Comparative Raman scattering study of crystal field excitations in Co-based quantum magnets
Not provided.
Materials Science↗
Engineering topics
Zhong, Ruidan
Publications and source records attributed to Zhong, Ruidan.
Time-reversal symmetry breaking superconductivity between twisted cuprate superconductors
Twisted interfaces between stacked van der Waals (vdW) cuprate crystals present a platform for engineering superconducting order parameters by adjusting stacking angles. Using a cryogenic assembly technique, we construct twisted vdW Josephson junctions (JJs) at atomically sharp interfaces between Bi 2 Sr 2 CaCu 2 O 8+x crystals, with quality approaching the limit set by intrinsic JJs. Near 45° twist angle, we observe fractional Shapiro steps and Fraunhofer patterns, consistent with the existence of two degenerate Josephson ground states related by time-reversal symmetry (TRS). By programming the JJ current bias sequence, we controllably break TRS to place the JJ into either of the two ground states, realizing reversible Josephson diodes without external magnetic fields. Furthermore, our results open a path to engineering topological devices at higher temperatures.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Low-energy quasi-circular electron correlations with charge order wavelength in Bi 2 Sr 2 CaCu 2 O 8+δ
Most resonant inelastic x-ray scattering (RIXS) studies of dynamic charge order correlations in the cuprates have focused on the high-symmetry directions of the copper oxide plane. However, scattering along other in-plane directions should not be ignored as it may help understand, for example, the origin of charge order correlations or the isotropic scattering resulting in strange metal behavior. Our RIXS experiments reveal dynamic charge correlations over the qx-qy scattering plane in underdoped Bi 2 Sr 2 CaCu 2 O 8+δ . Tracking the softening of the RIXS-measured bond-stretching phonon, we show that these dynamic correlations exist at energies below approximately 70 meV and are centered around a quasi-circular manifold in the q x -q y scattering plane with radius equal to the magnitude of the charge order wave vector, q CO . This phonon-tracking procedure also allows us to rule out fluctuations of short-range directional charge order (i.e., centered around [q x = ±q CO , q y = 0] and [q x = 0, q y = ±q CO ]) as the origin of the observed correlations.
36 MATERIALS SCIENCE↗
Geometrical frustration versus Kitaev interactions in BaCo 2 (AsO 4 ) 2
Recently, Co-based honeycomb magnets have been proposed as promising candidate materials to host the Kitaev spin liquid (KSL) state. One of the front-runners is BaCo 2 (AsO 4 ) 2 (BCAO), where it was suggested that the exchange processes between Co 2+ ions via the surrounding edge-sharing oxygen octahedra could give rise to bond-dependent Kitaev interactions. In this work, we present and analyze a comprehensive inelastic neutron scattering (INS) study of BCAO with fields in the honeycomb plane. Combining the constraints from the magnon excitations in the high-field polarized state and the inelastic spin structure factor measured in zero magnetic field, we examine two leading theoretical models: the Kitaev-type JK Γ Γ ′ model and the XXZ - J 1 - J 3 model. We show that the existing experimental data can be consistently accounted for by the XXZ - J 1 - J 3 model but not by the JK Γ Γ ′ model, and we discuss the implications of these results for the realization of a spin liquid phase in BCAO and more generally for the realization of the Kitaev model in cobaltates.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Fe 1+ y Te x Se1- x : A Delicate and Tunable Majorana Material
Here, we report the observation for the p z electron band and the band inversion in Fe 1 + y Te x Se 1–x with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y > 0) inhibits the topological band inversion in Fe 1 + y Te x Se 1 – x , which explains the absence of Majorana zero modes in previous reports for Fe 1+y Te x Se 1 – x with excess Fe. Based on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may be able to tune the topological transition via applying lattice strain or carrier doping.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in the Fe-based superconductor Fe 0.98 Te 0.5 Se 0.5
Here, we have performed inelastic neutron scattering measurements on optimally doped Fe 0.98 Te 0.5 Se 0.5 and 10% Cu-doped Fe 0.88 Cu 0.1 Te 0.5 Se 0.5 to investigate the substitution effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations (≤100 meV), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we find that the 10% Cu doping enlarges the effective fluctuating moment from 2.85 to 3.13 μB/Fe, although there is no long- or short-range magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced magnetic excitations in the 10% Cu doped sample which is in the insulating state indicates that the magnetic excitations must have some contributions from the local moments, reflecting the dual nature of the magnetism in iron-based superconductors. We attribute the substitution effects to the localization of the itinerant electrons induced by Cu dopants. Furthermore, these results indicate that the Cu doping does not act as electron donor as in a rigid-band shift model, but more as scattering centers that localize the system.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Hydrostatic pressure effect on the Co-based honeycomb magnet BaCo 2 (AsO 4 ) 2
