arXiv subjects

Y. J. Yan

Publications and source records attributed to Y. J. Yan.

At least 19 recordsLinked to original sources

arXiv2021

Two-impurity Kondo effect in potassium doped single-layer p-sexiphenyl films

We show that a self-assembled phase of potassium (K) doped single-layer para-sexiphenyl (PSP) film on gold substrate is an excellent platform for studying the two-impurity Kondo model. On K-doped PSP molecules well separated from others, we find a Kondo resonance peak near EF with a Kondo temperature of about 30 K. The Kondo resonance peak splits when another K-doped PSP molecule is present in the vicinity, and the splitting gradually increases with the decreased inter-molecular distance, with no signs of phase transition. Our data demonstrate how a Kondo singlet state gradually evolves into an antiferromagnetic singlet state due to the competition between Kondo screening and antiferromagnetic RKKY coupling, as described in the two-impurity Kondo model. Intriguingly, the antiferromagnetic singlet is destroyed quickly upon increasing temperature and transforms back to a Kondo singlet well below the Kondo temperature. Our data provide a comprehensive picture and quantitative constraints on related theories and calculations of two-impurity Kondo model.

cond-mat.str-el↗

arXiv2018

Robust and clean Majorana zero mode in the vortex core of high-temperature superconductor (Li0.84Fe0.16)OHFeSe

The Majorana fermion, which is its own anti-particle and obeys non-abelian statistics, plays a critical role in topological quantum computing. It can be realized as a bound state at zero energy, called a Majorana zero mode (MZM), in the vortex core of a topological superconductor, or at the ends of a nanowire when both superconductivity and strong spin orbital coupling are present. A MZM can be detected as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. However, in practice, clean and robust MZMs have not been realized in the vortices of a superconductor, due to contamination from impurity states or other closely-packed Caroli-de Gennes-Matricon (CdGM) states, which hampers further manipulations of Majorana fermions. Here using scanning tunneling spectroscopy, we show that a ZBCP well separated from the other discrete CdGM states exists ubiquitously in the cores of free vortices in the defect free regions of (Li0.84Fe0.16)OHFeSe, which has a superconducting transition temperature of 42 K. Moreover, a Dirac-cone-type surface state is observed by angle-resolved photoemission spectroscopy, and its topological nature is confirmed by band calculations. The observed ZBCP can be naturally attributed to a MZM arising from this chiral topological surface states of a bulk superconductor. (Li0.84Fe0.16)OHFeSe thus provides an ideal platform for studying MZMs and topological quantum computing.

cond-mat.supr-con↗

arXiv2017

Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe

In the iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently it was found that in iron selenide (FeSe), enhanced superconductivity (Tc over 40K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. Here by utilizing surface potassium dosing, scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we studied the electronic structure and superconductivity of (Li0.8Fe0.2OH)FeSe in the deep electron-doped regime. We find that a Γ-centered electron band, which originally lies above the Fermi level (EF), can be continuously tuned to cross EF and contribute a new electron pocket at Γ. When this Lifshitz transition occurs, the superconductivity in the M-centered electron pocket is slightly suppressed; while a possible superconducting gap with small size (up to ~5 meV) and a dome-like doping dependence is observed on the new Γ electron pocket. Upon further K dosing, the system eventually evolves into an insulating state. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors.

cond-mat.supr-con↗

arXiv2017

Observation of novel gapped phases in potassium doped single layer p-terphenyl on Au (111)

Recently, superconductivity in potassium (K) doped p-terphenyl (C18H14) has been suggested by the possible observation of the Meissner effect and subsequent photoemission spectroscopy measurements, but the detailed lattice structure and more-direct evidence are still lacking. Here we report a low temperature scanning tunneling microscopy/spectroscopy (STM/STS) study on K-doped single layer p-terphenyl films grown on Au (111). We observe several ordered phases with different morphologies and electronic behaviors, in two of which a sharp and symmetric low-energy gap of about 11 meV opens below 50 K. In particular, the gap shows no obvious response to a magnetic field up to 11 Tesla, which would caution against superconductivity as an interpretation in previous reports of K-doped p-terphenyl materials. Such gapped phases are rarely (if ever) observed in single layer hydrocarbon molecular crystals. Our work also paves the way for fabricating doped two-dimensional (2D) hydrocarbon materials, which will provide a platform to search for novel emergent phenomena.

cond-mat.supr-con↗

arXiv2016

Possible Nodeless Superconducting Gaps in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ and YBa$_2$Cu$_3$O$_{7-x}$ Revealed by Cross-Sectional Scanning Tunneling Spectroscopy

Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/ spectroscopy, we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) and YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) along their $c$-axes on a scale shorter than the interlayer spacing. By tunneling into the (100) plane of a Bi2212 single crystal and a YBCO film, we observe both U-shaped tunneling spectra with extended flat zero-conductance bottoms, and V-shaped gap structures, in different regions of each sample. On the YBCO film, tunneling into a (110) surface only reveals a U-shaped gap without any zero-bias peak. Our analysis suggests that the U-shaped gap is likely a nodeless superconducting gap. The V-shaped gap has a very small amplitude, and is likely proximity-induced by regions having the larger U-shaped gap.

