Quantum spin driven Yu-Shiba-Rusinov multiplets and fermion-parity-preserving phase transition in K3C60.

Magnetic impurities in superconductors are of increasing interest due to emergent Yu-Shiba-Rusinov (YSR) states and Majorana zero modes for fault-tolerant quantum computation. However, a direct relationship between the YSR multiple states and magnetic anisotropy splitting of quantum impurity spins remains poorly characterized. By using scanning tunneling microscopy, we systematically resolve individual transition-metal (Fe, Cr, and Ni) impurities induced YSR multiplets as well as their Zeeman effects in the K3C60 superconductor. The YSR multiplets show identical d orbital-like wave functions that are symmetry-mismatched to the threefold K3C60(1 1 1) host surface, breaking point-group symmetries of the spatial distribution of YSR bound states in real space. Remarkably, we identify an unprecedented fermion-parity-preserving quantum phase transition between ground states with opposite signs of the uniaxial magnetic anisotropy that can be manipulated by an external magnetic field. These findings can be readily understood in terms of anisotropy splitting of quantum impurity spins, and thus elucidate the intricate interplay between the magnetic anisotropy and YSR multiplets.

Chiral Charge Density Wave and Backscattering-Immune Orbital Texture in Monolayer 1T-TiTe2.

Nontrivial electronic states are attracting intense attention in low-dimensional physics. Though chirality has been identified in charge states with a scalar order parameter, its intertwining with charge density waves (CDW), film thickness, and the impact on the electronic behaviors remain less well understood. Here, using scanning tunneling microscopy, we report a 2 × 2 chiral CDW as well as a strong suppression of the Te-5p hole-band backscattering in monolayer 1T-TiTe2. These exotic characters vanish in bilayer TiTe2 in a non-CDW state. Theoretical calculations prove that chirality comes from a helical stacking of the triple-q CDW components and, therefore, can persist at the two-dimensional limit. Furthermore, the chirality renders the Te-5p bands with an unconventional orbital texture that prohibits electron backscattering. Our study establishes TiTe2 as a promising playground for manipulating the chiral ground states at the monolayer limit and provides a novel path to engineer electronic properties from an orbital degree.

Critical nematic correlations throughout the superconducting doping range in Bi2-zPbzSr2-yLayCuO6+x.

Charge modulations have been widely observed in cuprates, suggesting their centrality for understanding the high-Tc superconductivity in these materials. However, the dimensionality of these modulations remains controversial, including whether their wavevector is unidirectional or bidirectional, and also whether they extend seamlessly from the surface of the material into the bulk. Material disorder presents severe challenges to understanding the charge modulations through bulk scattering techniques. We use a local technique, scanning tunneling microscopy, to image the static charge modulations on Bi2-zPbzSr2-yLayCuO6+x. The ratio of the phase correlation length ξCDW to the orientation correlation length ξorient points to unidirectional charge modulations. By computing new critical exponents at free surfaces including that of the pair connectivity correlation function, we show that these locally 1D charge modulations are actually a bulk effect resulting from classical 3D criticality of the random field Ising model throughout the entire superconducting doping range.

Direct observation of nodeless superconductivity and phonon modes in electron-doped copper oxide Sr1-xNdxCuO2.

The microscopic understanding of high-temperature superconductivity in cuprates has been hindered by the apparent complexity of crystal structures in these materials. We used scanning tunneling microscopy and spectroscopy to study the electron-doped copper oxide compound Sr1- x Nd x CuO2, which has only bare cations separating the CuO2 planes and thus the simplest infinite-layer structure of all cuprate superconductors. Tunneling conductance spectra of the major CuO2 planes in the superconducting state revealed direct evidence for a nodeless pairing gap, regardless of variation of its magnitude with the local doping of trivalent neodymium. Furthermore, three distinct bosonic modes are observed as multiple peak-dip-hump features outside the superconducting gaps and their respective energies depend little on the spatially varying gaps. As well as the bosonic modes, with energies identical to those of the external, bending and stretching phonons of copper oxides, our findings reveal the origin of the bosonic modes in lattice vibrations rather than spin excitations.

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Semiconductor-Metal Phase Transition and Emergent Charge Density Waves in 1T-ZrX2 (X = Se, Te) at the Two-Dimensional Limit.

