In bulk materials superconductivity is remarkably robust with respect to non-magnetic disorder. In the two-dimensional limit however, the quantum condensate suffers from the effects produced by disorder and electron correlations which both tend to destroy superconductivity . The recent discovery of superconductivity in single atomic layers of Pb, the striped incommensurate (SIC) and $\sqrt{7}\times\sqrt{3}$ Pb/Si(111), opened an unique opportunity to probe the influence of well-identified structural disorder on two-dimensional superconductivity at the atomic and mesoscopic scale. In these two ultimate condensates we reveal how the superconducting spectra loose their conventional character, by mapping the local tunneling density of states. We report variations of the spectral properties even at scales significantly shorter than the coherence length. Furthermore, fine structural differences between the two monolayers, such as their atomic density, lead to very different superconducting behaviour. The denser SIC remains globally robust to disorder, as are thicker Pb films, whereas in the slighly more diluted $\sqrt{7}\times\sqrt{3}$ system superconductivity is strongly fragilized. A consequence of this weakness is revealed at monoatomic steps of $\sqrt{7}\times\sqrt{3}$, which disrupt superconductivity at the atomic scale. This effect witnesses that each individual step edge is a Josephson barrier. At a mesoscopic scale the weakly linked superconducting atomic terraces of $\sqrt{7}\times\sqrt{3}$ form a native network of Josephson junctions. We anticipate the Pb/Si(111) system to offer the unique opportunity to tune the superconducting coupling between adjacent terraces, paving a new way of designing atomic scale quantum devices compatible with silicon technology.
Silicon, one of the most abundant elements found on Earth, has been an excellent choice of the semiconductor industry for ages. Despite it's remarkable applications in modern semiconductor-based electronic devices, the potential of cubic silicon in superconducting electronics remained a challenge because even heavily doped silicon crystals do not superconduct under normal conditions. It is apparent that if superconductivity can be introduced in cubic silicon, that will bring a breakthrough in low-dissipation electronic circuitry. Motivated by this, attempts have been made by several research g…
The discoveries of superconductivity in heavily boron-doped diamond (C:B) in 2004 and silicon (Si:B) in 2006 renew the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily-boron doped silicon carbide (SiC:B). The sample used for that study consists of cubic and hexagonal SiC phase fractions and hence this lead to the question which of them participates in the superconductivity. …
We study within a first-principle approach the band structure, vibrational modes and electron-phonon coupling in boron, aluminum and phosphorus doped silicon in the diamond phase. Our results provide evidences that the recently discovered superconducting transition in boron doped cubic silicon can be explained within a standard phonon-mediated mechanism. The importance of lattice compression and dopant related stretching modes are emphasized. We find that T$_C$ can be increased by one order of magnitude by adopting aluminum doping instead of boron.
In 2004 the discovery of superconductivity in heavily boron-doped diamond (C:B) led to an increasing interest in the superconducting phases of wide-gap semiconductors. Subsequently superconductivity was found in heavily boron-doped cubic silicon (Si:B) and recently in the stochiometric ''mixture'' of heavily boron-doped silicon carbide (SiC:B). The latter system surprisingly exhibits type-I superconductivity in contrast to the type-II superconductors C:B and Si:B. Here we will focus on the specific heat of two different superconducting samples of boron-doped SiC. One of them contains cubic and…
The discoveries of superconductivity in heavily boron-doped diamond, silicon and silicon carbide renewed the interest in the ground states of charge-carrier doped wide-gap semiconductors. Recently, aluminium doping in silicon carbide successfully yielded a metallic phase from which at high aluminium concentrations superconductivity emerges. Here, we present a specific-heat study on superconducting aluminium-doped silicon carbide. We observe a clear jump anomaly at the superconducting transition temperature 1.5 K indicating that aluminium-doped silicon carbide is a bulk superconductor. An analy…
Adsorption of one-third monolayer of Sn on an atomically-clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation-doped and metallized using heavily-doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7 +- 0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed TC, Mo…
Scanning tunneling spectroscopies in the subKelvin temperature range were performed on superconducting Silicon epilayers doped with Boron in the atomic percent range. The resulting local differential conductance behaved as expected for a homogeneous superconductor, with an energy gap dispersion below +/- 10%. The spectral shape, the amplitude and temperature dependence of the superconductivity gap follow the BCS model, bringing further support to the hypothesis of a hole pairing mechanism mediated by phonons in the weak coupling limit.
