Non-superconducting State Research Articles

Magnetic field expulsion in optically driven YBa2Cu3O6.48.

Coherent optical driving in quantum solids is emerging as a research frontier, with many reports of interesting non-equilibrium quantum phases1-4 and transient photo-induced functional phenomena such as ferroelectricity5,6, magnetism7-10 and superconductivity11-14. In high-temperature cupratesuperconductors, coherent driving of certain phonon modes has resulted in a transient state with superconducting-like optical properties, observed far above their transition temperatureTc and throughout the pseudogap phase15-18. However, questions remain on the microscopic nature of this transient state and how to distinguish it from a non-superconducting state with enhanced carrier mobility. For example, it is not known whether cuprates driven in this fashion exhibit Meissner diamagnetism. Here we examine the time-dependent magnetic field surrounding an optically driven YBa2Cu3O6.48 crystal by measuring Faraday rotation in a magneto-optic material placed in the vicinity of the sample. For a constant applied magnetic field and under the same driving conditions that result in superconducting-like optical properties15-18, a transient diamagnetic response was observed. This response is comparable in size with that expected in an equilibrium type II superconductor of similar shape and size with a volume susceptibility χv of order -0.3. This value is incompatible with a photo-induced increase in mobility without superconductivity. Rather, it underscores the notion of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the drive.

Unusual metallic state in superconducting A15-type La4H23.

Hydride superconductors continue to fascinate the communities of condensed matter physics and material scientists because they host the promising near room-temperature superconductivity. Current research has concentrated on the new hydride superconductors with the enhancement of the superconducting transition temperature (T c). The multiple extreme conditions (high pressure/temperature and magnetic field) will introduce new insights into hydride superconductors. The study of transport properties under very high magnetic fields facilitates the understanding of superconductivity in conventional hydride superconductors. In the present work, we report experimental evidence of an unusual metal state in a newly synthesized cubic A15-type La4H23 that exhibits superconductivity with a T c reaching 105K at 118GPa. A large negative magnetoresistance is observed in strong pulsed magnetic fields in the non-superconducting state of this compound below 40K. Moreover, we construct the full magnetic phase diagram of La4H23 up to 68 T at high pressure. The present work reveals anomalous electronic structural properties of A15-La4H23 under high magnetic fields, and therefore has great importance with regard to advancing the understanding of quantum transport behaviors in hydride superconductors.

Observation and quantification of the pseudogap in unitary Fermi gases.

The microscopic origin of high-temperature superconductivity in cuprates remains unknown. It is widely believed that substantial progress could be achieved by better understanding of the pseudogap phase, a normal non-superconducting state of cuprates1,2. In particular, a central issue is whether the pseudogap could originate from strong pairing fluctuations3. Unitary Fermi gases4,5, in which the pseudogap-if it exists-necessarily arises from many-body pairing, offer ideal quantum simulators to address this question. Here we report the observation of a pair-fluctuation-driven pseudogap in homogeneous unitary Fermi gases of lithium-6 atoms, by precisely measuring the fermion spectral function through momentum-resolved microwave spectroscopy and without spurious effects from final-state interactions. The temperature dependence of the pairing gap, inverse pair lifetime and single-particle scattering rate are quantitatively determined by analysing the spectra. We find a large pseudogap above the superfluid transition temperature. The inverse pair lifetime exhibits a thermally activated exponential behaviour, uncovering the microscopic virtual pair breaking and recombination mechanism. The obtained large, temperature-independent single-particle scattering rate is comparable with that set by the Planckian limit6. Our findings quantitatively characterize the pseudogap in strongly interacting Fermi gases and they lend support for the role of preformed pairing as a precursor to superfluidity.

A novel heater-triggered HTS semi-persistent current switch with high switching-off resistance

A semi-persistent current switch (semi-PCS) with a simple fabrication process, high switching-off resistance, and minimal power dissipation during operation is required to realize the semi-persistent mode in a high-temperature superconducting (HTS) magnet. In this study, a semi-PCS with high switching-off resistance was developed by optimizing the HTS tape structure in a heater-triggered semi-PCS. This optimization was achieved by selectively removing a portion of the conductive and superconducting layers on the surface and edges of the HTS tapes. The microstructure of the treated HTS tape was then analyzed. The resistance of the semi-PCS in a non-superconducting state was measured, and it was connected to an insulated coil for performance testing. To further evaluate its impact, an equivalent electrical circuit model was developed and used to predict the effect of the semi-PCS when applied to a larger superconducting coil. Furthermore, the study explored the impact of the excitation rates on the power dissipation and heat generation of the semi-PCS.

