The lack of energy dissipation and abrupt electrical phase transition of superconductors favorite them for nanoscale technologies, including radiation detectors, and quantum technologies. Moreover, understanding the nanoscale behavior of superconductivity is significant for revealing the onset of collective-electron behavior in nature. Nevertheless, the limited number of accessible superconductors restricts availability of the superconducting properties, encumbering the realization of their potential. Superconducting nanowire single photon detectors (SNSPDs) sense single-IR photons faster and more efficient with respect to competing technologies. However, these advantageous properties are material-dependent causing an undesirable speed-efficiency payoff. Usually, SNSPDs based on granular materials are faster, while those based on amorphous materials are more efficient. Here we optimized ultrathin films of granular NbN on SiO2 and of amorphous W5Si3. We showed that hybrid superconducting nanowire single photon detectors (SNSPDs) made of 2-nm-thick W5Si3 films over 2-nm-thick NbN films exhibit advantageous coexistence of timing (< 5-ns reset time and 52-ps timing jitter) and efficiency (> 96% quantum efficiency) performance. We propose that the governing mechanism of this hybridization is the presence of a dual superconducting behavior: native superconductivity of each of the films and superconductivity that is induced from the neighboring film via the proximity effect. In addition to improvement in SNSPDs performance, our results suggest that such hybridization can expand the range of available superconducting properties, impacting nano-superconducting technologies. Lastly, this hybridization may be used to tune the amorphous character of superconducting films and to illuminate the elusive onset of collective-electron behavior near the superconducting-to-insulating transition.
We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconducting pairing. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario - evidence for the presence of a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the s…
We explore ways in which the close proximity between graphene sheets and monolayers of 2D superconductors can lead to hybridization between their collective excitations. We consider heterostructures formed by combinations of graphene sheets and 2D superconductor monolayers. The broad range of energies in which the graphene plasmon can exist, together with its tunability, makes such heterostrucutres promising platforms for probing the many-body physics of superconductors. We show that the hybridization between the graphene plasmon and the Bardasis-Schrieffer mode of a 2D superconductor results …
The Roth's two-pole approximation has been used by the present authors to study the effects of the hybridization in the superconducting properties of a strongly correlated electron system. The model used is the extended Hubbard model which includes the $d-p$ hybridization, the $p$-band and a narrow $d$-band. The present work is an extension a previous reference [Intern. Journ. of Modern Phys. B, Vol. 18 No. 2 (2004) 241]. Nevertheless, some important correlation functions necessary to estimate the Roth's band shift, are included together with the temperature $T$ and the Coulomb interaction $U$…
Motivated by the recent realization of the infinite-layer nickelate superconductivity (SC), we quantitatively investigate a two-orbital $d$-$s$ model by dynamic cluster quantum Monte Carlo calculations. Focusing on the impact of inter-orbital hybridization on the superconducting properties, our simulations indicate that the $d$-$s$ hybridization strength has a decisive role in the suppression of the $d$-wave pairing in the doping regime relevant to infinite-layer nickelates. Although we confirm on the single-orbital description at weak hybridization, at relatively large hybridization, there ex…
The main proposal of this paper is to analyze the size of the Cooper pairs composed by unbalanced mass fermions from different electronic bands along the BCS-BEC crossover and study the binding energy of the pairs. We are considering an interaction between fermions with different masses leading to an inter-band pairing. In addiction to the attractive interaction we have an hybridization term to couple both bands, which in general acts unfavorable for the pairing between the electrons. We get first order phase transitions as the hybridization break the Cooper pairs for the the $s$-wave symmetry…
Within the Ginzburg-Landau model we study the critical field and temperature enhancement for crossing superconducting channels formed either along the sample edges or domain walls in thin-film magnetically coupled superconducting - ferromagnetic bilayers. The corresponding Cooper pair wave function can be viewed as a hybridization of two order parameter (OP) modes propagating along the boundaries and/or domain walls. Different momenta of hybridized OP modes result in the formation of vortex chains outgoing from the crossing point of these channels. Near this crossing point the wave functions o…
Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates
