arXiv subjects

Can-Li Song

Publications and source records attributed to Can-Li Song.

At least 55 records · Page 3Linked to original sources

arXiv2018

Surface symmetry breaking and disorder effects on superconductivity in perovskite BaBi3 epitaxial films

The structural or electronic symmetry breaking of the host lattice is a recurrent phenomenon in many quantum materials, including superconductors. Yet, how these broken symmetry states affect the electronic pair wave function of superconductivity have been rarely elucidated. Here, using low-temperature scanning tunneling microscopy and first-principles calculations, we identify the broken rotational symmetry via stripe ordering on the (001) surface of perovskite BaBi3 films grown by molecular beam epitaxy, and show that it consequently leads to anisotropic superconductivity with twofold symmetry. In contrast, the structural disorder smears out the anisotropy of electron pairing and fills superconducting subgap density of states as the film thickness is reduced. A quasi-long range model of superconducting fluctuations is revealed to describe the tunneling conductance spectra of thin BaBi3 films well, and to exemplify how disorders contribute to the low-energy quasiparticle excitations in superconductors. Our findings help understand the effects of symmetry breaking states and disorders on superconductivity, particularly the existing tunneling conductance spectra there.

cond-mat.supr-con↗

arXiv2018

Atomic Visualization of Copper Oxide Structure in Infinite-Layer Cuprate SrCuO2

We report on atomic-scale visualization of the structure of infinite-layer cuprate SrCuO2 thin films grown on Nb-doped SrTiO3 substrates by molecular beam epitaxy. In-situ scanning tunneling microscopy study reveals stoichiometric copper oxide (CuO2) plane with a 2 x 2 surface reconstruction, prompted by preferential clustering of four adjacent CuO2 plaquettes. By imaging the subsurface Sr atoms, intra-unit-cell rotational symmetry breaking is observed, which, together with the adjacent CuO2 clustering, can be well accounted for by a periodic up-down buckling of oxygen ions on the CuO2 plane. Further post-annealing leads to an incommensurate stripe structure of the surface layer. Our findings provide important structural information for deeply understanding the electronic structure of superconducting CuO2 plane as well as high temperature superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2018

Anisotropic superconductivity and elongated vortices with unusual bound states in quasi-one-dimensional nickel-bismuth compounds

We report low temperature scanning tunneling microscopy and spectroscopy studies of Ni-Bi films grown by molecular beam epitaxy. Highly anisotropic and twofold symmetric superconducting gaps are revealed in two distinct composites, Bi-rich NiBi3 and near-equimolar NixBi, both sharing quasi-one-dimensional crystal structure. We further reveal axially elongated vortices in both phases, but Caroli-de Gennes-Matricon states solely within the vortex cores of NiBi3. Intriguingly, although the localized bound state splits energetically off at a finite distance ~10 nm away from a vortex center along the minor axis of elliptic vortex, no splitting is found along the major axis. We attribute the elongated vortices and unusual vortex behaviors to the combined effects of twofold superconducting gap and Fermi velocity. The findings provide a comprehensive understanding of the electron pairing and vortex matter in quasi-one-dimensional superconductors

cond-mat.supr-con↗

arXiv2018

Observation of Interface Superconductivity in a SnSe2-Epitaxial Graphene van der Waals Heterostructure

We report on the direct observation of interface superconductivity in single-unit-cell SnSe2 films grown on graphitized SiC(0001) substrate by means of van der Waals epitaxy. Tunneling spectrum in the superconducting state reveals rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the presence of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of SnSe2 and graphene. Our finding opens up novel strategies to hunt for and understand interface superconductivity based on van der Waals heterostructures.

cond-mat.supr-con↗

arXiv2018

Observation of tunneling gap in epitaxial ultrathin films of pyrite-type copper disulfide

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide. Layer by layer growth of CuS2 films with a preferential orientation of (111) on SrTiO3(001) and Bi2Sr2CaCu2O8+δ substrates is achieved by molecular beam epitaxy growth. For ultrathin films on both kinds of substrates, we observed symmetric tunneling gap around Fermi level that persists up to ~ 15 K. The tunneling gap degrades with either increasing temperature or increasing thickness, suggesting new matter states at the extreme two dimensional limit.

