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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

We report the observation for the pz electron band and the band inversion in Fe1+yTexSe1-x with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y>0) inhibits the topological band inversion in Fe1+yTexSe1-x, which explains the absence of Majorana zero modes in previous reports for Fe1+yTexSe1-x with excess Fe. Basing 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 able to tune the topological transition via applying lattice strain or carrier doping.

The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-temperature superconductor, FeTe1 – xSex (refs. 1–11). This superconductor undergoes both a topological transition3,4, 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 peaks12,13. 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 FeTe1 – xSex 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. Our experiments reveal a tendency of FeTe1 – xSex, 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. The interplay of superconductivity and nematicity of electrons remains unclear in a wide range of materials. Now, more evidence emerges that nematic fluctuations can be pinned into a static phase by disorder, which hinders the superconductivity.

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 FeTe0.55Se0.45. 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., Braiding Majorana modes is essential for topological quantum computing, but it remains difficult to find a suitable platform. Here, the authors report the evidence of hybridization between field-induced and magnetic adatom induced Majorana modes in an iron-based superconductor FeTe0.55Se0.45, providing a possible single-material platform for braiding Majorana modes.

The single-crystal growth, crystal structure, heat capacity, and anisotropic magnetization characterization of Rb3Yb(PO4)2, a Yb-based triangular lattice material, are presented. Single-crystal X-r...

The A15 structure of superconductors is a prototypical type-II superconductor that has generated considerable interest since the early history of superconducting materials. This paper discusses the superconducting properties of previously unreported V3−x Co x Si (0 ⩽ x ⩽ 0.30) alloys. It is found that the lattice parameter decreases with increasing cobalt-doped content and leads to an decreased residual resistivity ratio value of the V3−x Co x Si system. Meanwhile, the superconducting transition temperature (Tc ) also decreases with increasing cobalt-doped content. Furthermore, the fitted data show that the increase of cobalt-doped content also reduces the lower/upper critical fields of the V3−x Co x Si system. Type-II superconductivity is demonstrated in all V3−x Co x Si samples. With higher Co-doped content, V3−x Co x Si (0.15 ⩽ x ⩽ 0.30) alloys may have superconducting and structural phase transitions in low-temperature regions. As the electron/atom ratio increases, the Tc variation trend of V3Si is as pronounced as in crystalline alloys and monotonically follows the trend observed for amorphous superconductors.

Searching for Kitaev quantum spin liquids (QSLs) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in $5d$-electron iridates and $4d$-electron ${\mathrm{RuCl}}_{3}$. Recently, theoretical studies suggested that $3{d}^{7}$ Co-based honeycomb materials with a high spin state $S=3/2$ and effective orbital angular momentum $L=1$ could also be promising candidates of a Kitaev QSL. One of the candidates, ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, was revisited recently. The long-range magnetic order in ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ can be suppressed by a very weak in-plane magnetic field, suggesting its proximity to a Kitaev QSL. Here, we perform time-domain terahertz spectroscopy measurements to study the magnetic excitations on ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long-range magnetic order is suppressed by a weak field ${H}_{c2}$. The spectra beyond ${H}_{c2}$ are dominated by single-magnon and two-magnon excitations. Therefore, in the measured low-temperature region, the field-induced state is not a Kitaev QSL. We also compared the excitation spectra of ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ with that of the widely studied $4d$-electron Kitaev candidate ${\mathrm{RuCl}}_{3}$ and addressed their similarities in magnetic excitations.

A previously unreported 1D iridate, K3Ir2O6, has been grown by a flux method in O2-rich environment, and its crystal structure was determined via single crystal structural analysis. It exhibits straight chains of face-sharing [IrO6] octahedra, which are arranged along the crystallographic c axis, separated by nonmagnetic K ions. No magnetic transitions are observed during measured range, and the material is electrically insulating. Potentially interesting electronic behavior for K3Ir2O6 is supported by electronic structure calculations. A structurally related material, K16.3Ir8O30, which displays similar fundamental geometric units but in a different spatial arrangement-zigzag chains-based on edge and face sharing [IrO6] octahedra, is also reported. Both materials are of interest for probing the properties of a 1D system with strong spin-orbit coupling.