A charge density wave (CDW) is a collective quantum phenomenon in metals and features a wavelike modulation of the conduction electron density. A microscopic understanding and experimental control of this many-body electronic state in atomically thin materials remain hot topics in materials physics. By means of material engineering, we realized a dimensionality and Zr intercalation induced semiconductor-metal phase transition in 1T-ZrX2 (X = Se, Te) ultrathin films, accompanied by a commensurate 2 × 2 CDW order. Furthermore, we observed a CDW energy gap of up to 22 meV around the Fermi level. Fourier-transformed scanning tunneling microscopy and angle-resolved photoemission spectroscopy reveal that 1T-ZrX2 films exhibit the simplest Fermi surface among the known CDW materials in TMDCs, consisting only of a Zr 4d derived elliptical electron conduction band at the corners of the Brillouin zone.

Direct Visualization of Ambipolar Mott Transition in Cuprate CuO_{2} Planes.

Identifying the essence of doped Mott insulators is one of the major outstanding problems in condensed matter physics and the key to understanding the high-temperature superconductivity in cuprates. We report real space visualization of Mott insulator-metal transition in Sr_{1-x}La_{x}CuO_{2+y} cuprate films that cover both the electron- and hole-doped regimes. Tunneling conductance measurements directly on the copper-oxide (CuO_{2}) planes reveal a systematic shift in the Fermi level, while the fundamental Mott-Hubbard band structure remains unchanged. This is further demonstrated by exploring the atomic-scale electronic response of CuO_{2} to substitutional dopants and intrinsic defects in a sister compound Sr_{0.92}Nd_{0.08}CuO_{2}. The results may be better explained in the framework of self-modulation doping, similar to that in semiconductor heterostructures, and form a basis for developing any microscopic theories for cuprate superconductivity.

Direct Observation of Full-Gap Superconductivity and Pseudogap in Two-Dimensional Fullerides.

Alkali-fulleride superconductors with a maximum critical temperature T_{c}∼40 K exhibit a similar electronic phase diagram to that of unconventional high-T_{c} superconductors. Here we employ cryogenic scanning tunneling microscopy to show that trilayer K_{3}C_{60} displays fully gapped strong coupling s-wave superconductivity, accompanied by a pseudogap above T_{c}∼22 K and within vortices. A precise control of the electronic correlations and potassium doping enables us to reveal that superconductivity occurs near a superconductor-Mott-insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of the superconductivity below half-filling, indicates that alkali fullerides are predominantly phonon-mediated superconductors, although the electronic correlations also come into play.

Interface enhanced superconductivity in monolayer FeSe films on MgO(001): charge transfer with atomic substitution.

Interface enhanced superconductivity over 50 K has been discovered in monolayer FeSe films grown on several TiO2-terminated oxide substrates. Whether such phenomenon exists in other oxide substrates remains an extremely interesting topic. Here we report enhanced superconductivity with an onset transition temperature of 18 K in monolayer FeSe on MgO(001) substrate by transport measurement. Scanning transmission electron microscopy investigation on the interface structure indicates that FeSe films grow epitaxially on MgO(001) and that overlayer Fe atoms diffuse into the top two layers of MgO and substitute Mg atoms. Our density functional theory calculations reveal that this substitution promotes the charge transfer from the MgO substrate to the FeSe films, an essential process that also occurs in monolayer FeSe on TiO2-terminated oxides and contributes to the enhanced superconductivity therein. Our finding suggests that superconductivity enhancement in monolayer FeSe films on oxides substrates is rather general as long as charge transfer is allowed at the interface, thus pointing out an explicit direction for searching for new high temperature superconductivity by interface engineering.

Experimental signature of topological superconductivity and Majorana zero modes on ß-Bi2Pd thin films.

The search for Majorana fermions in topological superconductors is one of paramount research targets in physics today. Using a cryogenic scanning tunneling microscopy, we here report the signature of topologically nontrivial superconductivity on a single material of ß-Bi2Pd films grown by molecular beam epitaxy. The superconducting gap associated with spinless odd-parity pairing opens on the surface and appears much larger than the bulk one due to the Dirac-fermion enhanced parity mixing of surface pair potential. Zero bias conductance peaks, probably from Majorana zero modes supported by such superconducting states, are identified at magnetic vortices. The superconductivity exhibits resistance to nonmagnetic defects, characteristic of time-reversal-invariant topological superconductors. Our study reveals ß-Bi2Pd as a prime platform to generate, manipulate and braid Majorana zero modes for quantum computation.

High-Temperature Superconductivity in Single-Unit-Cell FeSe Films on Anatase TiO_{2}(001).

