Zero-resistance State Research Articles

Two-band superconductivity in Pt-intercalated ZrTe2, a Dirac semimetal

This study presents compelling evidence of superconductivity with 3.5 K onset and zero-resistance state at 2.25 K in the Dirac semimetal compound ZrTe2, upon intercalation with platinum in the van der Waals gap. The temperature dependence of the upper critical field and the normalized superfluid density can be modeled effectively using a two-gap model, consistent with the possibility of topological superconductivity.

Collapse of metallicity and high-Tc superconductivity in the high-pressure phase of FeSe0.89S0.11

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressure-temperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. This high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities.

Enhanced Superconductivity in 2H-TaS2 Devices through in Situ Molecular Intercalation.

The intercalation of guest species into the gap of van der Waals materials often leads to the emergence of intriguing phenomena such as superconductivity. While intercalation-induced superconductivity has been reported in several bulk crystals, reaching a zero-resistance state in flakes remains challenging. Here, we show a simple method for enhancing the superconducting transition in tens-of-nanometers thick 2H-TaS2 crystals contacted by gold electrodes through in situ intercalation. Our approach enables measuring the electrical characteristics of the same flake before and after intercalation, permitting us to precisely identify the effect of the guest species on the TaS2 transport properties. We find that the intercalation of amylamine molecules into TaS2 flakes causes a suppression of the charge density wave and an increase in the superconducting transition with an onset temperature above 3 K. Additionally, we show that a fully developed zero-resistance state can be achieved in flakes by engineering the conditions of the chemical intercalation. Our findings pave the way for the integration of chemically tailored intercalation compounds in scalable quantum technologies.

Orphan high field superconductivity in non-superconducting uranium ditelluride.

Reentrant superconductivity is an uncommon phenomenon in which the destructive effects of magnetic field on superconductivity are mitigated, allowing a zero-resistance state to survive under conditions that would otherwise destroy it. Typically, the reentrant superconducting region derives from a zero-field parent superconducting phase. Here, we show that in UTe2 crystals extreme applied magnetic fields give rise to an unprecedented high-field superconductor that lacks a zero-field antecedent. This high-field orphan superconductivity exists at angles offset between 29o and 42o from the crystallographic b to c axes with applied fields between 37 T and 52 T. The stability of field-induced orphan superconductivity presented in this work defies both empirical precedent and theoretical explanation and demonstrates that high-field superconductivity can exist in an otherwise non-superconducting material.

Coherent states in microwave-induced resistance oscillations and zero resistance states

We investigate irradiated high-mobility two-dimensional electron systems (2DES) under low or moderated magnetic fields. These systems present microwave-induced magnetoresistance oscillations (MIRO) which, as we demonstrate, reveal the presence of coherent states of the quantum harmonic oscillator. We also show that the principle of minimum uncertainty of coherent states is at the heart of MIRO and zero-resistance states (ZRS). Accordingly, we are able to explain, based on coherent states, important experimental evidence of these photo-oscillations, such as their physical origin, their periodicity with the inverse of the magnetic field and their peculiar oscillations minima and maxima positions in regards of the magnetic field. Thus, remarkably enough, we come to the conclusion that 2DES, under low magnetic fields, become a system of quasiclassical states or coherent states and MIRO would be the smoking gun of the existence of these peculiar states in these systems. Published by the American Physical Society 2024

Pressure-Induced Superconductivity In Polycrystalline La3Ni2O7−δ

We synthesized polycrystalline La3Ni2O7−δ (δ≈0.07) samples by using the sol-gel method without postannealing under high oxygen pressure, and then measured temperature-dependent resistivity under various hydrostatic pressures up to 18 GPa by using the cubic anvil and two-stage multianvil apparatus. We find that the density-wave-like anomaly in resistivity is progressively suppressed with increasing pressure and the resistivity drop corresponding to the onset of superconductivity emerges at pressure as low as ∼6 GPa. Zero resistivity is achieved at 9 GPa below Tczero≈6.6 K, which increases quickly with pressure to 41 K at 18 GPa. However, the diamagnetic response was not detected in the ac magnetic susceptibility measurements up to 15 GPa, indicating a filamentary nature of the observed superconductivity in the studied pressure range. The constructed T−P phase diagram reveals an intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. The observation of zero-resistance state in the polycrystalline La3Ni2O7−δ samples under high pressures not only corroborates the recent report of superconductivity in the pressurized La3Ni2O7 crystals but also facilitates further studies on this emerging family of nickelate high-Tc superconductors. Published by the American Physical Society 2024

