relaxation occurring at 4.4{\deg} twist angle, and formation of large, untwisted bilayer regions., Comment: 23 pages, 11 figures

In the past few years, phase-change materials have become increasingly important in nano-photonics and optoelectronics. The advantages of sizeable optical contrast between phases and the additional degree of freedom from phase switching have been the driving force. From multilevel reflectance to dynamic nanoprinting and structural colors, phase-change materials have achieved outstanding results with prospects for real-world applications. The local crystallization/amorphization of phase-change materials and the corresponding reflectance tunning by the crystallized/amorphized region size have potential applications for future dynamic display devices. Although the resolution is much higher than current display devices, the pixel sizes in those devices are limited by the locally switchable structure size. Here, we reduce the spot sizes further by using ion beams instead of laser beams and dramatically increase the pixel density, demonstrating the capability of having superior resolution. In addition, the power to sputter away materials can be utilized in creating nanostructures with relative height differences and local contrast. Our experiment focuses on one archetypal phase-change material, Sb$_2$Se$_3$, prepared by pulsed-laser deposition on a reflective gold substrate. We demonstrate that we can produce structural colors and achieve reflectance tunning by focused ion beam milling/sputtering of phase change materials at the nanoscale. Furthermore, we show that the local structuring of phase-change materials by focused ion beam can be used to produce high pixel density display devices with superior resolutions., Comment: nanophotonics, focused ion beam, nanoprinting, pulsed laser deposition, phase change materials

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.

Weakly disordered two-dimensional (2D) superconductors can host richer quantum phase transitions than their highly-disordered counterparts. This is due to the insertion of a metallic state in the transition between a superconductor and an insulator. Disorders were predicted to show profound influences on the boundaries surrounding this intermediate metallic phase, affecting the existence of a metallic ground state called Bose metal and the dynamic processes in a quantum Griffiths state. Here we present a study on quantum phase transitions of a clean 2D superconductor, MoS2, under a perpendicular magnetic field as a function of disorder strength that is tuned electrostatically. We found two universal scaling behaviors independent of disorders: a power-law scaling applies for all metallic states, and an activated dynamical scaling characterizes transitions between quantum Griffiths state and weakly localized metal. A study over a range of disorder states in this unexplored clear regime suggests that a Bose metal ground state is expected only after reaching a critical cleanness. Whereas, stronger disorder stabilizes a true 2D superconductivity and enhances the quantum Griffiths phase., Comment: 21 pages, 4 figures

Two-dimensional (2D) perovskite materials have recently re-attracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. In this report, the band-edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2x10^3cm/s and an intrinsic lifetime of 185ns. Biexciton emission is evidenced at room temperature, with binding energy of about 45meV and a lifetime of 80ps. At low temperature, exciton state splitting is observed, which is caused by the electron-hole exchange interaction. Transient photoluminescence resolves the low-lying dark exciton state, with a bright/dark splitting energy estimated to be 10meV. This work contributes to understand the complex scenario of the elementary photoexcitations in 2D perovskites., Comment: 15 Pages, 5 Figures and supplementary material

A ferroelectric tunnel barrier in between two ferromagnetic electrodes (multiferroic tunnel junction, MFTJ), is one of the most promising concepts for future microelectronic devices. In parallel, Hafnia based ferroelectrics are showing great potential for device miniaturization down to the nanoscale. Here we utilize ferroelectric Hf0.5Zr0.5O2 (HZO) with thickness of only 2 nm, epitaxially grown on La0.7Sr0.3MnO3 (LSMO) ferromagnetic electrodes, as a large band-gap insulating barrier integrated in MFTJs with cobalt top electrodes. As previously reported for other MFTJs with similar electrodes, the tunneling magnetoresistance (TMR) can be tuned and its sign can even be reversed by the bias voltage across the junction. We demonstrate four non-volatile resistance states generated by magnetic and electric field switching with high reproducibility in this system.

We fabricate ion-gated field-effect transistors (iFET) on mechanically exfoliated multilayer MoS$_2$. We encapsulate the flake by Al$_2$O$_3$, leaving the device channel exposed at the edges only. A stable Li$^+$ intercalation in the MoS$_2$ lattice is induced by gating the samples with a Li-based polymeric electrolyte above $\sim$ 330 K and the doping state is fixed by quenching the device to $\sim$ 300 K. This intercalation process induces the emergence of anomalies in the temperature dependence of the sheet resistance and its first derivative, which are typically associated with structural/electronic/magnetic phase transitions. We suggest that these anomalies in the resistivity of MoS$_2$ can be naturally interpreted as the signature of a transition to a charge-density-wave phase induced by lithiation, in accordance with recent theoretical calculations., Comment: 8 pages, 4 figures

We report modifications of the temperature-dependent transport properties of $\mathrm{MoS_2}$ thin flakes via field-driven ion intercalation in an electric double layer transistor. We find that intercalation with $\mathrm{Li^+}$ ions induces the onset of an inhomogeneous superconducting state. Intercalation with $\mathrm{K^+}$ leads instead to a disorder-induced incipient metal-to-insulator transition. These findings suggest that similar ionic species can provide access to different electronic phases in the same material., Comment: 5 pages, 3 figures

Recently emerging two-dimensional (2D) superconductors in atomically thin layers and at heterogeneous interfaces are attracting growing interest in condensed matter physics. Here, we report that ion-gated ZrNCl surface, exhibiting a dome-shaped phase diagram with a maximum critical temperature of 14.8 kelvin, behaves as a superconductor persisting to the 2D limit. The superconducting thickness estimated from the upper critical fields is ~ 1.8 nanometers, which is thinner than one-unit-cell. The majority of the vortex phase diagram down to 2 kelvin is occupied by a metallic state with a finite resistance, owing to the quantum creep of vortices caused by extremely weak pinning and diminishing disorder. Our findings highlight the potential of electric-field-induced superconductivity, establishing a new platform for accessing quantum phases in clean 2D superconductors., Comment: 33 pages, 10 figures

Symmetry-breaking has been known to play a key role in noncentrosymmetric superconductors with strong spin-orbit-interaction (SOI). The studies, however, have been so far mainly focused on a particular type of SOI, known as Rashba SOI, whereby the electron spin is locked to its momentum at a right-angle, thereby leading to an in-planar helical spin texture. Here we discuss electric-field-induced superconductivity in molybdenum disulphide (MoS2), which exhibits a fundamentally different type of intrinsic SOI manifested by an out-of-plane Zeeman-type spin polarization of energy valleys. We find an upper critical field of approximately 52 T at 1.5 K, which indicates an enhancement of the Pauli limit by a factor of four as compared to that in centrosymmetric conventional superconductors. Using realistic tight-binding calculations, we reveal that this unusual behaviour is due to an inter-valley pairing that is symmetrically protected by Zeeman-type spin-valley locking against external magnetic fields. Our study sheds a new light on the interplay of inversion asymmetry with SOI in confined geometries, and its unprecedented role in superconductivity., Comment: 37 pages, 11 figures, http://meetings.aps.org/Meeting/MAR15/Session/G11.11