We report on the observation of high-temperature (T_{c}) superconductivity and magnetic vortices in single-unit-cell FeSe films on anatase TiO_{2}(001) substrate by using scanning tunneling microscopy. A systematic study and engineering of interfacial properties has clarified the essential roles of substrate in realizing the high-T_{c} superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. By visualizing and tuning the oxygen vacancies at the interface, we find their very limited effect on the superconductivity, which excludes interfacial oxygen vacancies as the primary source for charge transfer between the substrate and FeSe films. Our findings have placed severe constraints on any microscopic model for the high-T_{c} superconductivity in FeSe-related heterostructures.

Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation.

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-Tc Fe-SC, exhibits prevalent defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is a prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a √5 × âˆš5 superstructure in FeSe and related alkali-doped compounds.

Observation of Double-Dome Superconductivity in Potassium-Doped FeSe Thin Films.

We report on the emergence of two disconnected superconducting domes in alkali-metal potassium- (K-)doped FeSe ultrathin films grown on graphitized SiC(0001). The superconductivity exhibits hypersensitivity to K dosage in the lower-T_{c} dome, whereas in the heavily electron-doped higher-T_{c} dome it becomes spatially homogeneous and robust against disorder, supportive of a conventional Cooper-pairing mechanism. Furthermore, the heavily K-doped multilayer FeSe films all reveal a large superconducting gap of ∼14 meV, irrespective of film thickness, verifying the higher-T_{c} superconductivity only in the topmost FeSe layer. The unusual finding of a double-dome superconducting phase is a step towards the mechanistic understanding of superconductivity in FeSe-derived superconductors.

Mapping the Electronic Structure of Each Ingredient Oxide Layer of High-T\{c} Cuprate Superconductor Bi{2}Sr{2}CaCu{2}O{8+δ}.

Understanding the mechanism of high transition temperature (T{c}) superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy (STM), we report on layer-by-layer probing of the electronic structures of all ingredient planes (BiO, SrO, CuO{2}) of Bi{2}Sr{2}CaCu_2}O{8+δ} superconductor prepared by argon-ion bombardment and annealing technique. We show that the well-known pseudogap (PG) feature observed by STM is inherently a property of the BiO planes and thus irrelevant directly to Cooper pairing. The SrO planes exhibit an unexpected van Hove singularity near the Fermi level, while the CuO{2} planes are exclusively characterized by a smaller gap inside the PG. The small gap becomes invisible near T{c}, which we identify as the superconducting gap. The above results constitute severe constraints on any microscopic model for high T{c} superconductivity in cuprates.

Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO3.

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO3(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a Γ-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at Γ and explain how the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.

Probing Dirac fermion dynamics in topological insulator Bi2Se3 films with a scanning tunneling microscope.

Scanning tunneling microscopy and spectroscopy have been used to investigate the femtosecond dynamics of Dirac fermions in the topological insulator Bi2Se3 ultrathin films. At the two-dimensional limit, bulk electrons become quantized and the quantization can be controlled by the film thickness at a single quintuple layer level. By studying the spatial decay of standing waves (quasiparticle interference patterns) off steps, we measure directly the energy and film thickness dependence of the phase relaxation length lϕ and inelastic scattering lifetime τ of topological surface-state electrons. We find that τ exhibits a remarkable (E - EF)(-2) energy dependence and increases with film thickness. We show that the features revealed are typical for electron-electron scattering between surface and bulk states.

Detection of a superconducting phase in a two-atom layer of hexagonal Ga film grown on semiconducting GaN(0001).

The recent observation of the superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide-band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature Tc as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy and ex situ electrical magnetotransport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductors, with great potential for understanding the interface superconductivity.

Molecular beam epitaxy growth and post-growth annealing of FeSe films on SrTiO3: a scanning tunneling microscopy study.

Low temperature scanning tunneling microscopy and spectroscopy are used to investigate the atomic and electronic structure evolution of FeSe films grown on SrTiO3 as a function of post-growth annealing. Single unit cell FeSe films are found to bond strongly with the underlying substrate, and become superconductive with diminishing chemical bond disorders at the interface via post-annealing. For thicker FeSe films, post-annealing removes excess Se in the films and leads to a transition from semiconductor into metallic behaviors. In double and multilayer films, strain-induced complex textures are observed and suggested to be the main cause for the absent superconductivity.

Imaging the electron-boson coupling in superconducting FeSe films using a scanning tunneling microscope.

Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films. The mode appears below Tc as a "dip-hump" feature at energy Ω∼4.7kBTc beyond the superconducting gap Δ. Spectra on strained regions of the FeSe films reveal simultaneous decreases in Δ and Ω. This contrasts with all previous reports on other high-Tc superconductors, where Δ locally anticorrelates with Ω. A local strong coupling model is found to reconcile the discrepancy well, and to provide a unified picture of the electron-boson coupling in unconventional superconductors.