Here the honeycomb antiferromagnet BaCo 2 (AsO 4 ) 2 , in which small in-plane magnetic fields (H1 ≈ 0.26T and H2 ≈ 0.52T at T = 1.8 K < T N ≈ 5.4K) induce two magnetic phase transitions, has attracted attention as a possible candidate material for the realization of Kitaev physics based on the 3d element Co 2+ . Here, we report on the change in the transition temperature T N and the critical fields H1 and H2 of BaCo 2 (AsO 4 ) 2 with hydrostatic pressure up to ~20 kbar, as determined from magnetization and specific heat measurements. Within this pressure range, a marginal increase in the magnetic ordering temperature is observed. At the same time, the critical fields are changed significantly (up to ~25 –35%). Specifically, we find that H1 is increased with hydrostatic pressure, i.e., the antiferromagnetic state is stabilized, whereas H2, which was previously associated with a transition into a proposed Kitaev spin-liquid state, decreases with increasing pressure. We discuss to what extent these results are compatible with suggested models with sizable third-nearest-neighbor exchange. Overall, the results put constraints on the magnetic models that are used to describe the low-temperature magnetic properties of BaCo 2 (AsO 4 ) 2 .
2-dimensional systems↗
Little-Parks like oscillations in lightly doped cuprate superconductors
Understanding the rich and competing electronic orders in cuprate superconductors may provide important insight into the mechanism of high-temperature superconductivity. Here, by measuring Bi 2 Sr 2 CaCu 2 O 8+x in the extremely underdoped regime, we obtain evidence for a distinct type of ordering, which manifests itself as resistance oscillations at low magnetic fields (≤10 T) and at temperatures around the superconducting transition. By tuning the doping level p continuously, we reveal that these low-field oscillations occur only when p < 0.1. The oscillation amplitude increases with decreasing >p but the oscillation period stays almost constant. We show that these low-field oscillations can be well described by assuming a periodic superconducting structure with a mesh size of about 50 nm. Such a charge order, which is distinctly different from the well-established charge density wave and pair density wave, seems to be an unexpected piece of the puzzle on the correlated physics in cuprates.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Search for Q ~0 Order near a Forbidden Bragg Position in Bi 2.1 Sr 1.9 CaCu 2 O 8+x with Resonant Soft X-ray Scattering
Identifying what broken symmetries are present in the cuprates has become a major area of research. Many authors have reported evidence for so-called “Q ~ 0” order that involves broken inversion, mirror, chiral, or time-reversal symmetry that is uniform in space. Not all these observations are well understood and new experimental probes are needed. Here we use resonant soft x-ray scattering (RSXS) to search for Q ~ 0 order in Bi 2.1 Sr 1.9 CaCu 2 O 8+x (Bi-2212) by measuring the region of a forbidden Bragg peak, (0,0,3), which is normally extinguished by symmetry but may become allowed on resonance if valence band order is present. Using circularly polarized light, we found that this reflection becomes allowed on the CuL 3 resonance for temperatures T c < T < T*, though remains absent in linear polarization and at other temperatures. This observation suggests the existence of spatially uniform valence band order near the pseudogap temperature. In addition, we observed periodic oscillations in the specular reflectivity from the sample surface that resemble thin film interference fringes, though no known film is present. These fringes are highly resonant, appear in all polarizations, and exhibit a period that depends on the location where the beam strikes the sample surface. We speculate that these fringes arise from interaction between some intrinsic valence band instability and extrinsic structural surface morphologies of the material. Our study supports the existence of some kind of Q ~ 0 broken symmetry state in Bi-2212 at intermediate temperatures. Further studies using a microfocused beam, which could disentangle microscopic effects from macroscopic heterogeneities, are needed to ascertain the universality of the effect.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Surface orbital order and chemical potential inhomogeneity of the iron-based superconductor Fe Te 0.55 Se 0.45 investigated with special STM tips
