Superconducting Phase Transition Research Articles

Pressure-Induced Superconductivity and Topological Quantum Phase Transitions in the Topological Semimetal ZrTe2.

Topological transition metal dichalcogenides (TMDCs) have attracted much attention due to their potential applications in spintronics and quantum computations. In this work, the structural and electronic properties of topological TMDCs candidate ZrTe2 are systematically investigated under high pressure. A pressure-induced Lifshitz transition is evidenced by the change of charge carrier type as well as the Fermi surface. Superconductivity is observed at around 8.3GPa without structural phase transition. A typical dome-shape phase diagram is obtained with the maximum Tc of 5.6K for ZrTe2 . Furthermore, the theoretical calculations suggest the presence of multiple pressure-induced topological quantum phase transitions, which coexists with emergence of superconductivity. The results demonstrate that ZrTe2 with nontrivial topology of electronic states displays new ground states upon compression.

Calorimetric evidence for two phase transitions in Ba1−xKxFe2As2 with fermion pairing and quadrupling states

Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (Tc). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z2 time-reversal symmetry was reported in Ba1−xKxFe2As2. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z2 time-reversal symmetry is broken ({T}_{{{{{{{{rm{c}}}}}}}}}^{{{{{{{{rm{Z2}}}}}}}}} , > , {T}_{{{{{{{{rm{c}}}}}}}}}). Here, we report on detecting two anomalies in the specific heat of Ba1−xKxFe2As2 at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating the breakdown of Z2 symmetry. The second anomaly at the lower temperature coincides with the transition to a zero-resistance state, indicating the onset of superconductivity. Our data provide the first example of the appearance of a specific heat anomaly above the superconducting phase transition associated with the broken time-reversal symmetry due to the formation of the novel fermion order.

300 mm CMOS-compatible superconducting HfN and ZrN thin films for quantum applications

The rising interest in increased manufacturing maturity of quantum processing units is pushing the development of alternative superconducting materials for semiconductor fab process technology. However, these are often facing CMOS process incompatibility. In contrast to common CMOS materials, such as Al, TiN, and TaN, reports on the superconductivity of other suitable transition-metal nitrides are scarce, despite potential superiority. Here, we demonstrate fully CMOS-compatible fabrication of HfN and ZrN thin films on state-of-the-art 300 mm semiconductor process equipment, utilizing reactive DC magnetron sputtering on silicon wafers. Measurement of mechanical stress and surface roughness of the thin films demonstrates process compatibility. We investigated the materials phase and stoichiometry by structural analysis. The HfN and ZrN samples exhibit superconducting phase transitions with critical temperatures up to 5.84 and 7.32 K, critical fields of 1.73 and 6.40 T, and coherence lengths of 14 and 7 nm, respectively. A decrease in the critical temperature with decreasing film thickness indicates mesoscopic behavior due to geometric and grain-size limitations. The results promise a scalable application of HfN and ZrN in quantum computing and related fields.

Oscillating paramagnetic Meissner effect and Berezinskii-Kosterlitz-Thouless transition in underdoped Bi2Sr2CaCu2O8+δ.

Superconducting phase transitions in two dimensions lie beyond the description of the Ginzburg-Landau symmetry-breaking paradigm for three-dimensional superconductors. They are Berezinskii-Kosterlitz-Thouless (BKT) transitions of paired-electron condensate driven by the unbinding of topological excitations, i.e. vortices. The recently discovered monolayers of layered high-transition-temperature ([Formula: see text]) cuprate superconductor Bi2Sr2CaCu2O8+δ (Bi2212) meant that this 2D superconductor promised to be ideal for the study of unconventional superconductivity. But inhomogeneity posed challenges for distinguishing BKT physics from charge correlations in this material. Here, we utilize the phase sensitivity of scanning superconducting quantum interference device microscopy susceptometry to image the local magnetic response of underdoped Bi2212 from the monolayer to the bulk throughout its phase transition. The monolayer segregates into domains with independent phases at elevated temperatures below [Formula: see text]. Within a single domain, we find that the susceptibility oscillates with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up to [Formula: see text]. The finite modulation period, as well as the broadening of the peaks when approaching [Formula: see text] from below, suggests well-defined vortices that are increasingly screened by the dissociation of vortex-antivortex plasma through a BKT transition. In the multilayers, the susceptibility oscillation differs in a small temperature regime below [Formula: see text], consistent with a dimensional crossover led by interlayer coupling. Serving as strong evidence for BKT transition in the bulk, we observe a sharp jump in phase stiffness and paramagnetism at small fields just below [Formula: see text]. These results unify the superconducting phase transitions from the monolayer to the bulk underdoped Bi2212, and can be collectively referred to as the BKT transition with interlayer coupling.

