Evidence For Superconductivity Research Articles

Exploration of superconductivity in LK-99 synthesized under different cooling conditions

The claim of superconductivity above room temperature at ambient pressure in Cu-doped lead apatite (LK-99, Pb10-xCux(PO4)6O) has attracted considerable attention. In this paper, we present the results of synthesized LK-99 using a systematic approach based on quenching method variations. A slower cooling rate resulted in an increased LK-99 phase and a decreased Cu2S phase, with the slowest cooling method achieving the highest concentration of the LK-99 phase. Magnetic levitation was observed in certain specimens, with one end adhering to the magnet and the other end floating, which was distinguished from the Meissner effect. The majority of samples synthesized under different conditions exhibited no response to the magnet. The slowest cooled sample exhibited mostly insulating behavior. The samples synthesized in the other conditions had metallic behaviors, which contained a significant amount of Cu2S. No evidence of superconductivity was observed within the temperature range of 4 K–300 K in our samples.

Near Ambient Superconductivity by 15N as Needles in the Haystack: Lower Temperatures and Pressures for Possible 15N Enrichment in LuH2

The author has previously noted the effects of stable isotopes having different nuclear magnetic moments on chemistry, catalysis, biochemistry, thermodynamics, optics, superconductivity and more [1]. In this controversy surrounding reported room temperature superconductivity at near ambient pressures by nitrogen doped lutetium hydride, the author hopes to convince and reason that the different synthesis conditions of the original work of Dias and coworkers [2] at low temperature, mild pressures, diamond anvil cell compression and prolong annealing may lead to selective doping of the lutetium hydride by 15N. The later attempted replication of Dias and coworkers by Hai-hu Wen and coworkers [3] may have caused different outcomes as Hai-hu Wen and coworkers appeared to try Dias work and then switched to a different synthetic method whereby Wen and coworkers instead applied high pressures and high temperatures to the reacting hydrogen, nitrogen and lutetium to produce a nitrogen doped lutetium hydride with similar lattice structure as the originally reported by Dias and coworkers [2] but lacking observed superconductivity and evidence of superconductivity by diamagnetism. The author here by his theory notes the possibility that the different later high pressure, high temperature synthesis by Wen and coworkers doped their sample with 14N rather than 15N as originally enriched in Dias’s sample. Thereby the author notes by his theory [1] that whereas 15N doped lutetium hydride manifests higher superconductivity due to its negative nuclear magnetic moment (NMM), the 14N doped lutetium hydride should not manifest superconductivity at the higher temperatures due to its positive NMM.

Room-temperature superconductivity — or not? Comment on Nature 586, 373 (2020) by E. Snider et al.

Recently, the discovery of room-temperature superconductivity was announced for a carbonaceous sulfur hydride (CSH) under high pressure [E. Snider et al., Nature 586, 373 (2020)]. The evidence for superconductivity was based on resistance and magnetic susceptibility measurements. In the figures showing the susceptibility it was stated that “the background signal, determined from a nonsuperconducting CSH sample at 108 GPa, has been subtracted from the data”. From a thorough data analysis we show that the data are incompatible with the notion that the susceptibility data are obtained from the “measured voltage” using a background correction. On the other hand, the data are compatible with the reverse procedure, namely the “measured voltage” is obtained by adding a “background signal” containing noise to what was reported as the background-corrected susceptibility. For all six of the reported pressures our analysis leads to the conclusion that: (i) the reported background-corrected susceptibility data are pathological, (ii) they were not obtained by the method described in this paper nor by any one of the alternative three methods that were subsequently provided by the authors and (iii) the “measured voltage” data are not raw data.

Effects of external pressure on the narrow-gap semiconductor Ce3Cd2As6

Here we report the magnetic and electronic properties of recently discovered Ce$_{3}$Cd$_{2}$As$_{6}$. At ambient pressure, Ce$_{3}$Cd$_{2}$As$_{6}$ presents a semiconducting behavior with an activation gap of 74(1)~meV. At 136~K, a sudden increase of the electrical resistivity and a peak in specific heat are consistent with a charge density wave transition. At low temperatures, antiferromagnetic order of the Ce$^{3+}$ ions occurs below $T_{\rm N} = 4.0$~K with a magnetic hard axis along the $c$-axis and a $\Gamma_{6} = |\pm1/2\rangle$ doublet ground state. The application of external pressure strongly suppresses the charge density wave order, which is completely suppressed above 0.8(1)~GPa, and induces a metallic ground state. No evidence for superconductivity is detected above 2~K. Conversely, the antiferromagnetic state is favored by pressure, reaching a transition temperature of 5.3~K at 3.8(1)~GPa. Notably, the resistivity anomaly characterizing the antiferromagnetic order changes with increasing pressure, indicating that two different magnetic phases might be present in Ce$_{3}$Cd$_{2}$As$_{6}$ under pressure. This change in ordering appears to be associated to the crossing of the $T_{\rm CDW}$ and $T_{\rm N}$ lines.