College of Industrial Technology, Nihon University（日本大学工业技术学部）；Research Organization of Science and Technology, Ritsumeikan University（立命馆大学科学技术研究院）；Faculty of Engineering, Tottori University（鸟取大学工学部）；Department of Physics, The University of Osaka（大阪大学物理学系）
Epitaxial semiconductor-superconductor heterostructures are promising as a platform for gate-tunable superconducting electronics. Thus far, the superconducting properties in such hybrid systems have been predicted based on simplified hybridization models which neglect the electronic structure that can arise at the interface. Here, we demonstrate that the hybrid electronic structure derived at the interface between semiconducting black phosphorus and atomically thin films of lead can drastically modify the superconducting properties of the thin metallic film. Using ultra-low temperature scannin…
The profiles of local density of states (LDOS) around different vortex configurations in mesoscopic superconductors are studied taking account of the interference of quasiparticle waves experiencing Andreev reflection within the vortex cores and normal reflection at the boundaries or defects. For subgap energy levels these interference effects reveal themselves in a nontrivial dependence of the positions of the LDOS peaks on the intervortex distance and sample size: the peak positions generally do not coincide with the superconducting phase singularity points. The LDOS profiles are calculated …
Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. As such, YSR states have been very successfully modeled with an effective scattering potential (Kondo impurity model). Using a scanning tunneling microscope, we exploit the proximity of the tip to a surface impurity and its influence on the YSR state to make a quantitative connection between the YSR state energy and the impurity-substrate hybridization. We corroborate the coupling between impurity and substrate as a key energy scale for surface …
Ni-O hybridization-driven electronic reconstruction across the superconducting dome in an infinite-layer nickelate
National University of Singapore（新加坡国立大学）；Agency for Science, Technology and Research (A*STAR)（科学与技术研究局）；Shanghai University（上海大学）；Southern University of Science and Technology（南方科技大学）
Hybridization between surface flat bands and bulk bands in the topological nodal-line semimetal Sn$_{0.15}$NbSe$_{1.75}$ probed via soft-point-contact spectroscopy
We present the exact solution of a one-dimensional model of a spin-gapped correlated electron system with hybridization impurities exhibiting both magnetic and mixed-valence properties. The host supports superconducting fluctuations, with a spin gap. The localized electrons create a band of antiferromagnetic spin excitations inside the gap for concentrations x of the impurities below some critical value x_c. When x = x_c the spin gap closes and a ferrimagnetic phase appears. This is the first example of an exactly solvable model with coexisting superconducting and antiferromagnetic fluctuation…
We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clea…
Confinement at the helical edge of a topological insulator is possible in the presence of proximity-induced magnetic (F) or superconducting (S) order. The interplay of both phenomena leads to the formation of localized Majorana bound states (MBS) or likewise (under certain resonance conditions) the formation of ordinary Andreev bound states (ABS). We investigate the properties of bound states in junctions composed of alternating regions of F or S barriers. Interestingly, the direction of magnetization in F regions and the relative superconducting phase between S regions can be exploited to hyb…
The Hund's-rule-exchange induced and coexisting spin-triplet paired and magnetic states are considered within the doubly degenerate Hubbard model with interband hybridization. The Hartree-Fock approximation combined with the Bardeen-Cooper-Schrieffer (BCS) approach is analyzed for the case of square lattice. The calculated phase diagram contains regions of stability of the spin-triplet superconducting phase coexisting with either ferromagnetism or antiferromagnetism, as well as a pure superconducting phase. The influence of the inter-site hybridization on the stability of the considered phases…
Recent experiments announced an observation of topological superconductivity and Majorana quasiparticles in Shiba chains, consisting of an array of magnetic atoms deposited on top of a superconductor. In this work we study helical Shiba chains and generalize the microscopic theory of subgap energy bands to a regime where the decoupled magnetic impurity energy and the hybridization of different impurity states can be significant compared to the superconducting gap of the host material. From exact solutions of the Bogoliubov-de Gennes equation we extract expressions for the topological phase bou…
We investigate how hybridization (single-quasiparticle scattering) between two superconducting bands induces odd-frequency superconductivity in a multiband superconductor. An explicit derivation of the odd-frequency pairing correlation and its full frequency dependence is given. We find that the density of states is modified, from the sum of the two BCS spectra, at higher energies by additional hybridization gaps with strong coherence peaks when odd-frequency pairing is present.