In 2007, type-I superconductivity in heavily boron-doped silicon carbide was discovered. The question arose, if it is possible to achieve a superconducting phase by introducing dopants different from boron. Recently, aluminum-doped silicon carbide was successfully found to superconduct by means of resistivity and DC magnetization measurements. In contrast to boron-doped silicon carbide, the aluminum doped system is treated as a type-II superconductor because of the absence of an hysteresis in data measured upon decreasing and increasing temperature in finite magnetic fields. In this paper, res…
We present the findings of the superconductivity observed in the silicon nanostructures prepared by short time diffusion of boron on the n-type Si(100) surface. These Si-based nanostructures represent the p-type ultra-narrow self-assembled silicon quantum wells, 2nm, confined by the delta - barriers heavily doped with boron, 3nm. The EPR and the thermo-emf studies show that the delta - barriers appear to consist of the trigonal dipole centres, which are caused by the negative-U reconstruction of the shallow boron acceptors. Using the CV and thermo-emf techniques, the transport of two-dimension…
We show the possibility of inducing a superconductive phase transition in tetrahedrally coordinated semiconductors via field-effect (FET) doping by taking as an example the hydrogenated (111) silicon surface. We perform density functional theory computations of the electronic and vibrational properties of the system in the proper FET geometry, by taking into account the applied electric field and the induced charge density. Using a simplified superconductive model at $q=Γ$ and the McMillan/Allen-Dynes formula, we get an estimate of the superconductive critical temperature. We observe that, by …
We report a detailed study of specific heat, electrical resistivity and thermal expansion in combination with inelastic neutron and inelastic X-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 K and 1.5 K respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function and phonon anharmonicity from these…
The discoveries of superconductivity in the heavily-boron doped semiconductors diamond (C:B) in 2004 and silicon (Si:B) in 2006 have renewed the interest in the physics of the superconducting state of doped semiconductors. Recently, we discovered superconductivity in the closely related ''mixed'' system heavily boron-doped silcon carbide (SiC:B). Interestingly, the latter compound is a type-I superconductor whereas the two aforementioned materials are type-II. In this paper we present an extensive analysis of our recent specific-heat study, as well as the band structure and expected Fermi surf…
We present the findings of the superconductivity in the silicon nanostructures prepared by short time diffusion of boron after preliminary oxidation of the n-type Si (100) surface. These Si-based nanostructures represent the p-type high mobility silicon quantum well (Si-QW) confined by the delta - barriers heavily doped with boron. The ESR studies show that the delta - barriers appear to consist of the trigonal dipole centers, B(+)-B(-), which are caused by the negative-U reconstruction of the shallow boron acceptors, 2B(0)=>B(+)-B(-). The temperature and magnetic field dependencies of the res…
We present the findings of the superconductivity in the silicon nanostructures prepared by short time diffusion of boron of the n - type Si (100) surface. These Si - based nanostructures represent the p - type ultra-narrow self - assembled silicon quantum wells confined by the delta - barriers heavily with boron. The resistivity, thermo - emf and magnetic susceptibility studies show that the high temperature superconductivity observed seems to result from the single - hole tunnelling into the negative - U boron centres at the silicon quantum well - delta - barier interfaces.
Over the past 14 years MgB2 has gone from a startling discovery to a promising, applied superconductor. In this article we present a brief overview of the synthesis and the basic superconducting properties of this remarkable compound. In particular, the effects of pressure, substitutions and neutron irradiation on superconducting properties are discussed.