Saturating quantum efficiency of SNSPDs with disorder manipulation of NbN films

Quantum efficiency is one of the most important performance metrics for superconducting nanowire single-photon detectors (SNSPDs). Specifically, near-infrared NbN-SNSPDs with high quantum efficiency are extremely desirable in quantum communications. However, due to the high energy gap of NbN, it is difficult to achieve a saturated quantum efficiency. In this paper, we systematically investigated the primary determinants of fabricating highly saturated NbN-SNSPD by changing the stoichiometric ratio during the growth of NbN thin films. Through electron beam lithography and reactive ion etching processes, NbN-SNSPDs with a saturated quantum efficiency were fabricated. It is worth noting that the saturated quantum efficiency is observed to be closely related to the stoichiometric ratio of NbN films. Artificially increasing the disorder in NbN films can enhance the probability of superconducting phase transition during photon detection. Our work provides a consistently simple and effective method for the fabrication of highly efficient quantum devices, which is crucial for achieving higher precision in future quantum communications.

Ambipolar Superconductivity with Strong Pairing Interaction in Monolayer 1T'-MoTe2.

Gate tunable two-dimensional (2D) superconductors offer significant advantages in studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T'-MoTe2 monolayers with an intrinsic band gap of ∼7.3 meV using field effect doping. Despite large differences in the dispersion of the conduction and valence bands, superconductivity can be achieved easily for both electrons and holes. The onset of superconductivity occurs near 7-8 K for both charge carrier types. This temperature is much higher than that in bulk samples. Also the in-plane upper critical field is strongly enhanced and exceeds the BCS Pauli limit in both cases. Gap information is extracted using point-contact spectroscopy. The gap ratio exceeds multiple times the value expected for BCS weak-coupling. All of these observations suggest a strong enhancement of the pairing interaction.

Traveling discontinuity at the quantum butterfly front

We formulate a kinetic theory of quantum information scrambling in the context of a paradigmatic model of interacting electrons in the vicinity of a superconducting phase transition. We carefully derive a set of coupled partial differential equations that effectively govern the dynamics of information spreading in generic dimensions. Their solutions show that scrambling propagates at the maximal speed set by the Fermi velocity. At early times, we find exponential growth at a rate set by the inelastic scattering. At late times, we find that scrambling is governed by shock-wave dynamics with traveling waves exhibiting a discontinuity at the boundary of the light cone. Notably, we find perfectly causal dynamics where the solutions do not spill outside of the light cone.

Laser-induced structural modulation and superconductivity in SrTiO3

Under normal conditions, stoichiometric SrTiO3 is an excellent dielectric. It shows a structural phase transition, from cubic to tetragonal, below 105 K. In this structure, well separated domains hosting tetragonal phases with different long axes exist giving rise to the so-called X, Y, and Z domains. At very low temperatures, it becomes a quantum paraelectric in which local ferroelectric domains are found at the X, Y, and Z domain boundaries. Creation of oxygen vacancy in SrTiO3 makes it conducting with low carrier density which also undergoes an unconventional superconducting transition at sub-kelvin temperatures. We have created structural phase separation with clear domain boundaries (as in the X, Y, and Z domains) at room temperature on single crystals of SrTiO3 by irradiating the surface with high power density excimer laser pulses. We find that the domain boundaries are dominantly conducting, and the irradiated crystals undergo a superconducting phase transition below 180 mK indicating that the superconducting phase appears at the domain boundaries. This concurrence of local ferroelectricity and superconductivity in lightly doped SrTiO3 supports a ferroelectric fluctuation mediated Cooper pairing in the system. The results also point out the possibility of controlling ferroelectricity and superconductivity in functional electronic devices through surface engineering.