cond-mat.supr-con↗

arXiv2016

Surface electronic structure and evidence of plain s-wave superconductivity in (Li0.8Fe0.2)OHFeSe

(Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide superconductor with a Tc above 40 K, which is much higher than the Tc of bulk FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near EF is observed. Through quasi-particle interference (QPI) measurements, we clearly observed intra- and inter-pocket scatterings in between the electron pockets at the M point, as well as some evidence of scattering that connects gamma and M points. Upon applying magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an (s+-)wave pairing symmetry. Our experimental results suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.

cond-mat.supr-con↗

arXiv2016

Identification of prototypical Brinkman-Rice Mott physics in a class of iron chalcogenides superconductors

The 122$^{*}$ series of iron-chalcogenide superconductors, for example K$_x$Fe$_{2-y}$Se$_{2}$, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the 122$^{*}$ series of iron-chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of 122$^{*}$ iron-chalcogenides, including the isovalently `doped' K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$, Rb$_x$Fe$_{2-y}$Se$_{2-z}$Te$_z$ and (Tl,K)$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$. We found that the bandwidths of the low energy Fe \textit{3d} bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known 122$^{*}$-series iron chalcogenides can be understood {\it via} one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.

cond-mat.supr-con↗

arXiv2015

Signature of high temperature superconductivity in electron doped Sr2IrO4

Sr2IrO4 was predicted to be a high temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure and magnetic coupling constants. Here we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr2IrO4 with surface electron doping by depositing potassium (K) atoms. At the 0.5-0.7 monolayer (ML) K coverage, we observed a sharp, V-shaped gap with about 95% loss of density of state (DOS) at EFand visible coherence peaks. The gap magnitude is 25-30 meV for 0.5-0.6 ML K coverage and it closes around 50 K. These behaviors exhibit clear signature of superconductivity. Furthermore, we found that with increased electron doping, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogap-like state and possible superconducting state. Our data suggest possible high temperature superconductivity in electron doped Sr2IrO4, and its remarkable analogy to the cuprates.

cond-mat.supr-con↗

arXiv2015

Surface electronic structure and isotropic superconducting gap in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature ($T_c$) $\sim$ 40 K. We found that (Li$_{0.8}$Fe$_{0.2}$)OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe resembles that of Rb$_x$Fe$_{2-y}$Se$_2$ except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li$_{0.8}$Fe$_{0.2}$)OHFeSe and Rb$_x$Fe$_{2-y}$Se$_2$. Superconducting gap is clearly observed below $T_c$, with an isotropic distribution around the electron Fermi surface. Compared with $A_x$Fe$_{2-y}$Se$_2$ (\textit{A}=K, Rb, Cs, Tl/K), the higher $T_c$ in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the (Li$_{0.8}$Fe$_{0.2}$)OH layers.

cond-mat.supr-con↗

arXiv2015

Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by scanning tunneling microscopy

Single-layer FeSe film on SrTiO3(001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle interference (QPI) patterns reveal scatterings between and within the electron pockets, and put constraints on possible pairing symmetries. By introducing impurity atoms onto the sample, we show that the magnetic impurities (Cr, Mn) can locally suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K) cannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave paring symmetry whose order parameter has the same phase on all Fermi surface sections.

cond-mat.supr-con↗

arXiv2015

Scanning tunneling microscopy study of the possible topological surface states in BiTeCl

Recently, the non-centrosymmetric bismuth tellurohalides such as BiTeCl are being studied as possible candidates of topological insulators. While some photoemission studies showed that BiTeCl is an inversion asymmetric topological insulator, others showed that it is a normal semiconductor with Rashba splitting. Meanwhile, first-principle calculationsfailed to confirm the existence of topological surface states in BiTeCl so far. Therefore, the topological nature of BiTeCl requires further investigation. Here we report low temperature scanning tunneling microscopy study on the surface states of BiTeCl single crystals. On the tellurium-terminated surfaces with low defect density, strong evidences for topological surface states are found in the quasi-particle interference patterns generated by the scattering of these states, both in the anisotropy of the scattering vectors and the fast decay of the interference near step edges. Meanwhile, on samples with much higher defect densities, we observed surface states that behave differently. Our results help to resolve the current controversy on the topological nature of BiTeCl.

cond-mat.mes-hall↗

arXiv2014

Drastic electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ in the collapsed tetragonal phase

We report the electronic structure reconstruction of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$ = 0.1 and 0.15) in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ and the annealed CaFe$_2$As$_2$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface can still be observed in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$. Furthermore, we found at least three well separated electron-like bands around the zone corner in the CT phase of Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$, which are more dispersive than the electron-like bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.