Magnetic ground states of Ce3TiSb5, Pr3TiSb5 and Nd3TiSb5 determined by neutron powder diffraction and magnetic measurements

The R 3TiSb5 ternary compounds, with R a light rare earth (La to Sm) have been reported to crystallize with the anti-Hf5CuSn3-type hexagonal structure (Pearson’s symbol hP18; space-group P63/mcm, N. 193). An early article that reported possible superconductivity in some of these intermetallic phases (namely those with R = La, Ce, and Nd) caught our attention. In this work, we have now refined the crystal structure of the R 3TiSb5 compounds with R = Ce, Pr and Nd by Rietveld methods using high-resolution neutron powder diffraction data. The magnetic ground states of these intermetallics have been investigated by low-temperature magnetization and high-intensity neutron diffraction. We find two different magnetic transitions corresponding to two related magnetic structures at T N1 = 4.8 K (k 1 = [0, 1/2, 1/8]) and T N2 = 3.4 K (k 2 = [0, 0, 1/8]), respectively for Ce3TiSb5. However, the magnetic ordering appears to occur following a peculiar hysteresis: the k 2-type magnetic structure develops only after the k 1-type phase fraction has first slowly ordered with time and the size of the ordered Ce3+ magnetic moment has become large enough to induce the second magnetic transition. At T = 1.5 K the maximum amplitude of the Ce moment in the coexisting phases amounts to μ Ce = 2.15 μ B. For Nd3TiSb5 an antiferromagnetic ordering below T N = 5.2 K into a relatively simpler commensurate magnetic structure with a magnetic moment of μ Nd = 2.14(3) μ B and magnetic propagation vector of k = [0, 0, 0], was determined. No evidence of superconductivity has been found in Nd3TiSb5. Finally, Pr3TiSb5 does not show any ordering down to 1.5 K in neutron diffraction while an antiferromagnetic ground state is detected in magnetization measurements. There is no sign of magnetic contribution from Ti atoms found in any of the studied compounds.

Absence of magnetic evidence for superconductivity in hydrides under high pressure

It is generally believed that magnetization measurements on sulfur hydride under high pressure performed in 2015 [1] provided “final convincing evidence of superconductivity” [2] in that material, in agreement with theoretical predictions [3,4]. Supported by this precedent, drops in resistance that were later observed in several other hydrides under high pressure [[2], 5] have been generally accepted as evidence of superconductivity without corroborating evidence from magnetic measurements. In this paper we challenge the original interpretation that the magnetic measurements on sulfur hydride performed in 2015 were evidence of superconductivity. We point out that a large paramagnetic contribution to the magnetic susceptibility was detected below Tc and argue that its temperature dependence rules out the possibility that it would be a background signal; instead the temperature dependence indicates that the paramagnetic behavior originated in the sample. We discuss possible explanations for this remarkable behavior and conclude that standard superconductors would not show such behavior. We also survey all the other published data from magnetic measurements on this class of materials and conclude that they do not provide strong evidence for superconductivity. Consequently, we call into question the generally accepted view that conventional superconductivity in hydrogen-rich materials at high temperature and pressure is a reality, and discuss the implications if it is not.

Hall-coefficient diagnostics of the surface state in pressurized SmB6

In this study, we report the first results of the high-pressure Hall coefficient (RH) measurements in the putative topological Kondo insulator SmB6 up to 37 GPa. Below 10 GPa, our data reveal that RH(T) exhibits a prominent peak upon cooling below 20 K. Remarkably, the temperature at which surface conduction dominates coincides with the temperature of the peak in RH(T). The temperature dependent resistance and Hall coefficient can be well fitted by a two-channel model with contributions from the metallic surface and the thermally activated bulk states. When the bulk of SmB6 becomes metallic and magnetic at ~ 10 GPa, both the RH(T) peak and the resistance plateau disappear simultaneously. Our results indicate that the RH(T) peak is a fingerprint to diagnose the presence of a metallic surface state in SmB6. The high-pressure magnetic state of SmB6 is robust to 180 GPa, and no evidence of superconductivity is observed in the metallic phase.

Proximity-induced surface superconductivity in Dirac semimetal Cd3As2

Cd3As2 is a three-dimensional Dirac semimetal with separated Dirac points in momentum space. In spite of extensive transport and spectroscopic studies on its exotic properties, the evidence of superconductivity in its surface states remains elusive. Here, we report the observation of proximity-induced surface superconductivity in Nb/Cd3As2 hybrid structures. Our four-terminal transport measurement identifies a pronounced proximity-induced pairing gap (gap size comparable to Nb) on the surfaces, which exhibits a flat conductance plateau in differential conductance spectra, consistent with our theoretical simulations. The surface supercurrent from Nb/Cd3As2/Nb junctions is also achieved with a Fraunhofer/SQUID-like pattern under out-of-plane/in-plane magnetic fields, respectively. The resultant mapping shows a predominant distribution on the top and bottom surfaces as the bulk carriers are depleted, which can be regarded as a higher dimensional analog of edge supercurrent in two-dimensional quantum spin Hall insulators. Our study provides the evidence of surface superconductivity in Dirac semimetals.