Establishing the presence and the nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, in the cuprates they remain undetected until now. Here using symmetry-resolved electronic Raman scattering in the cuprate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta}$$\end{document}Bi2Sr2CaCu2O8+δ, we report the observation of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase. While our data hint at the possible presence of an incipient nematic quantum critical point, the doping dependence of the nematic fluctuations deviates significantly from a canonical quantum critical scenario. The observed nematic instability rather appears to be tied to the presence of a van Hove singularity in the band structure., Solid evidence of quantum fluctuations associated to a quantum critical point in cuprate superconductors remains elusive. Here, Auvray et al. report Raman scattering evidence of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase in Bi\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2Sr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2CaCu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{8+\delta }$$\end{document}8+δ.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin–Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductors such as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions. Transport and scanning tunnelling microscopy studies of freestanding monolayers of an unconventional layered copper oxide establish that the superconducting properties of copper oxides are not changed in the 2D limit.

Vortices in topological superconductors may host Majorana zero modes (MZMs), which have been proposed as the building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform with the potential for realizing MZMs has been discovered in iron-based superconductors, without involving hybrid semiconductor–superconductor structures. Here, we report a detailed scanning tunnelling spectroscopy study of a FeTe0.55Se0.45 single crystal and show that this material hosts two distinct classes of vortex. These differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied to the presence or absence of a zero-bias conductance peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound state, suggesting the presence of regions with and without topological surface states, which coexist within the same crystal. Our findings provide strong evidence for the presence of MZMs in FeTe0.55Se0.45 and establish it as an excellent platform for further studies. The authors use STM to show that there are two different classes of zero-bias peak in vortex cores of Fe(Te,Se). One class is topological, one not. These are distinguished by a shift in the energy levels of the excited states.

Symmetry, dimensionality, and interaction are crucial ingredients for phase transitions and quantum states of matter. As a prominent example, the integer quantum Hall effect (QHE) represents a topological phase generally regarded as characteristic for two-dimensional (2D) electronic systems, and its many aspects can be understood without invoking electron-electron interaction. The intriguing possibility of generalizing QHE to three-dimensional (3D) systems was proposed decades ago, yet it remains elusive experimentally. Here, we report clear experimental evidence for the 3D QHE observed in bulk ZrTe5 crystals. Owing to the extremely high sample quality, the extreme quantum limit with only the lowest Landau level occupied can be achieved by an applied magnetic field as low as 1.5 T. Remarkably, in this regime, we observe a dissipationless longitudinal resistivity rho_xx=0 accompanied with a well-developed Hall resistivity plateau rho_xy=(1\pm0.1) h/e^2 (\lambda_(F,z)/2), where \lambda_(F,z) is the Fermi wavelength along the field direction (z axis). This striking result strongly suggests a Fermi surface instability driven by the enhanced interaction effects in the extreme quantum limit. In addition, with further increasing magnetic field, both rho_xx and rho_xy increase dramatically and display an interesting metal-insulator transition, representing another magnetic field driven quantum phase transition. Our findings not only unambiguously reveal a novel quantum state of matter resulting from an intricate interplay among dimensionality, interaction, and symmetry breaking, but also provide a promising platform for further exploration of more exotic quantum phases and transitions in 3D systems.