High-temperature superconductivity and strange metallicity: Simple observations with (possibly) profound implications

Bednorz and Müller's discovery of superconductivity in the cuprate perovskite La2-xBaxCuO4 gave the modern condensed matter community arguably two of its greatest gifts – high-temperature superconductivity and strange metallicity; certainly two of its most profound and challenging problem. While it was the multi-fold increase in Tc that rightly catapulted cuprates to the summit of global media attention and scientific curiosity, it is the nature of their normal (non-superconducting) state that has sustained interest over the intervening years. This in turn has inspired condensed-matter experimentalists and theoreticians to revolutionise their respective fields. Yet despite such boundary-pushing endeavors, the origin of both phenomena remains elusive. In this brief note, I offer a personal reflection on the link between strange metallicity and high-Tc superconductivity, highlighting a number of simple observations that may hold profound implications for their ultimate resolution.

Microstructure, magnetoresistivity and carrier mobility of HTSC-YBCO tape with pinning centers generated by electron irradiation

Artificial pinning center lattice formed in HTSC both chemically and by irradiation is believed to increase critical current and field although Abrikosov vortex core is in the normal state. The paper presents SEM-EDS, XRD and Hall effect (80-300 K at magnetic field 0.55 Tesla) data in YBCO microfilm on 276-steel tape with metal coating exposed to 5 MeV electrons (1014 cm-2 at 400 nA) in air. No long-living radioisotope was generated. This irradiation resulted in structure modification of microinterfaces YBCO-AgCu, ten times decrease in the magneto-resistance >T c, increase in the 2nd type phase transition steep. The charge carrier mobility μ decreased by a few orders of value, especially at T <100 K and 280 K, where Cooper pairs and magnons condensate, respectively. Within 80-300 K the tape is in mixed magnetic states of YBCO and steel substrate, thereby providing effective flux pinning by highly correlated non-superconducting state.

Non-Fermi-Liquid Behavior of Superconducting SnH4.

The chemical interaction of Sn with H2 by X-ray diffraction methods at pressures of 180-210GPa is studied. A previously unknown tetrahydride SnH4 with a cubic structure (fcc) exhibiting superconducting properties below TC = 72 K is obtained; the formation of a high molecular C2/m-SnH14 superhydride and several lower hydrides, fcc SnH2 , and C2-Sn12 H18 , is also detected. The temperature dependence of critical current density JC (T) in SnH4 yields the superconducting gap 2Δ(0)= 21.6 meV at 180GPa. SnH4 has unusual behavior in strong magnetic fields: B,T-linear dependences of magnetoresistance and the upper critical magnetic field BC2 (T) ∝ (TC - T). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of fcc SnH4 in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Grüneisen model and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

Development of 50-kW-Class High-Temperature Superconducting Induction/Synchronous Motor With Continuous Drive Characteristics from Room Temperature

We investigate the rotational characteristics of a 50-kW-class high-temperature superconducting induction/synchro- nous motor in superconducting and non-superconducting states. The prototype motor was first placed in liquid nitrogen, and the efficiency contour was obtained. The liquid nitrogen was then gradually removed from the motor under the partial load (10 kW) condition. This showed that continuous operation is possible even at temperatures exceeding 130 K. Furthermore, we succeeded in driving at 11% of the rated output, even at room temperature. As a result, a practical high-temperature superconducting motor that can continue to operate with reduced output even when the cooling system fails could be realized.

The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca10(Pt4As8)((Fe0.97Pt0.03)2As2)5 superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A□) gradually diminishes. At a critical pressure, A□ and superconducting transition temperature (Tc) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A□ with Tc obeys the relation of Tc ~ (A□)0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-Tc superconductivity.

Ferroelectric polarization of Pb0.92La0.08(Zr0.52Ti0.48)O3 modulating the electrical properties of YBa2Cu3O7−ẟ

Pb0.92La0.08(Zr0.52Ti0.48)O3/YBa2Cu3O7−ẟ (PLZT/YBCO) heterostructure was prepared via the photosensitive sol–gel method (P-SG) combined with pulsed laser deposition (PLD). Test results show that the preparation process of PLZT film does not significantly cause degradation of the superconductivity of underlying YBCO film, and the PLZT has good ferroelectric properties in the temperature range of 300–50 K. With decreasing temperature, ferroelectric domain formation and inversion become more difficult, which causes coercive field (Ec) of PLZT to increase. However, remanent polarization (Pr) remains almost constant with decreasing temperature. Furthermore, it is found that the critical transition temperature (Tc) of YBCO decreases and resistance of non-superconducting state of YBCO increases as positive electric field applied to PLZT increases, while Tc and the resistance of non-superconducting state of YBCO do not change when negative electric fields are applied. It is believed that polarization charges regulate Fermi level of PLZT, which results in the promotion (hindrance) of electron injection into YBCO for positive (negative) applied electric fields, thereby regulating Tc and the resistance of non-superconducting state of YBCO. When positive applied electric fields are removed, the superconducting properties of YBCO in PLZT/YBCO structure cannot be restored to their initial values due to the remanent polarization of PLZT.

