arXiv subjects

Xiao-Jia Chen

Publications and source records attributed to Xiao-Jia Chen.

At least 19 recordsLinked to original sources

arXiv2024

Probing superconducting gap in CeH$_9$ under pressure

The recent discovery of superconductivity in hydrogen-rich compounds has garnered significant experimental and theoretical interest because of the record-setting critical temperatures. As the direct observation of the superconducting (SC) gap in these superhydrides is rare, the underlying mechanism behind its occurrence has yet to be settled down. Here, we report a successful synthesis of the $\textit{P6$_3$}$/$\textit{mmc}$ phase of CeH$_9$ that exhibits the SC transition with SC critical temperature of about 100 K at a pressure of about 100 GPa. The observation of the zero electrical resistance and the critical current demonstrates that the SC phase is realized in Ce-based superhydride. Quasiparticle scattering spectroscopy (QSS) reveals the Andreev reflection at zero bias voltage, a hallmark of superconductivity, in the differential conductance. The obtained SC gap-to-$\textit{T}$$_c$ ratio of 4.36 and temperature dependence of SC gap are consistent with the prediction from the Bardeen-Cooper-Schrieffer theory with a moderate coupling strength. The successful realization of QSS under Megabar conditions is expected to provide a desired route to the study of the mechanism of superconductivity as well as the establishment of the SC phase in superhydride high-$\textit{T}$$_c$ systems.

cond-mat.supr-con↗

arXiv2023

Lattice effect on the superexchange interaction in antiferromagnetic Bi$_2$Sr$_2$CaCu$_2$O$_8$

The in-plane superexchange interaction $J$ of cuprate superconductors has long been suggested to be an important parameter for exploring their high-temperature superconductivity. The bilayer Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is the most studied system with high-quality single crystals in the wide doping range with the the same structure and phase. So far, the lattice parameter dependence of $J$ in its antiferromagnetic parent compound Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ has not been established. By combining Raman scattering and x-ray diffraction techniques on the same sample in the same pressure environment, we obtain the evolution of both the two-magnon spectrum and the structural parameters with pressure up to nearly 30 GPa, The relationship between pressure or the in-plane lattice parameter and $J$ is thus established for Bi$_2$Sr$_2$CaCu$_2$O$_{8}$. Over the studied pressure range, superconductivity does not appear in this parent compound based on a sensitive magnetic measurement technique. The effects of pressure and chemical doping on the superexchange interaction and structure and their implications for superconductivity are discussed from the comparison of the obtained experimental data with the existing experiments. The results and findings provide valuable information for the understanding of superconductivity and the future theory developments for superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2023

Universal phonon softening in the pseudogap state of Tl$_2$Ba$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta}$

Exploring the origin of the pseudogap is important for the understanding of superconductivity in cuprates. Here we report a systematical experimental study on the phonon vibrational properties of Tl$_2$Ba$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta}$ ($n$=1,2,3) single crystals based on the Raman scattering measurements over the temperature range from 10 to 300 K. The temperature evolution of the frequency and linewidth of the observed phonon modes in each member of this family does not follow the expected self-energy effect when entering the superconducting state. Instead, these phonon modes exhibit a universal softening behavior below the temperature around 150 K, which is higher above the superconducting transition. From the comparison with the existing experimental data for various orders, we find that the observed starting temperature for the phonon softening corresponds to the onset opening temperature of the pseudogap. This finding indicates a large lattice effect in the pseudogap state and the non-negligible spin-phonon coupling for such a phonon softening.

cond-mat.supr-con↗

arXiv2022

Transport evidence for the surface state and spin-phonon interaction in FeTe$_{0.5}$Se$_{0.5}$

The iron chalcogenides have been proved to be intrinsic topological superconductors to implement quantum computation because of their unique electronic structures. The topologically nontrivial surface states of FeTe$_{0.5}$Se$_{0.5}$ have been predicted by several calculations and then confirmed by high-resolution photoemission and scanning tunneling experiments. However, so far, the shreds of the electrical transport evidence for topological surface states are still in absence. By carrying out electrical transport experiments, we observe a topological transition with a nonlinear Hall conductivity and simultaneous linear magnetoresistance near the superconducting transition temperature. Furthermore, we observe a sign reversal of the Hall coefficient accompanied by a concurrently softening of the ${A}_{1g}$ phonon mode at about 40 K, indicating a nematic transition. The synchronized phonon softening with nematicity manifests an enhanced fluctuation state through spin-phonon interaction. Our results solidly corroborate the topological surface states of FeTe$_{0.5}$Se$_{0.5}$ and provide an understanding of the mechanism of the superconductivity in iron chalcogenides.