We study the low-energy transport properties of a hybrid device composed by a native quantum dot coupled to both ends of a topological superconducting nanowire section hosting Majorana zero-modes. The account of the coupling between the dot and the farthest Majorana zero-mode allows to introduce the topological quality factor, characterizing the level of topological protection in the system. We demonstrate that Coulomb interaction between the dot and the topological superconducting section leads to the onset of the additional overlap of the wavefunctions describing the Majorana zero-modes, lea…
We study the effects of random nonmagnetic impurities on the superconducting transition temperature $T_c$ in a two-band superconductor, where we assume the equal-time spin-singlet s-wave pair potential in each conduction band and the hybridization between the two bands as well as the band asymmetry. In the clean limit, the phase of hybridization determines the stability of two states: called $s_{++}$ and $s_{+-}$. The interband impurity scatterings decrease $T_c$ of the two states exactly in the same manner when the Hamiltonian preserves time-reversal symmetry. We find that a superconductor wi…
We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi2. Compared with the layered AlB2 structure of graphite or diboride-like superconductors, in the hexagonal structure of binary silicides the sp3 arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, obtaining the enhancement of the superconducting transition temperature from 4 K to 8.7 K when the silicon planes flatten out. By performing ab-initio calculations based on density fun…
Quantitative magneto-optical imaging of a type-II superconductor thin film cooled under zero, homogeneous, and inhomogeneous applied magnetic fields, indicates that the latter procedure leads to an enhancement of the screening capacity. Such an observation is corroborated by both B-independent and B-dependent critical state model analyses. Furthermore, repulsive (attractive) vortex-(anti)vortex interactions were found to have a decisive role in the shielding ability, with initial states prepared with vortices resulting in a shorter magnetic flux front penetration depth than those prepared with…
In this paper we study the effects of hybridization in the superconducting properties of a two-band system. We consider the cases that these bands are formed by electronic orbitals with angular momentum, such that, the hybridization $V(\mathbf{k})$ among them can be symmetric or antisymmetric under inversion symmetry. We take into account only intra-band attractive interactions in the two bands and investigate the appearance of an induced inter-band pairing gap. We show that (inter-band) superconducting orderings are induced in the total absence of attractive interaction between the two bands,…
In this work, we measure longitudinal dressed states of a superconducting qubit, the single Cooper-pair box, and an intense microwave field. The dressed states represent the hybridization of the qubit and photon degrees of freedom, and appear as avoided level crossings in the combined energy diagram. By embedding the circuit in an rf oscillator, we directly probe the dressed states. We measure their dressed gap as a function of photon number and microwave amplitude, finding good agreement with theory. In addition, we extract the relaxation and dephasing rates of these states.
In this communication we consider generalities of the proximity effect in a contact between a conventional $s$-wave superconductor (S) nano-island and a thin film of a topological insulator (TI). A local hybridization coupling mechanism is considered and a corresponding model is corroborated that captures not only the induced unconventional superconductivity in a TI, but also predicts the spreading of topologically protected surface states into the superconducting over-layer. This dual nature of the proximity effect leads specifically to a modified description of topological superconductivity …
Hybridization of Bogoliubov-quasiparticles between adjacent CuO$_2$ layers in the triple-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ studied by ARPES
Indium Arsenide (InAs) near surface quantum wells (QWs) are promising for the fabrication of semiconductor-superconductor heterostructures given that they allow for a strong hybridization between the two-dimensional states in the quantum well and the ones in the superconductor. In this work we present results for InAs QWs in the quantum Hall regime placed in proximity of superconducting NbTiN. We observe a negative downstream resistance with a corresponding reduction of Hall (upstream) resistance, consistent with a very high Andreev conversion. We analyze the experimental data using the Landau…
Magnetic adatom chains on surfaces constitute fascinating quantum spin systems. Superconducting substrates suppress interactions with bulk electronic excitations but couple the adatom spins to a chain of subgap Yu-Shiba-Rusinov (YSR) quasiparticles. Using a scanning tunneling microscope, we investigate such correlated spin-fermion systems by constructing Fe chains adatom by adatom on superconducting NbSe$_2$. The adatoms couple entirely via the substrate, retaining their quantum spin nature. In dimers, we observe that the deepest YSR state undergoes a quantum phase transition due to Ruderman-K…
From band reconstruction to Bogoliubov dispersion: How dz2-band enhances iron-based superconductivity
State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University（清华大学物理系低维量子物理国家重点实验室）；Frontier Science Center for Quantum Information（量子信息前沿科学中心）；Department of Materials Science and Engineering, University of Utah（犹他大学材料科学与工程系）；School of Physics, Beihang University（北京航空航天大学物理学院）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Southern University of Science and Technology（南方科技大学）；Hefei National Laboratory（合肥国家实验室）
The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have non-uniform density while preserving the local order…
We measure the effect of uniaxial pressure on the superconducting transition temperature Tc in CeIrIn5. We find a linear change in Tc with both a-axis and c-axis pressure, with slopes of 56 mK/kbar and -66 mK/kbar, respectively. By comparing results from doping studies and different types of pressure measurements, we separate the influences of hybridization and dimensionality on Tc. We find the true geometric influence, for constant hybridization, is dTc/d(c/a)=44 K.