cond-mat.supr-con↗

arXiv2014

Anomalous impurity effects in the iron-based superconductor KFe$_2$As$_2$

High-quality K(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals have been grown by using KAs flux method. Instead of increasing the superconducting transition temperature $T_{\rm c}$ through electron doping, we find that Co impurities rapidly suppress $T_{\rm c}$ down to zero at only $x \approx$ 0.04. Such an effective suppression of $T_{\rm c}$ by impurities is quite different from that observed in Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ with multiple nodeless superconducting gaps. Thermal conductivity measurements in zero field show that the residual linear term $κ_0/T$ only change slightly with $3.4\%$ Co doping, despite the sharp increase of scattering rate. The implications of these anomalous impurity effects are discussed.

cond-mat.supr-con↗

arXiv2013

Calorimetric study in single crystal of CsFe$_2$As$_2$

We measured resistivity and specific heat of high-quality CsFe$_2$As$_2$ single crystals, which were grown by using a self-flux method. The CsFe$_2$As$_2$ crystal shows sharp superconducting transition at 1.8 K with the transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. Analysis on the superconducting-state specific heat supports unconventional pairing symmetry in CsFe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2013

Power-law Temperature Dependent Hall Angle in the Normal State and its Correlation with Superconductivity in iron-pnictides

We report Hall measurement of the normal state in K- and Co-doped BaFe$_2$As$_2$, as well NaFe$_{1-x}$Co$_x$As. We found that a power-law temperature dependence of Hall angle, cot$θ_{\rm H}$$\propto$ $T^β$, prevails in normal state with temperature range well above the structural, spin-density-wave and superconducting transitions for the all samples with various doping levels. The power $β$ is nearly 4 for the parent compounds and the heavily underdoped samples, while around 3 for the superconducting samples. The $β$ suddenly changes from 4 to 3 at a doping level that is close to the emergence of superconductivity. It suggests that the $β$ of $\sim 3$ is clearly tied to the superconductivity. Our data suggest that, similar to cuprates, there exists a connection between the physics in the normal state and superconductivity of iron-pnictides. These findings shed light on the mechanism of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2012

A crossover in phase diagram of NaFe$_{1-x}$Co$_x$As determined by electronic transport measurements

We report electronic transport measurements on single crystals of NaFe$_{1-x}$Co$_x$As system. We found that the cotangent of Hall angle, cot$θ_{\rm H}$, follows $T^4$ for the parent compound with filamentary superconductivity and $T^2$ for the heavily-overdoped non-superconducting sample. While it exhibits approximately $T^3$-dependence in all the superconducting samples, suggesting this behaivor is associated with bulk superconductivity in ferropnictides. A deviation develops below a characteristic temperature $T^*$ well above the structural and superconducting transitions, accompanied by a departure from power-law temperature dependence in resistivity. The doping dependence of $T^*$ resembles the crossover line of pseudogap phase in cuprates.

cond-mat.supr-con↗

arXiv2012

Physical properties in hole-doped SrFe$_{2-x}$Cu$_x$As$_2$ single crystals

We report the structural, magnetic and electronic transport properties of SrFe$_{2-x}$Cu$_x$As$_2$ single crystals grown by self-flux technique. SrCu$_2$As$_2$ and SrFe$_2$As$_2$ both crystallize in ThCr$_2$Si$_2$-type (122-type) structure at room temperature, but exhibit distinct magnetic and electronic transport properties. The x-ray photoelectron spectroscopy(XPS) Cu-2p core line position, resistivity, susceptibility and positive Hall coefficient indicate that SrCu$_2$As$_2$ is an sp-band metal with Cu in the 3d$^{10}$ electronic configuration corresponding to the valence state Cu$^{1+}$. The almost unchanged Cu-2p core line position in SrFe$_{2-x}$Cu$_x$As$_2$ compared with SrCu$_2$As$_2$ indicates that partial Cu substitutions for Fe in SrFe$_2$As$_2$ may result in hole doping rather than the expected electron doping. No superconductivity is induced by Cu substitution on Fe sites, even though the structural/spin density wave(SDW) transition is gradually suppressed with increasing Cu doping.

cond-mat.str-el↗

arXiv2012

Pressure effects on superconducting properties of single-crystalline Co doped NaFeAs

Resistivity and magnetic susceptibility measurements under external pressure were performed on single-crystals NaFe1-xCoxAs (x=0, 0.01, 0.028, 0.075, 0.109). The maximum Tc enhanced by pressure in both underdoped and optimally doped NaFe1-xCoxAs is the same, as high as 31 K. The overdoped sample with x = 0.075 also shows a positive pressure effect on Tc, and an enhancement of Tc by 13 K is achieved under pressure of 2.3 GPa. All the superconducting samples show large positive pressure coefficient on superconductivity, being different from Ba(Fe1-xCox)2As2. However, the superconductivity cannot be induced by pressure in heavily overdoped non-superconducting NaFe0.891Co0.109As. These results provide evidence for that the electronic structure is much different between superconducting and heavily overdoped non-superconducting NaFe1-xCoxAs, being consistent with the observation by angle-resolved photoemission spectroscopy.

cond-mat.supr-con↗