cond-mat.supr-con↗

arXiv2017

An extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on SrTiO3

Determination of the pairing symmetry in monolayer FeSe films on SrTiO3 is a requisite for understanding the high superconducting transition temperature in this system, which has attracted intense theoretical and experimental studies but remains controversial. Here, by introducing several types of point defects in FeSe monolayer films, we conduct a systematic investigation on the impurity-induced electronic states by spatially resolved scanning tunneling spectroscopy. Ranging from surface adsorption, chemical substitution to intrinsic structural modification, these defects generate a variety of scattering strength, which renders new insights on the pairing symmetry.

cond-mat.supr-con↗

arXiv2017

Experimental Observation of Topological Superconductivity and Majorana Zero Modes on beta-Bi2Pd Thin Films

Using a cryogenic scanning tunneling microscopy, we report the observation of topologically nontrivial superconductivity on a single material of β-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. Majorana zero modes (MZMs), supported by such superconducting states, are identified at magnetic vortices. The superconductivity and MZMs exhibit resistance to nonmagnetic defects, characteristic of time-reversal-invariant topological superconductors. Our results demonstrate a simple platform to generate, manipulate and braid MZMs for quantum computation.

cond-mat.supr-con↗

arXiv2016

Electronic structure of the ingredient planes of cuprate superconductor Bi2Sr2CuO6+δ: a comparison study with Bi2Sr2CaCu2O8+δ

By means of low-temperature scanning tunneling microscopy, we report on the electronic structures of BiO and SrO planes of Bi2Sr2CuO6+δ (Bi-2201) superconductor prepared by argon-ion bombardment and annealing. Depending on post annealing conditions, the BiO planes exhibit either pseudogap (PG) with sharp coherence peaks and an anomalously large gap of 49 meV or van Hove singularity (VHS) near the Fermi level, while the SrO is always characteristic of a PG-like feature. This contrasts with Bi2Sr2CaCu2O8+δ (Bi-2212) superconductor where VHS occurs solely on the SrO plane. We disclose the interstitial oxygen dopants (δ in the formulas) as a primary cause for the occurrence of VHS, which are located dominantly around the BiO and SrO planes, respectively, in Bi-2201 and Bi-2212. This is supported by the contrasting structural buckling amplitude of BiO and SrO planes in the two superconductors. Our findings provide solid evidence for the irrelevance of PG to the superconductivity in the two superconductors, as well as insights into why Bi-2212 can achieve a higher superconducting transition temperature than Bi-2201, and by implication, the mechanism of cuprate superconductivity.

cond-mat.supr-con↗

arXiv2016

High-temperature superconductivity in single-unit-cell FeSe films on anatase TiO2(001)

We report on the observation of high-temperature ($T_\textrm{c}$) 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-$T_\textrm{c}$ 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-$T_\textrm{c}$ superconductivity in FeSe-related heterostructures.

cond-mat.supr-con↗

arXiv2016

Nodeless pairing in superconducting copper-oxide monolayer films on Bi2Sr2CaCu2O8+δ

The pairing mechanism of high-temperature superconductivity in cuprates remains the biggest unresolved mystery in condensed matter physics. To solve the problem, one of the most effective approaches is to investigate directly the superconducting CuO2 layers. Here, by growing CuO2 monolayer films on Bi2Sr2CaCu2O8+δ substrates, we identify two distinct and spatially separated energy gaps centered at the Fermi energy, a smaller U-like gap and a larger V-like gap on the films, and study their interactions with alien atoms by low-temperature scanning tunneling microscopy. The newly discovered U-like gap exhibits strong phase coherence and is immune to scattering by K, Cs and Ag atoms, suggesting its nature as a nodeless superconducting gap in the CuO2 layers, whereas the V-like gap agrees with the well-known pseudogap state in the underdoped regime. Our results support an s-wave superconductivity in Bi2Sr2CaCu2O8+δ, which, we propose, originates from the modulation-doping resultant two-dimensional hole liquid confined in the CuO2 layers.

cond-mat.supr-con↗

arXiv2016

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-$T_c$ 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 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 $\sqrt{5}$$\times$$\sqrt{5}$ superstructure in FeSe and related alkali-doped compounds.