The superconducting state is achieved by the condensation of Cooper pairs and is protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the superconducting gap function. Thus, it is very pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the superconducting gap may have a d-wave function {\Delta} = {\Delta}_0cos2{\theta}. This gap function has an alternative sign change by every pi/2 in the momentum space when the in-plane azimuthal angle theta is scanned. It is very hard to visualize this sign change. Early experiments for recommending or proving this d-wave gap function were accomplished by the specially designed phase sensitive measurements based on the Josephson effect. Here we report the measurements of scanning tunneling spectroscopy in one of the model cuprate system Bi2Sr2CaCu2O8+{\delta} and conduct the analysis of phase-referenced quasiparticle interference (QPI). Due to the unique quasiparticle excitations in the superconducting state of cuprate, we have seen the seven basic scattering vectors that connect each pair of the terminals of the banana-shaped contour of constant quasiparticle energy (CCE). The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate a very effective way for determining the sign change of unconventional superconductors., Comment: 36 pages, 10 figures

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{j}}_{\text{eff}}=\frac{1}{2}$ moments in the triangular-lattice magnet ${\mathrm{Na}}_{2}\mathrm{Ba}\mathrm{Co}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{j}}_{\text{eff}}=\frac{1}{2}$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as an additional frustrating agent, altogether putting forward ${\mathrm{Na}}_{2}\mathrm{Ba}\mathrm{Co}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$ as a promising Kitaev spin-liquid material.

Understanding the rich and competing electronic orders in cuprate superconductors may provide important insight into the mechanism of high-temperature superconductivity. Here, by measuring Bi2Sr2CaCu2O8+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 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.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions. Using time-domain terahertz spectroscopy, we observed a broad magnetic continuum over a wide range of temperatures and fields in the honeycomb cobalt-based magnet BaCo

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates., Comment: 6 pages, 3 figures; Supplementary Materials available upon request

In strongly correlated systems the strength of Coulomb interactions between electrons, relative to their kinetic energy, plays a central role in determining their emergent quantum mechanical phases. We perform resonant x-ray scattering on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, a prototypical cuprate superconductor, to probe electronic correlations within the CuO$_2$ plane. We discover a dynamic quasi-circular pattern in the $x$-$y$ scattering plane with a radius that matches the wave vector magnitude of the well-known static charge order. Along with doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along $x$ and $y$ can dynamically rotate into any other in-plane direction. This quasi-circular spectrum, a hallmark of Brazovskii-type fluctuations, has immediate consequences to our understanding of rotational and translational symmetry breaking in the cuprates. We discuss how the combination of short- and long-range Coulomb interactions results in an effective non-monotonic potential that may determine the quasi-circular pattern., Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Communications. The final authenticated version is available online at: https://doi.org/10.1038/s41467-020-20824-7. Supplementary materials are available through the published version in Nature Communications

The realization of quantum spin liquids on a triangular lattice magnet remains challenging due to quenched disorders present in real materials. Here, we report local-probe signatures of spin-liquid-like excitations in the structurally perfect triangular antiferromagnet ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$. Our zero-field and longitudinal-field muon spin relaxation measurements reveal both spatially and temporally correlated spins with no hint of static magnetism down to 80 mK, consistent with a quantum spin liquid. Comprehensive $^{23}\mathrm{Na}$ nuclear magnetic resonance experiments enable us to identify a field-induced crossover of magnetic excitations from a spinonlike to a gapped magnon at the critical field ${\ensuremath{\mu}}_{0}{H}_{C}=1.65$ T. Our findings uncover the temporal, thermal, and magnetic-field characteristics of the magnetic excitations in the triangular-lattice quantum spin liquid candidate ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$.

Magnetic susceptibility, magnetization, and specific-heat measurements under both in-plane and out-of-plane field on ${\mathrm{K}}_{2}{\mathrm{Co}}_{2}{(\mathrm{Se}{\mathrm{O}}_{3})}_{3}$ single crystals are reported. The Co dimers in this compound, which are made from face-sharing effective spin-1/2 $\mathrm{Co}{\mathrm{O}}_{6}$ 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. The magnetic fluctuations present due to geometric frustration are more resistant to an in-plane field than a perpendicular-to-plane field.

Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. Here, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor FeTe0.55Se0.45 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 FeTe0.55Se0.45., 18 pages, 6 figures, supplementary Material included

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 fusion 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 Majorana zero mode induced by magnetic Fe adatoms deposited on the surface and the realization of its fusion with a nearby field-induced Majorana vortex in iron-based superconductor FeTe0.55Se0.45. 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.