Possible Raman signature of broken symmetry states near the quantum critical point in P doped BaFe[formula omitted]As[formula omitted]: Experiment and theory

Single crystals of the iron-pnictide compound BaFe2(As1−xPx)2 for x∼0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram, have been studied. Transport measurements confirm the presence of a magneto-structural transition at 60 K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16 K. Micro-Raman spectra of the compound exhibit a phonon mode at 211 cm−1 and two broad modes (BMs) at 515 cm−1 and 635 cm−1, which corresponds to an energy difference of 15 meV. The temperature dependent Raman measurements reveal that the intensity of these BMs drop over the temperature range between 80 K and 300 K. In the non-superconducting state, strong anisotropic behaviour is observed for these BMs in polarization dependent Raman scattering. Electronic structure calculations for doped and undoped BaFe2As2 show that while Fe dxz and dyz orbitals do not split in the tetragonal phase, the splitting energy is 13.5 meV in the orthorhombic phase of the doped system, which is reasonably close to the experimentally observed value of the energy separation of the BMs. We believe that the reported BMs are consistent with the signature of electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2＋ due to local breaking of the C4 symmetry in the doped system.

Signatures of a Quantum Griffiths Phase Close to an Electronic Nematic Quantum Phase Transition.

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe_{0.89}S_{0.11} using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the nonsuperconducting normal state that features a continuously varying dynamical exponent over a large temperature range. We interpret our results in terms of a quantum Griffiths phase caused by nematic islands that result from the local distribution of Se and S atoms. At low temperatures, the Griffiths phase is masked by the emergence of a Fermi liquid phase due to a strong nematoelastic coupling and a Lifshitz transition that changes the topology of the Fermi surface.

Observation of nearly identical superconducting transition temperatures in the pressurized Weyl semimetals MIrTe4 ( M=Nb and Ta)

Here we report the observation of pressure-induced superconductivity in type-II Weyl semimetal (WSM) candidate NbIrTe4 and the evolution of its Hall coefficient (RH), magnetoresistance (MR), and lattice with increasing pressure to ~57 GPa. These results provide a significant opportunity to investigate the universal high-pressure behavior of ternary WSMs, including the sister compound TaIrTe4 that has been known through our previous studies. We find that the pressure-tuned evolution from the WSM to the superconducting (SC) state in these two compounds exhibit the same trend, i.e., a pressure-induced SC state emerges from the matrix of the non-superconducting WSM state at ~ 27 GPa, and then the WSM state and the SC state coexist up to 40 GPa. Above this pressure, an identical high-pressure behavior, characterized by almost the same value of RH and MR in its normal state and the same value of Tc in its SC state, appears in both compounds. Our results not only reveal a universal connection between the WSM state and SC state, but also demonstrate that NbIrTe4 and TaIrTe4 can make the same contribution to the normal and SC states that inhabit in the high-pressure phase, although these two compounds have dramatically different topological band structure at ambient pressure.

Experimental realization of an all-(RE)BaCuO hybrid trapped field magnet lens generating a 9.8 T concentrated magnetic field from a 7 T external field

In this work, we have verified experimentally an all-(RE)BaCuO hybrid trapped field magnet lens (HTFML) using only one cryocooler and a special technique named the ‘loose contact method’. In the experimental setup, only the inner magnetic lens was tightly connected to the cold stage and cooled at all times, and the outer trapped field magnet (TFM) cylinder was loosely connected to the cold stage before the magnetizing process by introducing a gap between the outer TFM and cold stage of the cryocooler. As a result, the superconducting state for zero-field cooled magnetization of the inner magnetic lens and the non-superconducting (normal) state for field-cooled magnetization of the outer TFM cylinder can co-exist at the same time. A maximum concentrated field of B c = 9.8 T was achieved for the magnetizing process with an applied field of B app = 7 T in the present HTFML, consistent with the numerical estimation in our previous conceptual study. These results validate the HTFML concept as a compact and desktop-type magnet device that can provide 10 T-class magnetic field enhancement from the viewpoint of the magnetizing method. However, during magnetization with a higher B app of 10 T, thermal instability of the outer stacked TFM cylinder caused flux jumps to occur, resulting in mechanical fracture of multiple bulks. These results suggest that the further development of a practical cooling method that can realize a stable and controllable cooling process for each part of the HTFML is necessary based on fundamental studies relating to the thermal stability of the large stacked TFM cylinder.

