arXiv subjects

Ilya Eremin

Publications and source records attributed to Ilya Eremin.

At least 19 recordsLinked to original sources

arXiv2026

Bose condensation and Bogoliubov excitation in resonator-embedded superconducting qubit network

Superconducting qubit networks (SQNs) embedded in a low-dissipative resonator is a promising device allowing one not only to establish the collective quantum dynamics on a macroscopic scale but also to greatly enhance the sensitivity of detectors of microwave photons. A quantum ac Stark effect provided by coupling between an SQN and microwave photons of a resonator, leads to a strong nonlinear interaction between photons. Here, we present a two-tone spectroscopy experiment in which a set of 10 superconducting flux qubits is coupled to the input R- resonator and the output T- transmission line. An external microwave pump field close to the resonance frequency populates macroscopically the resonator mode as a Bose-Einstein condensate, while a second probe beam scans the resonances referred also as Bogoliubov-like excitations. The corresponding excitation frequency measured from the transmission coefficient, |S21(f)| displays an abrupt change of the resonant dip position once the power of the pump field overcomes a critical value Pcr. This sharp shift occurs in a narrow region of pump frequencies, and can be tuned by an applied magnetic field. It is a signature of bistability of the photon number inside the resonator, in agreement with theory.

quant-ph↗

arXiv2025

Unprecedentedly large gap in HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ with the highest $T_c$ at ambient pressure

In cuprate superconductors, the highest $T_c$ is possessed by the HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg-1223) system at ambient pressure, but the reason remains elusive. Here we report the scanning tunneling measurements on the Hg-1223 single crystals with $T_c$ = 134 K. The observed gaps determined from the tunneling spectra (STS) can be categorized into two groups: the smaller gap $Δ_1$ ranges from about 45 to 70 meV, while the larger gap $Δ_2$ from about 65 to 98 meV. The STS was measured up to 200 K and the larger gap can persist well above $T_c$, indicating a pseudogap feature which may reflect the strong pairing energy in the inner layer. Interestingly, an extremely strong particle-hole asymmetry is observed in associating with a very robust coherence-like peak at the bias of the larger gap in the hole branch of the Bogoliubov dispersion. We argue that the observed asymmetry results may be from the interplay of a flat band (van Hove singularity) in the electronic spectrum and the large gap in the underdoped (inner) layer. A theoretical approach based on a trilayer model with an interlayer coupling can give a reasonable explanation. Our results provide deep insight into understanding the mechanism of superconductivity in cuprate superconductors.

cond-mat.supr-con↗

arXiv2023

Revisiting the metal-to-metal transition in $2H$-AgNiO$_2$

The layered delafossite compound AgNiO$_2$ with $2H$ stacking symmetry undergoes a structural metal-to-metal transition at $T_{\rm S}\sim 365$ K. It has been described in the past as a charge-ordering transition, where local $S=1$ spins are formed on part of the Ni sites. By means of first-principles many body calculations, we show that the transition is in fact a site-selective Mott transition on the Ni sublattice with only minor charge differentiation. Key to this finding is the uncovering of ligand-hole physics, rendering a Ni$^{2+}$ instead of a formal Ni$^{3+}$ oxidation state, in conjunction with strong local Coulomb repulsions.

cond-mat.str-el↗

arXiv2021

Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination

Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap $Δ_\mathbf{k}^α$, for all momenta $\mathbf{k}$ on the Fermi surface of every band $α$. While there are a variety of techniques for determining $|Δ_\mathbf{k}^α|$, no general method existed to measure the signed values of $Δ_\mathbf{k}^α$. Recently, however, a new technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where $Δ_\mathbf{k}^α$ has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the $Δ_\mathbf{k}^α$ it generates to the $Δ_\mathbf{k}^α$ determined from single-atom scattering in FeSe where $s_{\pm}$ energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for $Δ_\mathbf{k}^α$ of opposite sign.