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arXiv2022

Critical field measure for topological superconductivity

A promising direction for harnessing the laws of quantum mechanics to perform quantum computation is the topological quantum computation, for which topological superconductivity is one of the physical platforms. Intensive theoretical studies have been carried out and followed by tremendous experimental efforts in the realization of topological superconductivity, though debates remain mainly because of the inadequacy of the convincing detection techniques. Here we report a theoretical finding in a superconductor with surface or edge states, where the critical field is found to obey the unique power-law temperature dependence $B_{c3}\sim (T_c-T)^{\gamma}$ with $T_c$ being the onset critical temperature of superconductivity and the fractional exponent $\gamma= 2/3$, differing from the conventional values of $\gamma=1/2$ and $1$. The topological surface superconductivity is hence expected in the three-dimensional (3D) system and the topological edge superconductivity in the two-dimensional (2D) system. The application of this measure to the intrinsic topological superconductors FeTe$_{1-x}$Se$_{x}$ ($x\sim 0.5$) supports the validity of such an accessible, convenient, and reliable transport technique for the identification of topological superconductivity. The new form of theory on topological superconductivity together with the developed identification technique is expected to guide the search of topological superconductors in future.

cond-mat.supr-con↗

arXiv2022

Topological superconductivity in a topological insulator

Topological superconductivity is an exotic quantum phenomenon with coupled nontrivial topological order and superconductivity together. A direct idea for producing topological superconductors is to create superconductivity based on the well recognized topological insulators. The topological insulating states in highly efficient thermoelectric materials Bi$_2$Te$_3$ and Bi$_2$Se$_3$ and their alloy Bi$_{2}$Te$_{3-x}$Se$_{x}$ have been established from angle-resolved photoemission and transport experiments. Superconductivity was also observed based on these popular topological insulators by the application of pressure chemical dopant and heterostructures. However, the experiments mainly focusing on Bi$_{2}$Se$_3$ doped by metals have not provided the consistent evidence to support the topological superconductivity. Here we carry out a systematic high-pressure study on a topological insulator Bi$_{2}$Te$_{2.7}$Se$_{0.3}$ to provide the convincing evidence for the expected topological superconductivity. Four phases with different structures are found upon compression. The topological surface state is identified in the entire initial phase, while superconductivity is found to coexist with such a state of the compressed material after its passing the electronic topological transition, followed by three other superconducting phases without topological character. For these superconducting phases, we observe that the upper critical field follows with the temperature in the critical exponent ${2/3}$ for the first one with the topological surface state and $1$ for the left. These observations support the realization of the topological superconductivity in the initial phase according to the theoretically proposed critical field measure. This work also points out a big pool and new direction for finding topological superconductors from topological thermoelectric materials.

cond-mat.supr-con↗

arXiv2022

Superconductivity of Cs$_3$C$_{60}$ at atmosphere pressure

Pressure as a clean and efficient tool can bring about unexpected extraordinary physical and chemical properties of matters. The recent discoveries of superconductivity at nearly room temperature in hydrides highlight the power of pressure in this aspect. Capturing such Tc superconductivity at atmosphere pressure for the technological applications is highly desired. The large-scale growth of diamond through the chemical vapor deposition away from the usual high-pressure and high-temperature conditions fuels such a hope. Similar to hydrides, Cs-doped C$_{60}$ was also found to exhibit superconductivity by the application of pressure with a comparable Tc of 40 K as MgB$_2$. Here, we report the successful realization of superconductivity in Cs-doped C$_{60}$ at atmosphere pressure. The phase is characterized to have the primitive cubic structure in the space group of Pa-3 with the stoichiometry of Cs$_3$C$_{60}$. The superconductivity is evidenced from the observations of both the Meissner effect and zero-resistance state. Although the pressure effects on superconductivity are different for the newly discovered Cs$_{3}$C$_{60}$ compared to the known two phases with fcc and A15 structure, the evolution of Tc with the volume for all these superconductors follows the same universal trend, suggesting the same pairing mechanism of the superconductivity. Such a trend together with the nearly linear Tc vs the lattice constant in the structure with smaller unit-cell volumes and the neighbouring antiferromagnetic state in the structure with larger unit-cell volumes invites the electron-phonon coupling and the electron correlations together to account for the superconductivity in Cs$_3$C$_{60}$. The present results and findings suggest a new route to capturing the superconductivity which takes place at high pressures to atmosphere pressure environment.