Fermions and bosons in nonsymmorphic PdSb2 with sixfold degeneracy

PdSb2 is a candidate for hosting 6-fold-degenerate exotic fermions (beyond Dirac and Weyl fermions).The nontrivial band crossing protected by the nonsymmorphic symmetry plays a crucial role in physical properties. We have grown high-quality single crystals of PdSb2 and characterized their physical properties under several stimuli (temperature, magnetic field, and pressure). While it is a diamagnetic Fermi-liquid metal under ambient pressure, PdSb2 exhibits a large magnetoresistance with continuous increase up to 14 T, which follows the Kohler's scaling law at all temperatures. This implies one-band electrical transport, although multiple bands are predicted by first principles calculations. By applying magnetic field along the [111] direction, de Haas-van Alphen oscillations are observed with frequency of 102 T. The effective mass is nearly zero (0.045m0) with the Berry phase close to {\pi}, confirming that the band close to the R point has a nontrivial character. Under quasihydrostatic pressure (p), evidence for superconductivity is observed in the resistivity below the critical temperature Tc. The dome-shaped Tc versus p is obtained with maximum Tc~2.9 K. We argue that the formation of Cooper pairs (bosons) is the consequence of the redistribution of the 6-fold-degenerate fermions under pressure.

Evidence for Superconductivity above 260K in Lanthanum Superhydride at Megabar Pressures.

Recent predictions and experimental observations of high T_{c} superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including four-probe electrical transport measurements that display significant drops in resistivity on cooling up to 260K and 180-200GPa, and resistivity transitions at both lower and higher temperatures in other experiments. Additional current-voltage measurements, critical current estimates, and low-temperature x-ray diffraction are also obtained. We suggest that the transitions represent signatures of superconductivity to near room temperature in phases of lanthanum superhydride, in good agreement with density functional structure search and BCS theory calculations.

A library of atomically thin metal chalcogenides.

Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional superconductivity 5 , suggesting potential applications for functional devices6-10. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization 18 of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature 19 and this approach 20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of thesynthesized materials inour library are useful, as supported by evidence of superconductivity in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.

Superconducting and normal state properties of the systemsLa1−xMxPt4Ge12(M = Ce,Th)

Electrical resistivity, magnetization, and specific heat measurements were performed on polycrystalline samples of the filled-skutterudite systems ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}\phantom{\rule{4pt}{0ex}}(M=\text{Ce}$ and $\mathrm{Th})$. Superconductivity in ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ was quickly suppressed with Ce substitution and no evidence for superconductivity was found down to 1.1 K for $xg0.2$. Temperature-dependent specific heat data at low temperatures for ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}$ show a change from power-law to exponential behavior, which may be an indication for multiband superconductivity in ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$. A similar crossover was observed in the ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}$ system. However, the suppression rates of the superconducting transition temperatures ${T}_{c}$($x$) in the two systems are quite disparate, indicating a difference in the nature of superconductivity, which is conventional in ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ and unconventional in ${\mathrm{PrPt}}_{4}{\mathrm{Ge}}_{12}$. In comparison, a nearly linear and smooth evolution of ${T}_{c}$ with increasing Th was observed in the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}$ system, with no change of the superconducting energy gap in the temperature dependence of the specific heat, suggesting similar types of superconductivity in both the ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ and ${\mathrm{ThPt}}_{4}{\mathrm{Ge}}_{12}$ compounds.

Evidence for superconductivity in Li-decorated monolayer graphene

Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131-134]. Although experiments have shown an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to λ ≃ 0.58. On part of the graphene-derived π*-band Fermi surface, we then observe the opening of a Δ ≃ 0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with Tc ≃ 5.9 K.

Superconductivity in Sm-doped [n]phenacenes (n = 3, 4, 5).

We report here the discovery of a new aromatic hydrocarbon superconductor, Sm-doped chrysene, with Tc ∼ 5 K, and compare its behavior with those measured in the full series of Sm-doped [n]phenacene superconductors, with n = 3, 4, 5, thus determining the trend of Tc as a function of the number of fused benzene rings and for an odd or even number of units.