We report on first principles calculations of superconductivity in a single layer of lead on a silicon substrate including a full treatment of phononic and RPA screened coulomb interactions within the parameter free framework of Density Functional Theory for superconductors. A thorough investigation shows that several approximations that are commonly valid in bulk systems fail in this constrained 2D geometry. The calculated critical temperature turns out to be much higher than the experimental value of 1.86K. We argue that the only plausible explanation for the experimental Tc suppression is t…
Semiconductivity and superconductivity are remarkable quantum phenomena that have immense impact on science and technology, and materials that can be tuned, usually by pressure or doping, to host both types of quantum states are of great fundamental and practical significance. Here we show by first-principles calculations a distinct route for tuning semiconductors into superconductors by diverse large-range elastic shear strains, as demonstrated in exemplary cases of silicon and silicon carbide. Analysis of strain driven evolution of bonding structure, electronic states, lattice vibration, and…
Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origin and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critic…
We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by Gas Immersion Laser Doping. The doping concentration cB has been varied up to approx. 10 at.% by increasing the number of laser shots to 500. No superconductivity could be observed down to 40mK for doping level below 2.5 at.%. The critical temperature Tc then increased steeply to reach 0.6K for cB = 8 at%. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field Hc2(0) w…
Cuprates exhibit exceptionally strong superconductivity. To understand why, it is essential to elucidate the nature of the electronic interactions that cause pairing. Superconductivity occurs on the backdrop of several underlying electronic phases, including a doped Mott insulator at low doping, a strange metal at high doping, and an enigmatic pseudogap phase in between -- inside which a phase of charge-density-wave order appears. In this Article, we aim to shed light on the nature of these remarkable phases by focusing on the limit as $T \to 0$, where experimental signatures and theoretical s…
The investigation of the temperature dependences of microwave surface impedance and complex conductivity of V3Si single crystals with different stoichiometry allowed to observe a number of peculiarities which are in remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At the same time, they can be well described by two-band model of superconductivity, thus strongly evidencing the existence of two distinct energy gaps with zero-temperature values Delta1~1.8Tc and Delta2~0.95Tc in V3Si.
We report the superconductivity at above 30 K in a new FeSe-layer compound K0.8Fe2Se2 (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to BaFe2As2 and possesses the highest Tc for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this new FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.
Atomic layers deposited on semiconductor substrates introduce a platform for the realization of the extended electronic Hubbard model, where the consideration of electronic repulsion beyond the onsite term is paramount. Recently, the onset of superconductivity at 4.7K has been reported in the hole-doped triangular lattice of tin atoms on a silicon substrate. Through renormalization group methods designed for weak and intermediate coupling, we investigate the nature of the superconducting instability in hole-doped Sn/Si(111). We find that the extended Hubbard nature of interactions is crucial t…
Through the solid state reaction method, we synthesized a new BiSe2-based superconductor Sr0:5La0:5FBiSe2 with superconducting transition temperature Tc?3.8 K. A strong diamagnetic signal below Tc in susceptibility ?(T) is observed indicating the bulk nature of superconductivity. Different to most BiS2-based compounds where superconductivity develops from a semiconducting-like normal state, the present compound exhibits a metallic behavior down to Tc. Under weak magnetic field or pressure, however, a remarkable crossover from metallic to insulating behaviors takes place around Tmin where the r…
The 3-layer (3R) polytype of TaSe2-xTex shows a superconducting transition temperature that is between 6 and 17 times higher than that of the two-layer (2H) polytype. The remarkable difference in Tc, although clearly associated with a difference in polytype, could have been due to an electronic effect specific to the Te-Se substitution. Here we report that small amounts of Mo or W doping lead to a 2H to 3R polytype transition in Ta1-xMoxSe2 and Ta1-xWxSe2. The 3R polytype materials are again found to have substantially higher Tcs (~ 2 K for Ta0.9W0.1Se2 and Ta0.9Mo0.1Se2) than the 2H material …