cond-mat.supr-con↗

arXiv2016

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 ultra-thin films grown on graphitized SiC(0001). The superconductivity exhibits hypersensitivity to K dosage in the lower-Tc dome, whereas in the heavily electron-doped higher-Tc 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-Tc superconductivity only in the topmost FeSe layer. The unusual finding of a double-dome superconducting phase has stepped towards the mechanistic understanding of superconductivity in FeSe-derived superconductors.

cond-mat.supr-con↗

arXiv2016

Visualizing the elongated vortices in $γ$-Ga nanostrips

We study the magnetic response of superconducting $γ$-Ga via low temperature scanning tunneling microscopy and spectroscopy. The magnetic vortex cores rely substantially on the Ga geometry, and exhibit an unexpectedly-large axial elongation with aspect ratio up to 40 in rectangular Ga nano-strips (width $l$ $<$ 100 nm). This is in stark contrast with the isotropic circular vortex core in a larger round-shaped Ga island. We suggest that the unusual elongated vortices in Ga nanostrips originate from geometric confinement effect probably via the strong repulsive interaction between the vortices and Meissner screening currents at the sample edge. Our finding provides novel conceptual insights into the geometrical confinement effect on magnetic vortices and forms the basis for the technological applications of superconductors.

cond-mat.supr-con↗

arXiv2016

Bounds on Nanoscale Nematicity in Single-Layer FeSe/SrTiO$_3$

We use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer FeSe/SrTiO$_3$. We develop a $T$-matrix model of multi-orbital QPI to disentangle scattering intensities from Fe $3d_{xz}$ and $3d_{yz}$ bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer FeSe/SrTiO$_3$ film, we quantitatively exclude static $xz/yz$ orbital ordering with domain size larger than $δr^2$ = 20 nm $\times$ 20 nm, $xz/yz$ Fermi wave vector difference larger than $δk$ = 0.014 $π$, and energy splitting larger than $δE$ = 3.5 meV. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicity.

cond-mat.supr-con↗

arXiv2015

Atomically Resolved FeSe/SrTiO3(001) Interface Structure by Scanning Transmission Electron Microscopy

Interface-enhanced high-temperature superconductivity in one unit-cell (UC) FeSe films on SrTiO3(001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. By combined in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and ex-situ scanning transmission electron microscopy (STEM) studies, we report on atomically resolved structure including both lattice constants and actual atomic positions of the FeSe/STO interface under both non-superconducting and superconducting states. We observed TiO2 double layers (DLs) and significant atomic displacements in the top two layers of STO, lattice compression of the Se-Fe-Se triple layer, and relative shift between bottom Se and topmost Ti atoms. By imaging the interface structures under various superconducting states, we unveil a close correlation between interface structure and superconductivity. Our atomic-scale identification of FeSe/STO interface structure provides useful information on investigating the pairing mechanism of this interface-enhanced high-temperature superconducting system.

cond-mat.supr-con↗

arXiv2015

Mapping the Electronic Structure of Each Ingredient Oxide Layer of High-$T_\textrm{c}$ Cuprate Superconductor Bi2Sr2CaCu2O8+δ

Understanding the mechanism of high transition temperature (Tc) superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy, we report on layer-by-layer probing of the electronic structures of all ingredient planes (BiO, SrO, CuO2) of Bi2Sr2CaCu2O8+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 CuO2 planes are exclusively characterized by a smaller gap inside the PG. The small gap becomes invisible near Tc, which we identify as the superconducting gap. The above results constitute severe constraints on any microscopic model for high Tc superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2015

Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO$_{3}$

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO$_3$(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 suggest that the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.

cond-mat.supr-con↗

arXiv2015

Probing Dirac Fermion Dynamics in Topological Insulator Bi$_2$Se$_3$ Films with Scanning Tunneling Microscope

Scanning tunneling microscopy and spectroscopy have been used to investigate the femtosecond dynamics of Dirac fermions in the topological insulator Bi$_2$Se$_3$ ultrathin films. At two-dimensional limit, bulk electrons becomes quantized and the quantization can be controlled by film thickness at 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 phase relaxation length $l_ϕ$ and inelastic scattering lifetime $τ$ of topological surface-state electrons. We find that $τ$ exhibits a remarkable $(E-E_F)^{-2}$ energy dependence and increases with film thickness. We show that the features revealed are typical for electron-electron scattering between surface and bulk states.

cond-mat.mes-hall↗