Reemergence of superconductivity in pressurized quasi-one-dimensional superconductor K2Mo3As3

Here we report a pressure-induced reemergence of superconductivity in recently discovered superconductor K2Mo3As3, which is the first experimental case observed in quasi-one-dimensional superconductors. We find that, after full suppression of the ambient-pressure superconducting (SC-I) state at 8.7 GPa, an intermediary non-superconducting state sets in and prevails to the pressure up to 18.2 GPa, however, above this pressure a new superconducting (SC-II) state appears unexpectedly. High pressure x-ray diffraction measurements demonstrate that the pressure-induced dramatic change of the lattice parameter c contributes mainly to the emergence of the SC-II state. Combined with the theioretical calculations on band strcture, our results suggest that the reemergemce of superconductivity is associated with the change of the complicated interplay among different orbital electrons, driven by the pressure-induced unisotropic change of the lattice.

Signatures of a quantum Griffiths phase close to an electronic nematic quantum phase transition

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe0.89S0.11 using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the non-superconducting normal state which features a continuously varying dynamical exponent over a large temperature range. We interpret our results in terms of a quantum Griffiths phase caused by nematic islands that result from the local distribution of Se and S atoms. At low temperatures, the Griffiths phase is masked by the emergence of a Fermi liquid phase due to a strong nematoelastic coupling and a Lifshitz transition that changes the topology of the Fermi surface.

Origin of pseudo gap in high-TC and Iron based superconductors

Many high-Tc superconductors, Iron based superconductors and other recently discovered superconducting materials shows the signature of existence of a strange Pseudo Gap above the superconducting transition temperature. Various experimental techniques such as NMR, Tunneling and ARPES suggest that pseudo gap does not change with temperature. As evidenced from these experimental results, the Pseudo gap possess the d-wave pairing symmetry similar to superconducting energy gap. The physical properties shows unusual behavior in the non-superconducting state where pseudo gap exist. Based on experimental evidences we argue about the origin of pseudo gap in these superconductors. Although a complete understanding of mechanism is lacking, the existence of preformed cooper indicated by various experimental probes pairs seems to be responsible for the origin of Pseudo gap.

Stability and electronic properties of LaNiO2/SrTiO3 heterostructures

Infinite-layer nickelate thin films materialize an intriguing new platform for high-temperature unconventional superconductivity, with LaNiO2/SrTiO3 as reference setup. We discuss the relative stability of the elementary interfaces of this system and determine the corresponding electronic band structure. We find substantial changes compared to the bulk, in particular in relation to the 5d orbital contributions to the low-energy physics which can be totally replaced by purely Ni-3d flat bands. The d9 configuration characteristic of cuprates can thus be supplemented by an extra interfacial ingredient destabilizing the normal non-superconducting state in these heterostructures.

Non-destructive characterisation of dopant spatial distribution in cuprate superconductors

Proper characterisation of investigated samples is vital when studying superconductivity as impurities and doping inhomogeneities can affect the physical properties of the measured system. We present a method where a polarised neutron imaging setup utilises the precession of spin-polarised neutrons in the presence of a trapped field in the superconducting sample to spatially map out the critical temperature for the phase transition between superconducting and non-superconducting states. We demonstrate this method on a superconducting crystal of the prototypical high-temperature superconductor (La,Sr)2CuO4. The results, which are backed up by complementary magnetic susceptibility measurements, show that the method is able to resolve minor variations in the transition temperature across the length of the LSCO crystal, caused by inhomogeneities in strontium doping.

A comprehensive ARPES study on the type-II Dirac semimetal candidate Ir1−xPtxTe2

The transition metal dichalcogenide Ir1−xPtxTe2 displays both superconductivity and a topological band structure. Using angle-resolved photoemission spectroscopy, we obtain a comprehensive understanding of the three-dimensional electronic structure in the normal state of Ir1−xPtxTe2 for doping levels from x = 0.1 to 0.4, which spans the composition range of a superconducting state to a non-superconducting state. Many features of the electronic structure can be attributed to strong Te–Te interactions between the layers of the layered crystal structure and can be resolved by photon energy dependent measurements. We demonstrate that the type-II Dirac fermions can be successfully tuned via Pt doping, where the Dirac point lies close to the Fermi level for x = 0.1. The band evolution vs doping provides a clearer understanding of the relationship between the superconductivity and electronic structure. In addition, the β band in the superconducting samples locates the system close to a type-II van Hove singularity, where spin triplet paring symmetry has been predicted. Our results provide a comprehensive understanding of the band structure of Ir1−xPtxTe2, and we discuss the possibilities of the existence of topological superconductivity in this system.