Not Available
Wang, Dongfei (ORCID:0000000346452881)↗
Presence of s -Wave Pairing in Josephson Junctions Made of Twisted Ultrathin Bi 2 Sr 2 CaCu 2 O 8 + x Flakes
Not Available
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Nematic transition and nanoscale suppression of superconductivity in Fe(Te,Se)
The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-temperature superconductor, FeTe 1 – x Se x . This superconductor undergoes both a topological transition, linked to the electronic band inversion, and an electronic nematic phase transition, associated with rotation symmetry breaking, around the same Se composition where the superconducting transition temperature peaks. In this regime, nematic fluctuations and symmetry-breaking strain could be important, but this is yet to be fully explored. Using spectroscopic-imaging scanning tunnelling microscopy, we study the electronic nematic transition in FeTe 1 – x Se x as a function of composition. Near the critical Se composition, we find electronic nematicity in nanoscale regions. The superconducting coherence peaks are suppressed in areas where static nematic order is the strongest. By analysing atomic displacement in scanning tunnelling microscopy topographs, we find that small anisotropic strain can give rise to these strongly nematic localized regions. Furthermore, our experiments reveal a tendency of FeTe 1 – x Se x , near x ≈ 0.45, to form puddles hosting static nematic order, suggestive of nematic fluctuations pinned by structural inhomogeneity, and demonstrate the effect of anisotropic strain on superconductivity in this regime.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Observation of magnetic adatom-induced Majorana vortex and its hybridization with field-induced Majorana vortex in an iron-based superconductor
Braiding Majorana zero modes is essential for fault-tolerant topological quantum computing. Iron-based superconductors with nontrivial band topology have recently emerged as a surprisingly promising platform for creating distinct Majorana zero modes in magnetic vortices in a single material and at relatively high temperatures. The magnetic field-induced Abrikosov vortex lattice makes it difficult to braid a set of Majorana zero modes or to study the coupling of a Majorana doublet due to overlapping wave functions. Here we report the observation of the proposed quantum anomalous vortex with integer quantized vortex core states and the Majorana zero mode induced by magnetic Fe adatoms deposited on the surface. We observe its hybridization with a nearby field-induced Majorana vortex in iron-based superconductor FeTe 0.55 Se 0.45 . We also observe vortex-free Yu-Shiba-Rusinov bound states at the Fe adatoms with a weaker coupling to the substrate, and discover a reversible transition between Yu-Shiba-Rusinov states and Majorana zero mode by manipulating the exchange coupling strength. The dual origin of the Majorana zero modes, from magnetic adatoms and external magnetic field, provides a new single-material platform for studying their interactions and braiding in superconductors bearing topological band structures.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spin-Polarized Yu-Shiba-Rusinov States in an Iron-Based Superconductor
Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. In this study, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor FeTe 0.55 Se 0.45 using a spin-polarized scanning tunneling microscope. We clearly identify the spin signature of pairs of YSR bound states at finite energies within the superconducting gap having opposite spin polarization as theoretically predicted. In addition, we also observe zero-energy bound states for some of the adsorbed Fe atoms. In this case, a spin signature is found to be absent indicating the absence of Majorana bound states associated with Fe adatoms on FeTe 0.55 Se 0.45 .
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Frustrated spin-1/2 dimer compound K 2 Co 2 ( Se O 3 ) 3 with easy-axis anisotropy
Magnetic susceptibility, magnetization, and specific-heat measurements under both in-plane and out-of-plane field on K 2 Co 2 (SeO 3 ) 3 single crystals are reported. The Co dimers in this compound, which are made from face-sharing effective spin-1/2 CoO 6 octahedra, sit on the corners of a geometrically frustrating triangular planar lattice. No long-range magnetic ordering is found down to 0.35 K, which, together with the large negative Curie-Weiss temperature obtained from fitting the temperature-dependent magnetic susceptibility, indicates that the system is magnetically frustrated. Analysis of the field and orientation dependence of the magnetic and thermodynamic properties suggests the presence of an easy-axis anisotropy. Furthermore, the magnetic fluctuations present due to geometric frustration are more resistant to an in-plane field than a perpendicular-to-plane field.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