Probing the superconductivity ofPrPt4Ge12through Ce substitution

We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr${}_{1\ensuremath{-}x}$Ce${}_{x}$Pt${}_{4}$Ge${}_{12}$. Superconductivity is suppressed with increasing Ce concentration up to $x=0.5$, above which there is no evidence for superconductivity down to 1.1 K. The Sommerfeld coefficient $\ensuremath{\gamma}$ increases with increasing $x$ from $\ensuremath{\sim}$48 mJ/mol K${}^{2}$ up to $\ensuremath{\sim}$120 mJ/mol K${}^{2}$ at $x=$ 0.5, indicating an increase in strength of electronic correlations. The temperature dependence of the specific heat at low temperatures evolves from roughly ${T}^{3}$ for $x=$ 0 to e${}^{\ensuremath{-}\ensuremath{\Delta}/T}$ behavior for $x=$ 0.05 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity. Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements.

Temperature-pressure phase diagram of CeCoSi: Pressure-induced high-temperature phase

We have studied the temperature-pressure phase diagram of CeCoSi by electrical-resistivity experiments under pressure. Our measurements revealed a very unusual phase diagram. While at low pressures no dramatic changes and only a slight shift of the Ne\'{e}l temperature $T_N$ ($\approx 10$ K) are observed, at about 1.45 GPa a sharp and large anomaly, indicative of the opening of a spin-density-wave (SDW) gap, appears at a comparatively high temperature $T_S \approx 38$ K. With further increasing pressure $T_S$ shifts rapidly to low temperatures and disappears at about 2.15 GPa, likely continuously in a quantum critical point, but without evidence for superconductivity. Even more surprisingly, we observed a clear shift of $T_S$ to higher temperatures upon applying a magnetic field. We discuss two possible origins for $T_S$, either magnetic ordering of Co or a meta-orbital type of transition of Ce.

Search for Superconductivity in Doped Amorphous Carbon Thin Films

In this paper, we have searched for superconductivity by measuring ohmic resistivity as a function of temperature in amorphous carbon films deposited by pulsed laser deposition and doped by ion implantation with sulfur and phosphorus ions. The doping concentrations were varied from 0.0003 to 4 Vol% for sulfur and 0.0003 to 1 Vol% for phosphorus. Previous efforts have studied doping of carbon-family materials such as highly oriented pyrolytic graphite, diamond-like carbon, and graphite/graphene, which have yielded critical temperatures lower than 20 K. In this study, amorphous carbon films doped with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$2.55 \times 10^{12}\ \hbox{ions/cm}^{2}$</tex></formula> phosphorus concentration showed a distinct change from semiconducting to metallic behavior and a dramatic 10 000-fold decrease in resistivity below 100 K as compared to undoped films. Sulfur-doped films with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$2.55 \times 10^{12}\ \hbox{ions/cm}^{2}$</tex> </formula> concentration showed up to a 100-fold decrease in resistivity below 100 K as compared to undoped samples. While evidence of superconductivity was not observed, significant improvements in conductivity were noted below 100 K.

Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (muSR) measurements. Zero-field muSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field muSR is nearly flat approaching T = 0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d + id (chiral d-wave) order parameter is most consistent with our experimental data.

Synthesis of tetragonal mackinawite-type FeS nanosheets by solvothermal crystallization

Mackinawite, a metastable 1:1 compound of iron and sulfur that adopts an anti-PbO-type structure, is of interest because of its relationship to known iron chalcogenide superconductors, as well as its biogeochemical relevance. Colloidal nanosheets of mackinawite-type FeS were synthesized by first generating an amorphous Fe–S precursor via the aqueous room-temperature co-precipitation of Fe2+ and S2−, then solvothermally crystallizing it in ethylene glycol at 200°C in an autoclave. The product is highly crystalline, with lattice constants of a=3.674(3)Å and c=5.0354(3)Å. The nanosheets, with their surface normal oriented along the [001] direction, are irregularly faceted with edge lengths that range from 100nm to over 1μm and average thicknesses of approx. 30nm. The samples showed a ferromagnetic background signal with no evidence of superconductivity.

Low-temperature electrical resistivity of praseodymium at pressures up to 120 GPa

Under ambient conditions, praseodymium metal possesses a localized $4{f}^{2}$-electron configuration. Near 20 GPa, the lattice volume collapses by $~$10% into the $\ensuremath{\alpha}$-uranium crystal structure, and the electrical resistivity drops dramatically. This behavior is similar to that observed in cerium metal and has been taken as evidence for $4f$ delocalization, although the precise nature of such volume collapse transitions in rare-earth metals is still a matter of debate. Since cerium metal develops superconductivity in the collapsed phase, we undertook a search for superconductivity in praseodymium metal at high pressure. Using designer diamond anvils, we measured the electrical resistivity of high purity praseodymium metal to pressures above 1 Mbar and millikelvin temperatures. No evidence for superconductivity was found in any of the measurements. The lack of superconductivity may derive from magnetic pair-breaking effects related to incomplete screening/delocalization of the $4f$-electron state.