cond-mat.supr-con↗

arXiv2019

Pairing in the Two-Dimensional Hubbard Model from Weak to Strong Coupling

The Hubbard model is the simplest model that is believed to exhibit superconductivity arising from purely repulsive interactions, and has been extensively applied to explore a variety of unconventional superconducting systems. Here we study the evolution of the leading superconducting instabilities of the single-orbital Hubbard model on a two-dimensional square lattice as a function of onsite Coulomb repulsion $U$ and band filling by calculating the irreducible particle-particle scattering vertex obtained from dynamical cluster approximation (DCA) calculations, and compare the results to both perturbative Kohn-Luttinger (KL) theory as well as the widely used random phase approximation (RPA) spin-fluctuation pairing scheme. Near half-filling we find remarkable agreement of the hierarchy of the leading pairing states between these three methods, implying adiabatic continuity between weak- and strong-coupling pairing solutions of the Hubbard model. The $d_{x^2-y^2}$-wave instability is robust to increasing $U$ near half-filling as expected. Away from half filling, the predictions of KL and RPA at small $U$ for transitions to other pair states agree with DCA at intermediate $U$ as well as recent diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute limit, where it yields a $d_{xy}$ ground state instead of a $p$-wave state established by diagrammatic Monte Carlo and low-order perturbative methods, as well as our DCA calculations. We discuss the origins of this discrepancy, highlighting the crucial role of the vertex corrections neglected in the RPA approach. Overall, comparison of the various methods over the entire phase diagram strongly suggests a smooth crossover of the superconducting interaction generated by local Hubbard interactions between weak and strong coupling.

cond-mat.supr-con↗

arXiv2019

Quasiparticle Interference and Symmetry of Superconducting Order Parameter in Strongly Electron-Doped Iron-based Superconductors

Motivated by recent experimental reports of significant spin-orbit coupling (SOC) and a sign-changing order-parameter in the Li$_{1-x}$Fe$_x$(OHFe)$_{1-y}$Zn$_y$Se superconductor with only electron pockets present, we study the possible Cooper-pairing symmetries and their quasiparticle interference (QPI) signatures. We find that each of the resulting states - $s$-wave, $d$-wave and helical $p$-wave - can have a fully gapped density of states (DOS) consistent with angle-resolved photoemission spectroscopy (ARPES) experiments and, due to spin-orbit coupling, are a mixture of spin singlet and triplet components leading to intra- and inter-band features in the QPI signal. Analyzing predicted QPI patterns we find that only the spin-triplet dominated even parity $A_{1g}$ (s-wave) and $B_{2g}$ (d-wave) pairing states are consistent with the experimental data. Additionally, we show that these states can indeed be realized in a microscopic model with atomic-like interactions and study their possible signatures in spin-resolved STM experiments.

cond-mat.supr-con↗

arXiv2018

Fluctuation-induced Néel and Bloch skyrmions at topological insulator surfaces

Ferromagnets in contact with a topological insulator have become appealing candidates for spintronics due to the presence of Dirac surface states with spin-momentum locking. Because of this bilayer Bi$_2$Se$_3$-EuS structures, for instance, show a finite magnetization at the interface at temperatures well exceeding the Curie temperature of bulk EuS. Here we determine theoretically the effective magnetic interactions at a topological insulator-ferromagnet interface {\it above} the magnetic ordering temperature. We show that by integrating out the Dirac fermion fluctuations an effective Dzyaloshinskii-Moriya interaction and magnetic charging interaction emerge. As a result individual magnetic skyrmions and extended skyrmion lattices can form at interfaces of ferromagnets and topological insulators, the first indications of which have been very recently observed experimentally.

cond-mat.mtrl-sci↗

arXiv2018

Short-time dynamics in $s+is$-wave superconductor with incipient bands

Motivated by the recent observation of the time-reversal symmetry broken state in K-doped BaFe$_2$As$_2$ superconducting alloys, we theoretically study the collective modes and the short time dynamics of the superconducting state with $s+is$-wave order parameter using an effective four-band model with two hole and two electron pockets. The superconducting $s+is$ state emerges for incipient electron bands as a result of hole doping and appears as an intermediate state between $s^{\pm}$ (high number of holes) and $s^{++}$ (low number of holes). The amplitude and phase modes are coupled giving rise to a variety of collective modes. In the $s^{\pm}$ state, we find the Higgs mode at frequencies similar to a two-band model with an absent Leggett mode, while in the $s+is$ and $s^{++}$ state, we uncover a new coupled collective soft mode. Finally we compare our results with the $s+id$ solution and find similar behaviour of the collective modes.