A study of the layered triangular lattice materials ${\mathrm{K}}_{2}\mathrm{Co}{({\mathrm{SeO}}_{3})}_{2}$ and ${\mathrm{Rb}}_{2}\mathrm{Co}{({\mathrm{SeO}}_{3})}_{2}$ is reported. These isostructural compounds crystallize in the trigonal space group $R\ensuremath{-}3m$ (No. 166). The magnetic $\mathrm{C}{\mathrm{o}}^{2+}$ ions form a spin-1/2 triangular lattice and display strongly frustrated magnetism. We find no evidence in the susceptibility or heat capacity for in-plane ordering of the spins; rather the in-plane inverse susceptibility is typical of that seen for frustrated magnets. At around 10 K, a ferromagnetic component appears in the magnetic susceptibility perpendicular to the triangular planes. The heat capacity at low applied fields does not show sharp features, but in larger applied fields a sharp ferromagneticlike transition appears. Field- and direction-dependent magnetization measurements at 2 K indicate that both materials have a magnetic easy axis perpendicular to the triangular planes. The magnetic properties are sensitive to the field applied perpendicular to the triangular planes, which induces a metamagnetization transition at 0.75 T. Spin polarization develops along with the increasing out-of-plane field, resulting in a saturated ferromagnetic state at high fields. In contrast, the magnetic properties are resistant to the in-plane field. Our results suggest that these materials are embodiments of Ising spins on triangular lattices with easy-axis anisotropy.

We observe proximity-induced superconductivity in in situ prepared heterostructures constructed by topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films and high-temperature cuprate superconductors ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. The superconducting gap maximum is about 7.6 meV on the surface of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. This gap form is consistent with the ${\mathrm{\ensuremath{\Delta}}}_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information on the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

PbS2 and Pb0.9Ag0.1S2 were prepared through the reaction of PbS with excess sulfur (and silver) at 4 GPa and 600 °C. PbS2 crystallizes in the CuAl2 structure type, (I4/mcm (#140), a = 6.1106(5) A, c = 7.4949(6) A, Z = 4), determined by single crystal X-ray diffraction. Its structure consists of layers of [S2]2- dimers and Pb2+ in square antiprismatic coordination – a rare structure type for metal dichalcogenides. Electronic structure calculations suggest that the material should be an indirect band gap semiconductor. Consistent with the calculations, the temperature-dependent resistivity of undoped PbS2 is that of a degenerate semiconductor, while the Ag-doped material shows metallic temperature dependent resistivity down to 2 K. The materials are both diamagnetic. For the Ag-doped material, the Seebeck coefficient is small and shows nearly linear temperature-dependent behavior from 50 to 250 K.

We realize superconductor–insulator transitions (SIT) in mechanically exfoliated Bi2Sr2CaCu2O8+δ (BSCCO) flakes and address simultaneously their transport properties as well as the evolution of density of states. Back-gating via the solid ion conductor (SIC) engenders a SIT in BSCCO due to the modulation of carrier density by intercalated lithium ions. Scaling analysis indicates that the SIT follows the theoretical description of a two-dimensional quantum phase transition (2D-QPT). We further carry out tunneling spectroscopy in graphite(G)/BSCCO heterojunctions. We observe V-shaped gaps in the critical regime of the SIT. The density of states in BSCCO gets symmetrically suppressed by further going into the insulating regime. Our technique of combining solid state gating with tunneling spectroscopy can be easily applied to the study of other two-dimensional materials.