In this paper we have studied the effect of external magnetic field in the co-existing phase of superconducting and anti-ferromagnetism of rare earth nickel borocarbides. The anti-ferromagnetism in these systems might have originated due to both localized 'f' electrons as well as itinerant electrons which are responsible for conduction. On the other hand, superconductivity is due to spin density wave, arising out of Fermi surface instability. The anti-ferromagnetism order is mostly influenced by hybridization of the 'f' electron with the conduction electron. Here we have obtained the dependenc…
Using random-phase approximation spin-fluctuation theory, we study the influence of the hybridization between iron $d$-orbitals and pnictide $p$-orbitals on the superconducting pairing state in iron-based superconductors. The calculations are performed for a 16-orbital Hubbard-Hund tight-binding model of BaFe$_2$As$_2$ that includes the As-$p$ orbital degrees of freedom in addition to the Fe-$d$ orbitals and compared to calculations for a 10-orbital Fe-$d$ only model. In both models we find a leading $s^\pm$ pairing state and a subleading $d_ {x^2-y^2}$-wave state in the parent compound. Upon …
A superconducting ring with a pi junction made from superconductor/ferromagnetic-metal/superconductor (S-FM-S) exhibits a spontaneous current without an external magnetic field in the ground state. Such pi ring provides so-called quiet qubit that can be efficiently decoupled from the fluctuation of the external field. However, the usage of the FM gives rise to strong Ohmic dissipation. Therefore, the realization of pi junctions without FM is expected for qubit applications. We theoretically consider the possibility of the pi coupling for S/Eu-chalcogenides/S junctions based on the d-f Hamilton…
The 1111-type iron arsenide hydride CaFe1-xCoxAsH was synthesized by high-pressure solid-state reaction, and its electronic structure and superconducting properties were investigated. Bulk superconductivity was observed at x = 0.07-0.26. A maximum superconducting critical temperature Tcmax of 23 K was observed at x = 0.07. These values are in agreement with those of CaFe1-xCoxAsF. The calculated Fermi surface of CaFeAsH has a small three-dimensional 3D hole pocket around the Gamma point. This is a result of weak covalent bonding between the As 4p and H 1s orbitals. No such covalency exists in …
The key ingredient of high critical currents in a type-II superconductor is defect sites that 'pin' vortices. Contrary to earlier understanding on nano-patterned artificial pinning, here we show unequivocally the advantages of a random pinscape over an ordered array in a wide magnetic field range. We reveal that the better performance of a random pinscape is due to the variation of its local-density-of-pinning-sites (LDOPS), which mitigates the motion of vortices. This is confirmed by achieving even higher enhancement of the critical current through a conformally mapped random pinscape, where …
The effect of nanoscale-SiC doping of MgB2 was investigated using transport and magnetic measurements. It was found that there is a clear correlation between the critical temperature Tc, the resistivity r, the residual resistivity ratio, RRR = R(300K)/R(40K), the irreversibility field H* and the alloying state in the samples. SiC-doping introduced many nano-scale precipitates, provoking an increase of r(40K) from 1 mW-cm (RRR = 15) for the clean limit sample to 300 mW-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of Hc2 and H* with only minor effect on Tc. EELS a…
We explore the temperature effects in the superconducting phases of a hybridized two-band system. We show that for zero hybridization between the bands, there are two different critical temperatures. However, for any finite hybridization there are only one critical temperature at which the two gaps vanish simultaneously. We construct the phase diagrams of the critical temperature versus hybridization parameter $α$ and critical temperature versus critical chemical potential asymmetry $δμ$ between the bands, identifying the superconductor and normal phases in the system. We find an interesting r…
The binary ruthenate, RuO$_2$, has been the subject of intense interest due to its itinerant antiferromagnetism and strain-induced superconductivity. The strain mechanism and its effect on the microscopic electronic states leading to the normal and superconducting state, however, remain undisclosed. Here, we investigate highly-strained epitaxial (110) RuO$_2$ films using polarization-dependent oxygen K-edge X-ray absorption spectroscopy (XAS). Through the detection of pre-edge peaks, arising from O:$2p$ - Ru:$4d$ hybridization, we uncover the effects of epitaxial strain on the orbital/electron…