The superconducting state of an infinitely long superconducting cylinder surrounded by a medium which enhances its superconductivity near the boundary is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be due to the proximity of another superconductor or due to surface treatment. Quantities like the free energy, the magnetization and the Cooper-pair density are calculated. Phase diagrams are obtained to investigate how the critical field and the critical temperature depend on this surface enhancement for different values of the Ginzburg-Landau parameter κ. Increasing …
Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the ma…
Superconductivity forms out of the condensation of Cooper pairs of electrons. The mechanism by which Cooper pairs are created in non-conventional superconductors is often elusive because experimental signatures that connect a specific pairing mechanism to the properties of superconducting state are rare. The recently discovered superconducting oxide-insulator/KTaO$_3$ interface may offer clues about its origins. Here we observe distinct dependences of the superconducting transition temperature Tc on carrier density n$_{2D}$ for electron gases formed at KTaO$_3$ (111), (001) and (110) interface…
We have systematically investigated the crystal structure and the magnetic properties of BiS2-based superconductor Ce1-xNdxO1-yFyBiS2 (x = 0 - 1.0, y = 0.3, 0.5 and 0.7) and Nd1-zSmzO1-yFyBiS2 (x = 0 - 0.8, y = 0.3, 0.5 and 0.7). In the REOBiS2 system, both crystal structure and physical properties are tunable by mixing the RE (RE = rare earth) site with Ce, Nd and Sm due to the difference of ionic radius of RE. In the Ce1-xNdxO1-yFyBiS2 system, bulk superconductivity is observed for x = 1.0 with y = 0.3 and x = 0.5 - 1.0 with y = 0.5. The transition temperature (Tc) increases with increasing …
Here we report the observation of extraordinary superconductivity in a pressurized commercial niobium-titanium alloy. We find that its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record high pressure up to which a known superconducting state can continuously survives. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (HC2) at 1.8 K has be…
Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin-orbi…
Using Monte Carlo techniques, we study a simple model which exhibits a competition between superconductivity and other types of order in two dimensions. The model is a site-diluted XY model, in which the XY spins are mobile, and also experience a repulsive interaction extending to one, two, or many shells of neighbors. Depending on the strength and range of the repulsion and spin concentration, the spins arrange themselves into a remarkable variety of patterns at low temperatures $T$, including phase separation, checkerboard order, and straight or labyrinthine patterns of stripes, which someti…
We have analysed the effect of intrinsic doping inhomogeneity and the presence of stripes in high-T_c superconductors on coupling λby using a simple analytically solvable model with an angular dependent λ(ϕ) represented by a square-well form. We have found that the introduction of the Coulomb repulsion λ_C, increasing the ''contrast'' | λ| + | λ_C |, or the depth of the angular modulation of λ(ϕ), leads to a remarkable enhancement of T_c. This effect can be optimized by combining attractive (λ<0) and repulsive (λ>0) interactions along stripes and perpendicular to them.
We propose that mass-imbalanced superconductivity is realized in an effective two-channel Kondo lattice, and its characteristic property appears in electromagnetic responses such as the Meissner effect. Starting from an effective two-channel Kondo lattice model as a low-energy effective theory, and approximating it with two mean-field order parameter components in a self-consistent fashion, it is shown that the balance of the two components is sensitively reflected in the magnitude of the Meissner kernel, while thermodynamic properties are little affected by the balance. This remarkable behavi…
We report the temperature-pressure (T-P) phase diagram of CePt2In7 single crystals, especially the pressure evolution of the antiferromagnetic order and the emergence of superconductivity, which have been studied by electrical resistivity and ac calorimetry under nearly hydrostatic environments. Compared with its polycrystalline counterpart, bulk superconductivity coexists with antiferromagnetism in a much narrower pressure region. The possible existence of textured superconductivity and local quantum criticality also are observed in CePt2In7, exhibiting a remarkable similarity with CeRhIn5.
Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO$_{2}$, the role of electronic correlations in driving superconductivity, and the possible relationship betweeen the cuprates and the nickelates are still open questions. Here, by comparing LaNiO$_2$ and NdNiO$_2$ systematically within a parameter-free density functional framework, all-electron first-principles framework, we reveal the role Nd 4$f$ elect…
We report the discovery of two-phase unconventional superconductivity in CeRh$_2$As$_2$. Using thermodynamic probes, we establish that the superconducting critical field of its high-field phase is as high as 14 T, remarkable in a material whose transition temperature is 0.26 K. Furthermore, a $c$-axis field drives a transition between two different superconducting phases. In spite of the fact that CeRh$_2$As$_2$ is globally centrosymmetric, we show that local inversion-symmetry breaking at the Ce sites enables Rashba spin-orbit coupling to play a key role in the underlying physics. More detail…
We report the effect of carbon doping in Ti-stabilized nonstoichiometric molybdenum diboride (Mo$_{0.96}$Ti$_{0.04}$)$_{0.8}$B$_{2}$, which exhibits bulk superconductivity below $T_{\rm c}$ = 7.0 K. It is found that (Mo$_{0.96}$Ti$_{0.04}$)$_{0.8}$(B$_{1-x}$C$_{x}$)$_{2}$ maintains the AlB$_{2}$-type phase with a uniform elemental distribution for $x$ = 0.12 and 0.16. The substitution of carbon for boron leads to a slight increase in $a$-axis, a remarkable reduction in $c$-axis, the formation of planar defects along the (100) crystallographic planes, and a shift of the B 1$s$ peaks towards hig…
Fermion systems with flat bands can boost superconductivity by enhancing the density of states at the Fermi level. We use quasiexact numerical methods to show that repulsive interactions between spinless fermions in a one-dimensional (1D) flat-band system, the Creutz ladder, give a finite pairing energy that increases with repulsion, though charge quasi-order (QO) remains dominant. Adding an attractive component shifts the balance in favor of superconductivity and the interplay of two flat bands further yields a remarkable enhancement of superconductivity, well outside of known paradigms for 1…
CeRh$_2$As$_2$, a non-symmorphic heavy fermion material, was recently reported to host a remarkable phase diagram with two superconducting phases. In this material, the two inequivalent Ce sites per unit cell, related by inversion symmetry, introduce a sublattice structure corresponding to an extra internal degree of freedom. Here we propose a classification of the possible superconducting states in CeRh$_2$As$_2$ from the two Ce-sites perspective. Based on the superconducting fitness analysis and the quasiclassical Eilenberger equations, we discuss two limits: Rashba spin-orbit coupling and i…
We study the properties of $s$-wave superconductivity induced around a nematic quantum critical point in two-dimensional metals. The strong Landau damping and the Cooper pairing between incoherent fermions have dramatic mutual influence on each other, and hence should be treated on an equal footing. This problem is addressed by analyzing the self-consistent Dyson-Schwinger equations for the superconducting gap and Landau damping rate. We solve the equations at zero temperature without making any linearization, and show that the superconducting gap is maximized at the quantum critical point and…
High-temperature iron-based superconductivity develops in a structure with unusual lattice-orbital geometry, based on a planar layer of Fe atoms with 3d orbitals and tetrahedrally coordinated by anions. Here we elucidate the electronic role of anions in the iron-based superconductors utilizing state-of-the-art scanning tunneling microscopy. By measuring the local electronic structure, we find that As anion in Ba0.4K0.6Fe2As2 has a striking impact on the electron pairing. The superconducting electronic feature can be switched off/on by removing/restoring As atoms on Fe layer at the atomic scale…
Time-reversal symmetry breaking superconductivity with electronic glass in nickelate (La, Pr, Sm)3Ni2O7 films
Peking University（北京大学）；Southern University of Science and Technology（南方科技大学）；Renmin University of China（中国人民大学）；ShanghaiTech University（上海科技大学）；Boston College（波士顿学院）；Tsinghua University（清华大学）
Topological phononic insulators are the counterpart of three-dimensional quantum spin Hall insulators in phononic systems and, as such, their topological surfaces are characterized by Dirac cone-shaped gapless edge states arising as a consequence of a bulk-boundary correspondence. We propose a theoretical framework for the possible superconducting phase in these materials, where the attractive interaction between electrons is mediated by topological phonons in nontrivial boundary modes. Within the BCS limit, we develop a self-consistent two-band gap equation, whose solutions show that the supe…
