arXiv subjects

Y. -J. Sun

Publications and source records attributed to Y. -J. Sun.

8 recordsLinked to original sources

arXiv2025

Pomeranchuk instability in the nematic phase of two monolayer FeSe/SrTiO3

Nematicity, where electrons break rotational symmetry while preserving translational symmetry, is ubiquitous in strongly correlated quantum matters, including high-Tc cuprates and iron-based superconductors. A central question in nematicity is whether it is driven by Pomeranchuk instability in momentum space or orbital order (polarization) in real space, especially as nematicity intertwines with superconductivity. FeSe/SrTiO3 (STO), where nematicity occurs without long-range magnetic order, is an ideal platform for studying the nature and origin of the electronic nematicity. Here we present direct evidence of Pomeranchuk nematic order in two monolayer FeSe/STO using angle-resolved photoemission spectroscopy, revealing a remarkable degeneracy of dxz and dyz bands at the Brillouin zone center, but a significant band separation at the zone corner. This momentum-dependent nematicity demonstrates that nematicity in FeSe/STO originates from the Pomeranchuk instability, offering insights into the relationship between nematicity and superconductivity. Our results establish two-dimensional FeSe thin film as a powerful platform for investigating quantum physics under complex intertwinement.

cond-mat.str-el↗

arXiv2019

Experimental evidence of anomalously large superconducting gap on topological surface state of $β$-$\rm\mathbf{Bi_2Pd}$ film

Connate topological superconductor (TSC) combines topological surface states with nodeless superconductivity in a single material, achieving effective $p$-wave pairing without interface complication. By combining angle-resolved photoemission spectroscopy and $in$-$situ$ molecular beam epitaxy, we studied the momentum-resolved superconductivity in $β$-$\rm{Bi_2Pd}$ film. We found that the superconducting gap of topological surface state ($Δ_{TSS} \sim$ 3.8 meV) is anomalously enhanced from its bulk value ($Δ_b \sim$ 0.8 meV). The ratio of $2Δ_{TSS}/k_BT_c \sim14.2$, is substantially larger than the BCS value. By measuring $β$-$\rm{Bi_2Pd}$ bulk single crystal as a comparison, we clearly observed the upward-shift of chemical potential in the film. In addition, a concomitant increasing of surface weight on the topological surface state was revealed by our first principle calculation, suggesting that the Dirac-fermion-mediated parity mixing may cause this anomalous superconducting enhancement. Our results establish $β$-$\rm{Bi_2Pd}$ film as a unique case of connate TSCs with a highly enhanced topological superconducting gap, which may stabilize Majorana zero modes at a higher temperature.

cond-mat.supr-con↗

arXiv2019

Observation of topological transition in high-T$_{c}$ superconductor FeTe$_{1-x}$Se$_{x}$/SrTiO$_{3}$(001) monolayers

Superconductors with topological surface or edge states have been intensively explored for the prospect of realizing Majorana bound states, which obey non-Abelian statistics and are crucial for topological quantum computation. The traditional routes for making topological insulator/superconductor and semiconductor/superconductor heterostructures suffer fabrication difficulties and can only work at low temperature. Here, we use angle-resolved photoemission spectroscopy to directly observe the evolution of a topological transition of band structure nearby the Fermi level in two-dimensional high-T$_{c}$ superconductor FeTe$_{1-x}$Se$_{x}$/SrTiO$_{3}$(001) monolayers, fully consistent with our theoretical calculations. Furthermore, evidence of edge states is revealed by scanning tunneling spectroscopy with assistance of theoretical calculations. Our study provides a simple and tunable platform for realizing and manipulating Majorana states at high temperature.

cond-mat.supr-con↗

arXiv2019

New classes of chiral topological nodes with non-contractible surface Fermi arcs in CoSi