cond-mat.supr-con↗

arXiv2018

In-plane magnetic penetration depth of superconducting CaKFe$_4$As$_4$

The temperature dependence of the in-plane magnetic penetration depth ($λ_{ab}$) in an extensively characterized sample of superconducting CaKFe$_4$As$_4$ ($T_{\rm c}\simeq35$ K) was investigated using muon-spin rotation ($μ$SR). A comparison of $λ_{ab}^{-2}(T)$ measured by $μ$SR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between $μ$SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small supercondcting gaps with an average zero-temperature value of $Δ_{0} =$ 2.4(2) meV. Our data suggest that in CaKFe$_4$As$_4$ the $s^\pm$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

cond-mat.supr-con↗

arXiv2017

Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS$_2$-superconductors

We develop the realistic minimal electronic model for recently discovered BiS$_2$ superconductors including the spin-orbit coupling based on a first-principles band structure calculations. Due to strong spin-orbit coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes $p_x$, $p_y$, and $p_z$ orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation (LAHA). For small and intermediate doping concentrations we find the dominant instabilities to be $d_{x^2-y^2}$-wave, and $s_{\pm}$-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsion are of the same strength, which yields the strongly anisotropic behaviour of the superconducting gaps on the Fermi surface in agreement with recent ARPES findings. In addition, we find that the Fermi surface topology for BiS$_2$ layered systems at large electron doping can resembles the doped iron-based pnictide superconductors with electron and hole Fermi surfaces with sufficient nesting between them. This could provide further boost to increase $T_c$ in these systems.

cond-mat.supr-con↗

arXiv2017

s+is superconductivity with incipient bands: doping dependence and STM signatures

Motivated by the recent observations of small Fermi energies and comparatively large superconducting gaps, present also on bands not crossing the Fermi energy (incipient bands) in iron-based superconductors, we analyse the doping evolution of superconductivity in a four-band model across the Lifshitz transition including BCS-BEC crossover effects on the shallow bands. Similar to the BCS case we find that with hole doping the phase difference between superconducting order parameters of the hole bands changes from $0$ to $π$ through an intermediate $s+is$ state breaking time-reversal symmetry. The transition however occurs in the region where electron bands are incipient and chemical potential renormalization in the superconducting state leads to a significant broadening of the $s+is$ region. We further present the qualitative features of the $s+is$ state that can be observed in scanning tunnelling microscopy (STM) experiments also taking incipient bands into account.

cond-mat.supr-con↗

arXiv2017

Electronic properties, low-energy Hamiltonian and superconducting instabilities in CaKFe$_4$As$_4$

We analyze the electronic properties of the recently discovered stoichiometric superconductor CaKFe$_4$As$_4$ by combining an ab initio approach and a projection of the band structure to a lowenergy tight-binding Hamiltonian, based on the maximally localized Wannier orbitals of the 3d Fe states. We identify the key symmetries as well as differences and similarities in the electronic structure between CaKFe$_4$As$_4$ and the parent systems CaFe$_2$As$_2$ and KFe$_2$As$_2$. In particular, we find CaKFe4As4 to have a significantly more quasi-two-dimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in CaKFe$_4$As$_4$ by employing the leading angular harmonics approximation (LAHA) and find two potential A$_{1g}$-symmetry representation of the superconducting gap to be the dominant instabilities in this system.

cond-mat.supr-con↗

arXiv2017

Experimental Demonstration of the Sign Reversal of the Order Parameter in (Li1-xFex)OHFe1-yZnySe

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is spin-fluctuation driven and intimately related to their fermiology, specifically, hole and electron pockets separated by the same wave vector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs. This picture was questioned after the discovery of a new family, based on the FeSe layers, either intercalated or in the monolayer form. The critical temperatures there reach ~40 K, the same as in optimally doped bulk FeSe - despite the fact that intercalation removes the hole pockets from the Fermi level and, seemingly, undermines the basis for the spin-fluctuation theory and the idea of a sign-changing OP. In this paper, using the recently proposed phase-sensitive quasiparticle interference technique, we show that in LiOH intercalated FeSe compound the OP does change sign, albeit within the electronic pockets, and not between the hole and electron ones. This result unifies the pairing mechanism of iron based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.

cond-mat.supr-con↗

arXiv2017

Spin excitations and the Fermi surface of superconducting FeS

High-temperature superconductivity occurs near antiferromagnetic instabilities and nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scatter- ing and Fermi surface measurements to demonstrate that hydro-thermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitation is observed up to 100 meV. No direct coupling between spin excitation and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.