Excitonic insulator to superconductor phase transition in ultra-compressed helium

Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about 20 eV at ambient pressures. While the metallization pressure of helium has been accurately determined, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal regime (1 TPa = 10 Mbar). Here, we employ density functional theory and many-body perturbation calculations to fill up this knowledge gap. It is found that prior to reaching metallicity helium becomes an excitonic insulator (EI), an exotic state of matter in which electrostatically bound electron-hole pairs may form spontaneously. Furthermore, we predict metallic helium to be a superconductor with a critical temperature of ≈ 20 K just above its metallization pressure and of ≈ 70 K at 100 TPa. These unforeseen phenomena may be critical for improving our fundamental understanding and modeling of celestial bodies.

Renormalization formalism for superconducting phase transition with inner-Cooper-pair dynamics

As charge carrier of the macroscopic superconductivity, the Cooper pair is a composite particle of two paired electrons, which has both center-of-mass and inner-pair degrees of freedom. In most cases, these two different degrees of freedom can be well described by the macroscopic Ginzburg-Landau theory and the microscopic Bardeen-Cooper-Schrieffer (BCS) theory, respectively. Near the superconducting phase transition where the Cooper pair is fragile and unstable because of the small binding energy, there are non-trivial couplings between these two different degrees of freedom due to such as finite energy and/or momentum transfer. The non-trivial couplings make the original derivation of the Ginzburg-Landau theory from the BCS theory fail in principle as where these two different degrees of freedom should not be decoupled. In this article, we will present a renormalization formalism for an extended Ginzburg-Landau action for the superconducting phase transition where there is finite energy transfer between the center-of-mass and the inner-pair degrees of freedom of Cooper pairs. This renormalization formalism will provide a theoretical tool to study the unusual dynamical effects of the inner-pair time-retarded physics on the superconducting phase transition.

Pion-mediated Cooper pairing of neutrons: beyond the bare vertex approximation

In some quantum many particle systems, the fermions could form Cooper pairs by exchanging intermediate bosons. This then drives a superconducting phase transition or a superfluid transition. Such transitions should be theoretically investigated by using proper non-perturbative methods. Here we take the neutron superfluid transition as an example and study the Cooper pairing of neutrons mediated by neutral π-mesons in the low-density region of a neutron matter. We perform a non-perturbative analysis of the neutron–meson coupling and compute the pairing gap Δ, the critical density ρ c , and the critical temperature T c by solving the Dyson–Schwinger equation of the neutron propagator. We first carry out calculations under the widely used bare vertex approximation and then incorporate the contribution of the lowest-order vertex correction. This vertex correction is not negligible even at low densities and its importance is further enhanced as the density increases. The transition critical line on density-temperature plane obtained under the bare vertex approximation is substantially changed after including the vertex correction. These results indicate that the vertex corrections play a significant role and need to be seriously taken into account.

Lorentz violation emergence in a superconductivity phase transition scenario

In this letter, we use a condensation of topological defects mechanism to show how a Lorentz-violating (LV) term can emerge. The approach used here was originally developed to describe phase transitions due to a vortice proliferation in systems such as superconductors. First, as a consequence of our approach, we obtain a Podolsky-like LV term as a result of this work. Second, a condensation of topological defects in the theory restores the original Lorentz symmetric phase of the theory. The approach presented here can be seen as an early description of a mechanism to describe a phase transition between a Lorentz symmetric phase and a non-symmetric one. Besides the usage of this mechanism to show how LV terms can emerge, we also show how to extend the condensation mechanism to scalar theories. The condensation mechanism was originally designed for gauge theories. As a result, our scalar extension recovers the Ginzburg-Landau (GL) description of both regular and non-local superconductors. The GL description, in our approach, arises as a consequence of the condensation of topological defects.

Detection of electromagnetic phase transitions using a helical cavity susceptometer.