A central mystery in high temperature superconductivity is the origin of the so-called "strange metal," i.e., the anomalous conductor from which superconductivity emerges at low temperature. Measuring the dynamic charge response of the copper-oxides, $\chi''(q,\omega)$, would directly reveal the collective properties of the strange metal, but it has never been possible to measure this quantity with meV resolution. Here, we present the first measurement of $\chi''(q,\omega)$ for a cuprate, optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ ($T_c=91$ K), using momentum-resolved inelastic electron scattering. In the medium energy range 0.1-2 eV relevant to the strange metal, the spectra are dominated by a featureless, temperature- and momentum-independent continuum persisting to the eV energy scale. This continuum displays a simple power law form, exhibiting $q^2$ behavior at low energy and $q^2/\omega^2$ behavior at high energy. Measurements of an overdoped crystal ($T_c=50$ K) showed the emergence of a gap-like feature at low temperature, indicating deviation from power law form outside the strange metal regime. Our study suggests the strange metal exhibits a new type of charge dynamics in which excitations are local to such a degree that space and time axes are decoupled., Comment: 21 pages, 8 figures

Identifying what broken symmetries are present in the cuprates has become a major area of research. Many authors have reported evidence for so-called "$Q \sim 0$" 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 $Q \sim 0$ 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 Cu $L_3$ resonance for temperatures $T_c < T < T^\ast$, 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 $Q \sim 0$ broken symmetry state in Bi-2212 at intermediate temperatures, and calls for further study using a microfocused beam that could disentangle microscopic effects from macroscopic heterogeneities., 9 pages, 4 figures

The evidence for proximity-induced superconductivity in heterostructures of topological insulators and high-Tc cuprates has been intensely debated. We use molecular-beam epitaxy to grow thin films of topological insulator Bi2Te3 on a cuprate Bi2Sr2CaCu2O8+x, and study the surface of Bi2Te3 using low-temperature scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi2Te3 films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV spectra. By reducing the coverage of Bi2Te3 films to create nanoscale islands, we discover that this spectral feature dramatically evolves into a much larger hard gap, which can be understood as a Coulomb blockade gap. This conclusion is supported by the evolution of dI/dV spectra with the lateral size of Bi2Te3 islands, as well as by topographic measurements that show an additional barrier separating Bi2Te3 and Bi2Sr2CaCu2O8+x. We conclude that the prominent gap-like feature in dI/dV spectra in Bi2Te3 films is not a proximity-induced superconducting gap. Instead, it can be explained by Coulomb blockade effects, which take into account additional resistive and capacitive coupling at the interface. Our experiments provide a fresh insight into the tunneling measurements of complex heterostructures with buried interfaces.

International audience; Zirconium pentatetelluride, ZrTe 5 , shows remarkable sensitivity to hydrostatic pressure. In this work we address the high-pressure transport and optical properties of this compound, on samples grown by flux and chemical vapor transport. The high-pressure resistivity is measured up to 2 GPa, and the infrared transmission up to 9 GPa. The dc conductivity anisotropy is determined using a microstructured sample. Together, the transport and optical measurements allow us to discern band parameters with and without the hydrostatic pressure, in particular the Fermi level, and the effective mass in the less conducting, out-of-plane direction. The results are interpreted within a simple two-band model characterized by a Dirac-type, linear in-plane band dispersion, and a parabolic out-of-plane dispersion.

Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states, in which no long-range magnetic ordering appears at low temperatures due to strong quantum fluctuations of the magnetic moments. The existing QSL candidates all have their intrinsic disadvantages, however, and solid evidence for quantum fluctuations is scarce. Here, we report a previously unreported compound, N a 2 B a C o ( P O 4 ) 2 , a geometrically frustrated system with effective spin-1/2 local moments for Co 2+ ions on an isotropic 2-dimensional (2D) triangular lattice. Magnetic susceptibility and neutron scattering experiments show no magnetic ordering down to 0.05 K. Thermodynamic measurements show that there is a tremendous amount of magnetic entropy present below 1 K in 0-applied magnetic field. The presence of localized low-energy spin fluctuations is revealed by inelastic neutron measurements. At low applied fields, these spin excitations are confined to low energy and contribute to the anomalously large specific heat. In larger applied fields, the system reverts to normal behavior as evident by both neutron and thermodynamic results. Our experimental characterization thus reveals that this material is an excellent candidate for the experimental realization of a QSL state.