cond-mat.supr-con↗

arXiv2016

Dichotomy between in-plane magnetic susceptibility and resistivity anisotropies in extremely strained $BaFe_{2}As_{2}$

The in-plane resistivity and uniform magnetic susceptibility anisotropies of $BaFe_{2}As_{2}$ are obtained with a new method, in which a large symmetry-breaking uniaxial strain is applied using a substrate with a very anisotropic thermal expansion. The resistivity anisotropy and its corresponding elastoresistivity exhibit very similar diverging behavior as those obtained from piezo-stack experiments. This suggests that the resistivity anisotropy is more a direct measure of magnetism than of nematicity, since the nematic transition is no longer well-defined under a large strain. In strong contrast to the large resistivity anisotropy above $T_{N}$, the anisotropy of the in-plane magnetic susceptibility develops largely below $T_{N}$. Using an itinerant model, we show that the observed anisotropy ($χ_{b}>χ_{a}$) is determined by spin-orbit coupling and the orientation of the magnetic moments in the antiferromagnetic phase, and that the anisotropy is dominated by intra-orbital ($yz,yz$) contributions of the Umklapp susceptibility.

cond-mat.supr-con↗

arXiv2016

Surface state tunneling signatures in two-component superconductor UPt$_3$

Quasiparticle interference (QPI) imaging of Bogoliubov excitations in quasi-two dimensional unconventional superconductors has become a powerful technique for measuring the superconducting gap and its symmetry. Here, we present the extension of this method to three-dimensional superconductors and analyze the expected QPI spectrum for the two-component heavy fermion superconductor UPt$_3$ whose gap structure is still controversial. Starting from a 3D electronic structure and the three proposed chiral gap models $E_{1g,u}$ or $E_{2u}$, we perform a slab calculation that determines the 2D continuum Bogoliubov- de Gennes (BdG) surface quasiparticle bands and in addition the in-gap flat-band Andreev bound states that lead to surface Weyl arcs connecting the projected gap nodes. Both features are very distinct for the three models, in particular the most prominent $E_{2u}$ candidate is singled out by the existence of {\it two} Weyl arcs due to the double monopole node points. The signature of these distinct surface bound and continuum states that is left in QPI is derived and discussed. We show that it provides a fingerprint that may finally determine the true nodal structure of UPt$_3$ superconductor.

cond-mat.supr-con↗

arXiv2016

Anisotropic spin fluctuations in Sr$_2$RuO$_4$: role of spin-orbit coupling and induced strain

We analyze the spin anisotropy of the magnetic susceptibility of Sr$_2$RuO$4$ in presence of spin-orbit coupling and anisotropic strain using quasi-two-dimensional tight-binding parametrization fitted to the ARPES results. Similar to the previous observations we find the in-plane polarization of the low ${\bf q}$ magnetic fluctuations and the out-of-plane polarization of the incommensurate magnetic fluctuation at the nesting wave vector ${\bf Q}_1 = (2/3 π,2/3 π)$ but also nearly isotropic fluctuations near ${\bf Q}_2=(π/6,π/6)$. Furthermore, one finds that apart from the high-symmetry direction of the tetragonal Brillouin zone the magnetic anisotropy is maximal, i.e. $χ^{xx} \neq χ^{yy} \neq χ^{zz}$. This is the consequence of the orbital anisotropy of the $xz$ and $yz$ orbitals in the momentum space. We also study how the magnetic anisotropy evolves in the presence of the strain and find strong Ising-like ferromagnetic fluctuations near the Lifshitz transition for the $xy$-band.

cond-mat.supr-con↗

arXiv2016

Collective magnetic excitations of $C_{4}$ symmetric magnetic states in iron-based superconductors

We study the collective magnetic excitations of the recently discovered $C_{4}$ symmetric spin-density wave states of iron-based superconductors with particular emphasis on their orbital character based on an itinerant multiorbital approach. This is important since the $C_{4}$ symmetric spin-density wave states exist only at moderate interaction strengths where damping effects from a coupling to the continuum of particle-hole excitations strongly modifies the shape of the excitation spectra compared to predictions based on a local moment picture. We uncover a distinct orbital polarization inherent to magnetic excitations in $C_{4}$ symmetric states, which provide a route to identify the different commensurate magnetic states appearing in the continuously updated phase diagram of the iron-pnictide family.

cond-mat.supr-con↗