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic susceptibilities. To demonstrate its operation, we detected superconducting phase transition in Nb and YBa2Cu3O7-δ, metal-insulator transitions in V2O3, ferromagnetic transition in Gd, and magnetic field induced transformation in meta magnetic NiCoMnIn single crystals. This high sensitivity apparatus allows the detection of trace amounts of materials (10-9-cc) undergoing an electromagnetic transition in a very broad temperature (2-400K) and magnetic field (up to 90 kOe) ranges.

Effects of the intramolecular structural change on the superconducting phase of the TMTTF systems

The tetramethyltetrathiafulvalene (TMTTF) system is one of the key materials to reveal mechanisms for unconventional superconductivity which is driven by electronic correlation. Recent experimental studies show that only the SbF6 salt, which exhibit the strongest charge order among the PF6 and AsF6 and SbF6 salts have anomalously high superconducting phase transition pressure for the insulating-superconducting phase and superconducting-metallic phase transitions. To investigate the relationship between the superconductivity and the strength of the charge order of these salts, we constructed an extended Hubbard model with the periodic boundary condition that takes into account the site-deformation potential driven by the intramolecular structural change which causes the nonuniform charge order. The superconductivity was examined by the Drude weight and the charge gap obtained by exact diagonalization. It was found that site-deformation potentials driven by the intramolecular conformational change, even small changes, move the insulator-superconductor transition point to higher pressure, i.e. too large transfer integrals. Without the site-deformation potentials, the transfer integral at the transition point is too small as compared with those values deduced from the first-principles calculations at the structures under atmospheric pressure and it is difficult to reproduce the transition.

Sweeping across the BCS-BEC crossover, reentrant, and hidden quantum phase transitions in two-band superconductors by tuning the valence and conduction bands

Two-band electronic structures with a valence and a conduction band separated by a tunable energy gap and with pairing of electrons in different channels can be relevant to investigate the properties of two-dimensional multiband superconductors and electron-hole superfluids, as monolayer FeSe, recently discovered superconducting bilayer graphene, and double-bilayer graphene electron-hole systems. This electronic configuration allows also to study the coexistence of superconductivity and charge density waves in connection with underdoped cuprates and transition metal dichalcogenides. By using a mean-field approach to study the system above mentioned, we have obtained numerical results for superconducting gaps, chemical potential, condensate fractions, coherence lengths, and superconducting mean-field critical temperature, considering a tunable band gap and different filling of the conduction band, for parametric choice of the pairing interactions. By tuning these quantities, the electrons redistribute among valence and conduction band in a complex way, leading to a new physics with respect to single-band superconductors, such as density induced and band-selective BCS-BEC crossover, quantum phase transitions, and hidden criticalities. At finite temperature, this phenomenon is also responsible for the non-monotonic behavior of the superconducting gaps resulting in a superconducting-normal state reentrant transition, without the need of disorder or magnetic effects.

Universal time-dependent Ginzburg-Landau theory

We study the hydrodynamics of superconductors within the framework of Schwinger-Keldysh effective field theory (EFT). We show that in the vicinity of the superconducting phase transition the most general leading-order EFT satisfying the local Kubo-Martin-Schwinger condition is described by a version of the time-dependent Ginzburg-Landau (TDGL) equations augmented with stochastic terms. This version of TDGL is applicable in the gapless regime independent of any microscopic details. Within this approach, it is possible to include systematically the effects of nonuniform temperature and heat conductivity, as well as explicit or spontaneous breaking of time reversal. We also introduce a thermal version of the Josephson relation and use it to construct an exotic hydrodynamics describing a phase of matter where heat can flow without dissipation.