In condensed matter systems, chiral topological nodes are robust band crossing points in momentum space that carry nonzero Chern numbers. The chirality is manifested by the presence of surface Fermi arcs connecting the projections of nodes with opposite Chern numbers. In addition to the well-known Weyl nodes, theorists have proposed several other types of chiral topological nodes in condensed matter systems, but the direct experimental evidence of their existence is still lacking. Here, using angle-resolved photoemission spectroscopy, we reveal two types of new chiral nodes, namely the spin-1 nodes and charge-2 Dirac nodes, at the band crossing points near the Fermi level in CoSi, the projections of which on the (001) surface are connected by topologically protected surface Fermi arcs. As these chiral nodes in CoSi are enforced at the Brillouin zone (BZ) center and corner by the crystalline symmetries, the surface Fermi arcs connecting their projections form a non-contractible path traversing the entire (001) surface BZ, in sharp contrast to pairs of Weyl nodes with small separation. Our work marks the first experimental observation of chiral topological nodes beyond the Weyl nodes both in the bulk and on the surface in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO$_{3}$ (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides A$_{x}$Fe$_{2-y}$Se$_{2}$ and potassium doped FeSe, which also have a relatively high superconducting critical temperature ($T_c$), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing $T_c$. However, the fundamental reasons explaining this increase of $T_c$ remain unclear. Here we report a 15 K jump of $T_c$ accompanying a second Lifshitz transition, characterized by the emergence of an electron pocket at the Brillouin zone (BZ) centre, that is triggered by high electron doping following in-situ deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital-dependent with the $d_{xy}$ orbital playing a determining role in generating enhanced superconductivity in FeSe.

cond-mat.supr-con↗

arXiv2015

Observation of high-Tc superconductivity in rectangular FeSe/STO(110) monolayer

It is well known that superconductivity in Fe-based materials is favoured under tetragonal symmetry, whereas competing orders such as spin-density-wave (SDW) and nematic orders emerge or are reinforced upon breaking the fourfold (C4) symmetry. Accordingly, suppression of orthorhombicity below the superconducting transition temperature (Tc) is found in underdoped compounds. Epitaxial film growth on selected substrates allows the design of crystal specific lattice distortions. Here we show that despite the breakdown of the C4 symmetry induced by a 5% difference in the lattice parameters, monolayers of FeSe grown by molecular beam epitaxy (MBE) on the (110) surface of SrTiO3 (STO) substrates [FeSe/STO(110)] exhibit a large nearly isotropic superconducting (SC) gap of 16 meV closing around 60 K. Our results on this new interfacial material, similar to those obtained previously on FeSe/STO(001), contradict the common belief that the C4 symmetry is essential for reaching high Tc's in Fe-based superconductors.

cond-mat.supr-con↗

arXiv2013

Persistence of magnetic excitations in La2-xSrxCuO4 from the undoped insulator to the heavily overdoped non-superconducting metal

One of the most intensely studied scenarios of high-temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations [1]. Indeed, such excitations have been observed up to around optimal doping in the cuprates [2-7]. In the heavily overdoped regime neutron scattering measurements indicate that magnetic excitations have effectively disappeared [8-10], and this was argued to cause the demise of HTS with overdoping [1, 8, 10]. Here we use resonant inelastic x-ray scattering (RIXS), which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La2-xSrxCuO4 across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x = 0.40). For x = 0, well-defined magnon excitations are observed [11]. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations; other factor(s), such as the redistribution of spectral weight, must be considered.

cond-mat.supr-con↗

arXiv2012

Orbital Characters Determined from Fermi Surface Intensity Patterns using Angle-Resolved Photoemission Spectroscopy

In order to determine the orbital characters on the various Fermi surface pockets of the Fe-based superconductors Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and FeSe$_{0.45}$Te$_{0.55}$, we introduce a method to calculate photoemission matrix elements. We compare our simulations to experimental data obtained with various experimental configurations of beam orientation and light polarization. We show that the photoemission intensity patterns revealed from angle-resolved photoemission spectroscopy measurements of Fermi surface mappings and energy-momentum plots along high-symmetry lines exhibit asymmetries carrying precious information on the nature of the states probed, information that is destroyed after the data symmetrization process often performed in the analysis of angle-resolved photoemission spectroscopy data. Our simulations are consistent with Fermi surfaces originating mainly from the $d_{xy}$, $d_{xz}$ and $d_{yz}$ orbitals in these materials.

cond-mat.supr-con↗