The study of multi-band superconductivity is relevant for a variety of systems, from ultra cold atoms with population imbalance to particle physics, and condensed matter. As a consequence, this problem has been widely investigated bringing to light many new and interesting phenomena. In this work we point out and explore a correspondence between a two-band metal with a $k$-dependent hybridization and a uniformly polarized fermionic system in the presence of spin-orbit coupling (SOC). We study the ground state phase diagram of the metal in the presence of an attractive interaction. We find rema…
Orbital Hybridization-Induced Ising-Type Superconductivity in a Confined Gallium Layer
The Pennsylvania State University（宾夕法尼亚州立大学）；Shanghai Jiao Tong University（上海交通大学）；University of Oxford（牛津大学）；Zhejiang University（浙江大学）；Diamond Light Source（钻石光源研究所）；University of North Texas（北德克萨斯大学）
I propose that a dopant charge singlet bonding state may arise from the hybridization of molecular orbitals in a cluster containing 13 Cu atoms in the CuO2 plane of the superconducting cuprates. This singlet state forms a pre-formed pair with low binding energy that is spatially bounded and weakly interacting, and that can undergo Bose-Einstein condensation. I show that this model is able to account, in a quantitative and natural way, for many of the thermodynamic and electronic characteristics of the superconducting cuprates, including many of the key experimental ARPES, muSR and microwave re…
In multi-band systems, electrons from different orbitals coexist at the Fermi surface. An attractive interaction among these quasi-particles gives rise to inter-band or hybrid pairs which eventually condense in a superconducting state. These quasi-particles have a natural mismatch of their Fermi wave-vectors, $δk_F$, which depends on the strength of the hybridization between their orbitals. The existence of this natural scale suggests the possibility of inhomogeneous superconducting ground states in these systems, even in the absence of an applied magnetic field. Furthermore, since hybridizati…
In the endeavour to make quantum computers a reality, integrated superconducting circuits have become a promising architecture. A major challenge of this approach is decoherence originating from spurious atomic tunneling defects at the interfaces of qubit electrodes, which may resonantly absorb energy from the qubit's oscillating electric field and reduce the qubit's energy relaxation time $T_1$. Here, we show that qubit coherence can be improved by tuning dominating defects away from the qubit resonance using an applied DC-electric field. We demonstrate a method that optimizes the applied fie…
X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x=1-0.8) single crystals cleaved in situ in vacuum reveal characteristic Fe 4sp states, a lattice distortion and the Se K-edge spectra point to a strong Fe 3d-Se 4p hybridization giving rise to itinerant charge carriers. A formal charge of ~1.8+ for Fe and ~2.2- for Se were evaluated from these spectra in the FeSex (x=0.88). The charge balance between Fe and Se is assigned itinerant electrons located in the Fe-Se hybridization bond. As x decreases the 4p hole count increases and a crystal structure distortion is observed that …
Current crowding at bends of superconducting nanowire single-photon detectors is one of the main factors limiting the performance of meander-style detectors with large filling factors. In this paper, we propose a new concept to reduce influence of the current crowding effect, a so-called variable thickness SNSPD, which is composed of two regions with different thicknesses. A larger thickness of bends in comparison to the thickness of straight nanowire sections locally reduces the current density and reduces the suppression of the critical current caused by the current crowding. This allows var…
We study decoherence effects in qubits coupled to environments that exhibit resonant frequencies in their spectral function. We model the coupling of the qubit to its environment via the Caldeira-Leggett formulation of quantum dissipation/coherence, and study the simplest example of decoherence effects in circuits with resonances such as a dc SQUID phase qubit in the presence of an isolation circuit, which is designed to enhance the coherence time. We emphasize that the spectral density of the environment is strongly dependent on the circuit design, and can be engineered to produce longer deco…
We report on a pressure($P$)-induced evolution of superconductivity and spin correlations in CeIrIn$_5$ via the $^{115}$In nuclear-spin-lattice-relaxation rate measurements. We find that applying pressure suppresses dramatically the antiferromagnetic fluctuations that are strong at ambient pressure. At $P$ = 2.1 GPa, $T_{\rm c}$ increases to $T_{\rm c}$ = 0.8 K that is twice $T_{\rm c}$($P$ = 0 GPa), in the background of Fermi liquid state. This is in sharp contrast with the previous case in which negative, chemical pressure (replacing Ir with Rh) enhances magnetic interaction and increases $T…