The mixed-state entanglement in holographic p-wave superconductor model

In this paper, we investigate the mixed-state entanglement in a model of p-wave superconductivity phase transition using holographic methods. We calculate several entanglement measures, including holographic entanglement entropy (HEE), mutual information (MI), and entanglement wedge cross-section (EWCS). Our results show that these measures display critical behavior at the phase transition points, with the EWCS exhibiting opposite temperature behavior compared to the HEE. Furthermore, we explore the behavior of thermodynamics and holographic quantum information at the zeroth-order phase transition point and find that it is opposite to that observed in the first-order phase transition. Additionally, we find that the critical exponents of all entanglement measures are twice those of the condensate. Our findings also suggest that the EWCS is a more sensitive indicator of the critical behavior of phase transitions than the HEE. Lastly, we uncover a universal inequality in the growth rates of EWCS and MI near critical points in thermal phase transitions, such as p-wave and s-wave superconductivity, suggesting that MI captures more information than EWCS when a phase transition first occurs.

Fluctuation-driven excess noise near superconducting phase transition

We discuss intrinsic mechanisms of nonequilibrium excess noise in superconducting devices and transition edge sensors. In particular, we present an overview of fluctuation-driven contributions to the current noise in the vicinity of the superconducting transition. We argue that sufficiently close to the critical temperature fluctuations of conductivity may become correlated provided that the rate of quasiparticle relaxation is slow as compared to dynamics of superconducting fluctuations. In this regime, fluctuations of conductivity adiabatically follow the fluctuations of the electron distribution. This leads to a substantial enhancement of current noise. The corresponding spectral power density of noise has a Lorentzian shape in the frequency domain while its magnitude scales proportionally to the inelastic relaxation time. It also sensitively depends on the dephasing and Ginzburg–Landau timescales. Further estimates suggest that this mechanism dominates over the conventional temperature fluctuations in the same range of parameters. To describe these effects microscopically, we use the nonequilibrium Keldysh technique in the semiclassical approximation of superconductivity with Boltzmann–Langevin random forces to account for correlations of fluctuations.

Bulk Synthesis and Transport Properties of Rocksalt-Type Ti1–xMgxN Solid Solution

Owing to the decomposition issue of Mg3N2, many Mg-containing ternary nitrides were prepared by the hybrid arc evaporation/sputtering technique, which has the advantages including access to the unstable phases, high film purity, good density, and uniform film formation but the drawbacks of cost and long production cycle for the required targets. In the present study, we demonstrate that rocksalt-type Ti1-xMgxN, previously prepared exclusively by the thin-film methods, can be obtained as a disordered cubic phase by the conventional bulk synthesis method through a facile one-step reaction. Employing a combination of experimental measurements and theoretical calculations, we discover that the crystal structure and the physical properties of the as-synthesized Ti1-xMgxN solid solution can be tuned by the Mg content; a metal-to-semiconductor transition and also suppression of the superconducting phase transition are observed when the Mg and Ti content ratio increases to close to 1. Theoretical calculations indicate that the lattice distortions in the disordered Ti1-xMgxN induced by the different ionic sizes of Mg and Ti increase with the Mg content and the disordered cubic rocksalt structures become unstable. The ordered rocksalt-derived structures are more stable than the disordered rocksalt structures on composition x = 0.5. Furthermore, electronic structure calculations provide an insight into the low resistance behavior and transport property evolution of Ti1-xMgxN from the aspects of Ti3+ content, the cation distribution, or nitrogen defects. The results highlight the feasibility of the simple bulk route for the successful synthesis of Mg-containing ternary nitrides and the heterovalent ion substitution on modulating the properties of nitrides.

Electric Control of 2D Van Hove Singularity in Oxide Ultra-Thin Films.

Divergent density of states (DOS) can induce extraordinary phenomena such as significant enhancement of superconductivity and unexpected phase transitions. Moreover, van Hove singularities (VHSs) lead to divergent DOS in 2D systems. Despite recent interest in VHSs, only a few controllable cases have been reported to date. In this work, by utilizing an atomically ultra-thin SrRuO3 film, the electronic structure of a 2D VHS is investigated with angle-resolved photoemission spectroscopy and transport properties are controlled. By applying electric fields with alkali metal deposition and ionic-liquid gating methods, the 2D VHS and the sign of the charge carrier are precisely controlled. Use of a tunable 2D VHS in an atomically flat oxide film could serve as a new strategy to realize infinite DOS near the Fermi level, thereby allowing efficient tuning of electric properties.