Zero-Resistance State for a Potential High-Temperature Superconducting Nickelate

Zero-Resistance State for a Potential High-Temperature Superconducting Nickelate

Synthesis and superconductivity in yttrium-cerium hydrides at high pressures

Further increasing the critical temperature and/or decreasing the stabilized pressure are the general hopes for the hydride superconductors. Inspired by the low stabilized pressure associated with Ce 4f electrons in superconducting cerium superhydride and the high critical temperature in yttrium superhydride, we carry out seven independent runs to synthesize yttrium-cerium alloy hydrides. The synthetic process is examined by the Raman scattering and X-ray diffraction measurements. The superconductivity is obtained from the observed zero-resistance state with the detected onset critical temperatures in the range of 97-141 K. The upper critical field towards 0 K at pressure of 124 GPa is determined to be between 56 and 78 T by extrapolation of the results of the electrical transport measurements at applied magnetic fields. The analysis of the structural data and theoretical calculations suggest that the phase of Y0.5Ce0.5H9 in hexagonal structure with the space group of P63/mmc is stable in the studied pressure range. These results indicate that alloying superhydrides indeed can maintain relatively high critical temperature at relatively modest pressures accessible by laboratory conditions.

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.

Direct evidence of high Tc superconductivity in glossy carbon films

Recent achievements and predictions in the field of intrinsic superconductivity in column IV-based covalent semiconductors and the nature of coupling, which determines the prospects for higher transition temperatures as well as the influence of disorder, are debated issues that can determine the future of the field. With this context, here we report the experimental findings of intrinsic superconductivity in glossy carbon (GC) films. These films are one of the forms of carbon materials obtained by pyrolysis of Pongamia glabra oil under high pressure and temperature. A zero-resistance state for films was observed at a critical Zero Resistance temperature of 134 K. Such relatively high superconducting critical temperature of films puts it among the superconductors with the strongest pairing strength between electrons and therefore an ideal material for investigations of high critical temperature superconductors.

Inhomogeneous Superconductivity Onset in FeSe Studied by Transport Properties

Heterogeneous superconductivity onset is a common phenomenon in high-Tc superconductors of both the cuprate and iron-based families. It is manifested by a fairly wide transition from the metallic to zero-resistance states. Usually, in these strongly anisotropic materials, superconductivity (SC) first appears as isolated domains. This leads to anisotropic excess conductivity above Tc, and the transport measurements provide valuable information about the SC domain structure deep within the sample. In bulk samples, this anisotropic SC onset gives an approximate average shape of SC grains, while in thin samples, it also indicates the average size of SC grains. In this work, both interlayer and intralayer resistivity were measured as a function of temperature in FeSe samples of various thicknesses. To measure the interlayer resistivity, FeSe mesa structures oriented across the layers were fabricated using FIB. As the sample thickness decreases, a significant increase in superconducting transition temperature Tc is observed: Tc raises from 8 K in bulk material to 12 K in microbridges of thickness ∼40 nm. We applied analytical and numerical calculations to analyze these and earlier data and find the aspect ratio and size of the SC domains in FeSe consistent with our resistivity and diamagnetic response measurements. We propose a simple and fairly accurate method for estimating the aspect ratio of SC domains from Tc anisotropy in samples of various small thicknesses. The relationship between nematic and superconducting domains in FeSe is discussed. We also generalize the analytical formulas for conductivity in heterogeneous anisotropic superconductors to the case of elongated SC domains of two perpendicular orientations with equal volume fractions, corresponding to the nematic domain structure in various Fe-based superconductors.

Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport

The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical temperature superconductivity, in light of the similarities between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd 5d-Ni 3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we report about Nd1-x Sr x NiO2 (x = 0, 0.05 and 0.2) thin films synthesized with and without a SrTiO3 (STO) capping-layer, showing very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance (AMR) measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped STO-capped and uncapped samples. The results point at a key role of the capping-layer in controlling the magnitude and anisotropy of the AMR. We discuss this control in terms of a combined effect between the Nd 5d-Ni 3d hybridization and an intra-atomic Nd 4f-Nd 5d exchange coupling. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

Observation of Γ -Valley Moiré Bands and Emergent Hexagonal Lattice in Twisted Transition Metal Dichalcogenides

Twisted van der Waals heterostructures have recently been proposed as a condensed-matter platform for realizing controllable quantum models due to the low-energy moir\'e bands with specific charge distributions in moir\'e superlattices. Here, combining angle-resolved photoemission spectroscopy with sub-micron spatial resolution ({\mu}-ARPES) and scanning tunneling microscopy (STM), we performed a systematic investigation on the electronic structure of 5.1{\deg} twisted bilayer WSe2 that hosts correlated insulating and zero-resistance states. Interestingly, contrary to one's expectation, moir\'e bands were observed only at {\Gamma}-valley but not K-valley in {\mu}-ARPES measurements; and correspondingly, our STM measurements clearly identified the real-space honeycomb- and Kagome-shaped charge distributions at the moir\'e length scale associated with the {\Gamma}-valley moir\'e bands. These results not only reveal the unsual valley dependent moir\'e-modified electronic structure in twisted transition metal dichalcogenides, but also highlight the {\Gamma}-valley moir\'e bands as a promising platform for exploring strongly correlated physics in emergent honeycomb and Kagome lattices at different energy scales.

Theory of superconductivity mediated by Rashba coupling in incipient ferroelectrics

Experimental evidence suggests that superconductivity in ${\mathrm{SrTiO}}_{3}$ is mediated by a soft transverse ferroelectric mode which, according to conventional theories, has negligible coupling with electrons. A phenomenological Rashba-type coupling has been proposed on symmetry arguments but a microscopic derivation is lacking. Here, we fill this gap and obtain a linear coupling directly from a minimal microscopic model of the electronic structure. We find that the effective electron-electron pairing interaction has a strong momentum dependence. This yields an unusual situation in which the leading $s$-wave channel is followed by a subleading $p$-wave state which shows a stronger pairing instability than the $d$-wave state. The bare Rashba coupling constant is estimated for the lowest band of doped ${\mathrm{SrTiO}}_{3}$ with the aid of first-principles computations. Extrapolating the estimation to the multiband regime we show that it can explain bulk superconductivity. For densities where the Meissner effect has not been observed an extra softening due to inhomogeneities is needed to explain the zero resistance state.

2D system incorporating perforated Mg sheet sandwiched between Pb layer and Si(111)

We synthesized and characterized a new (6 × 6)-periodical 2D compound layer consisting of a Mg network sandwiched between a Pb atomic layer and Si(111) substrate. Angle-resolved photoelectron spectroscopy revealed the close similarity of its electronic structure with that typical for the Pb/Si(111) system. As a result of Mg intercalation, 0.45 eV band bending takes place due to electron doping from Mg (1.27 electron per atom). The electron–phonon coupling constant λ is enhanced (2.54 ± 0.1) compared to the pristine Pb/Si(111) monolayer (0.58 ± 0.1) as derived from temperature-dependent ARPES measurements of the lifetime broadening. Four-point probe transport measurements revealed the abrupt decrease of surface resistance that disappeared in the presence of a magnetic field. It might be an indication of superconducting phase transition. Although we were unable to observe a zero-resistance state and hence to measure Tc directly, we estimated the possible superconducting critical temperature Tc = 1.15 K using the Aslamazov–Larkin correction. This value is close to 1.1 K for the case of pristine Pb/Si(111) system.

Tracing the main elements and electron orbitals that induce superconducting phase transition

The experimental determination of the superconducting transition requires the observation of the emergence of a zero-resistance and perfect diamagnetism state. Based on the close relationship between superconducting transition temperature () and electron density of states (DOS), we take two typical superconducting materials Hg and ZrTe3 as samples and calculate their DOS vs. temperature under different pressures by using the first-principle molecular dynamics simulations. According to the analysis of the calculation results, the main contributors that induce superconducting transitions are deduced by tracing the variation of partial density of states near . The microscopic mechanism of pressure induced increasing is further analyzed.

Structural and electronic inhomogeneity of superconducting Nb-doped Bi2Se3

The crystal structure, electronic structure, and transport properties of crystals with the nominal composition ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound ${(\mathrm{Bi}\mathrm{Se})}_{1.1}\mathrm{Nb}{\mathrm{Se}}_{2}$. The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.

Integrating superconducting van der Waals materials on paper substrates.

Paper has the potential to dramatically reduce the cost of electronic components. In fact, paper is 10 000 times cheaper than crystalline silicon, motivating the research to integrate electronic materials on paper substrates. Among the different electronic materials, van der Waals materials are attracting the interest of the scientific community working on paper-based electronics because of the combination of high electrical performance and mechanical flexibility. Up to now, different methods have been developed to pattern conducting, semiconducting and insulating van der Waals materials on paper but the integration of superconductors remains elusive. Here, the deposition of NbSe2, an illustrative van der Waals superconductor, on standard copy paper is demonstrated. The deposited NbSe2 films on paper display superconducting properties (e.g. observation of Meissner effect and resistance drop to zero-resistance state when cooled down below its critical temperature) similar to those of bulk NbSe2.

Electrically induced breakdown of the quantum Hall effect at different Hall bar widths: Visualizing the edge- and bulk-dominated regimes within a quantum Hall plateau

We present systematic investigations of the electrically induced breakdown of the integer quantum Hall effect (QHE) in (Al,Ga)As-based Hall bars of different widths ranging from 7 to 70 $\ensuremath{\mu}\mathrm{m}$. It is striking that, for the narrow Hall bars, the threshold values for the applied voltage that induces the breakdown of the longitudinal zero-resistance state differ appreciably between the upper and lower sides of a quantum Hall (QH) plateau. When moving from the low magnetic field side of the QH plateau to higher magnetic fields, the threshold rises in a strongly superlinear manner until---at the expected integer value for the Landau level (LL) filling factor in the bulk of the two-dimensional electron system (2DES)---the threshold value abruptly drops to low values for the rest of the plateau. With increasing widths of the Hall bars, the zero-resistance state extends slightly further to higher magnetic field values. The threshold values on the low-magnetic field side are almost independent of the Hall bar width, whereas the threshold values on the high magnetic field side scale linearly with the Hall bar width. These observations correspond perfectly with the microscope picture of the QHE where the biased current flows in a dissipationless manner in electrically incompressible regions of the 2DES of locally the same LL filling factor, driven by the respective drop of the Hall voltage over the width of these incompressible regions. Owing to the self-consistent evolution of the electrically incompressible/compressible landscape within the 2DES as a function of magnetic field, a transition from an edge- to a bulk-dominated QH regime is described within a QH plateau. This microscopic picture was derived from sophisticated, long-lasting scanning probe experiments measuring Hall potential profiles on typically 15-$\ensuremath{\mu}$m-wide Hall bars. Conversely, performing systematic electrically induced breakdown measurements like those presented here would allow us to identify the presence of edge- and bulk-dominated regimes of the QHE in a wider variety of samples.

Effect of external electromagnetic radiation on the anomalous metallic behavior in superconducting Ta thin films

We investigated transport characteristics of superconducting Ta thin films with three configurations in rf radiation filters; no filter, only room-temperature filters, and low-temperature filters in addition to room-temperature filters. The transport properties near the transition temperature are strongly dependent on whether the room-temperature filter is installed or not. The entire transition is shifted to higher temperature with loading layers of the room-temperature filters. Once the zero-resistance state is achieved at B=0, no strong radiation effect is observed even with low-temperature filters installed. When magnetic field is turned on, the nonzero-resistance saturation at low temperatures is revealed without low-temperature filters, which has been considered to be magnetic-field-induced quantum metallic phase. However, the insertion of the additional low-temperature filter weakens the saturation of the resistance, the signature of the metallic behavior. This observation suggests that the previously reported anomalous metallic state in Ta films is mainly induced by the unfiltered radiation and, thus, the intrinsic metallic ground state should be limited to the narrow range of magnetic fields near the critical point, if exists.