cond-mat.supr-con↗

arXiv2022

Synthesis and Superconductivity in Yttrium-Cerium Hydrides at Moderate Pressures

Inspired by the high critical temperature in yttrium superhydride and the low stabilized pressure in superconducting cerium superhydride, we carry out four independent runs to synthesize yttrium-cerium alloy hydrides. The phases examined by the Raman scattering and x-ray diffraction measurements. The superconductivity is detected with the zero-resistance state at the critical temperature in the range of 97-140 K at pressures ranging from 114 GPa to 120$\pm$4 GPa. The maximum critical temperature of the synthesized hydrides is larger than those reported for cerium hydrides, while the corresponding stabilized pressure is much lower than those for superconducting yttrium hydrides. The structural analysis and theoretical calculations suggest that the phase of Y$_{0.5}$Ce$_{0.5}$H$_9$ has the space group $P6_3/mmc$ with the calculated critical temperature of 119 K, in fair agreement with the experiments. These results indicate that alloying superhydrides indeed can maintain relatively high critical temperature at modest pressures accessible by many laboratories.

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arXiv2022

Synthesis of Superconducting Phase of La$_{0.5}$Ce$_{0.5}$H$_{10}$ at High Pressures

Clathrate hydride \emph{Fm}\={3}\emph{m}-LaH$_{10}$ has been proven as the most extraordinary superconductor with the critical temperature $T_c$ above 250 K upon compression of hundreds of GPa in recent years. A general hope is to reduce the stabilization pressure and maintain the high $T_c$ value of the specific phase in LaH$_{10}$. However, strong structural instability distorts \emph{Fm}\={3}\emph{m} structure and leads to a rapid decrease of $T_c$ at low pressures. Here, we investigate the phase stability and superconducting behaviors of \emph{Fm}\={3}\emph{m}-LaH$_{10}$ with enhanced chemical pre-compression through partly replacing La by Ce atoms from both experiments and calculations. For explicitly characterizing the synthesized hydride, we choose lanthanum-cerium alloy with stoichiometry composition of 1:1. X-ray diffraction and Raman scattering measurements reveal the stabilization of \emph{Fm}\={3}\emph{m}-La$_{0.5}$Ce$_{0.5}$H$_{10}$ in the pressure range of 140-160 GPa. Superconductivity with $T_c$ of 175$\pm$2 K at 155 GPa is confirmed with the observation of the zero-resistivity state and supported by the theoretical calculations. These findings provide applicability in the future explorations for a large variety of hydrogen-rich hydrides.

cond-mat.supr-con↗

arXiv2022

Full Set of Superconducting Parameters of K$_3$C$_{60}$

The superconducting parameters are the key for building or identifying the theory responsible for the mechanism of superconductivity. Such parameters for fulleride superconductors have not been well established despite the tremendous efforts over the past 30 years. Here we provide a full set of parameters through a systematic study on a well-characterized K$_{3}$C$_{60}$ sample. The obtained high upper critical field of 33.0$\pm$0.5 T from the direct electrical transport measurements together with the relatively high critical temperature and large critical current density classifies K$_{3}$C$_{60}$ as a promising three-dimensional superconducting magnet material with the advantage of the rich carbon abundance on the Earth. This high upper critical field along with the large reduced superconducting energy gap and strong phonon self-energy effect supports the strong electron-phonon coupling interactions in this superconductor. The evaluation of all self-consistently obtained parameters suggests the unconventional nature of the superconductivity for K$_3$C$_{60}$ with the joint contributions from the strong electron-phonon coupling and electron correlations. These results and findings are important not only for fundamentally understanding the superconductivity in fullerides but also for future superconducting magnet developments and applications.

cond-mat.supr-con↗

arXiv2022

Robust superconductivity near constant temperature in rubidium-doped C$_{60}$

To establish the doping-dependent phase diagram in alkali-metal doped C$_{60}$, we synthesize Rb-doped C$_{60}$ samples with different stoichiometries by using the improved wet-chemistry technique. The doping levels determined from the Raman scattering spectra often show the appearance of three electrons corresponding to the band filling of three for the synthesized compounds no matter matter what dopants are used. The multiple phase coexistence with the unique Rb$_{3}$C$_{60}$ is identified from the refined x-ray diffraction patterns. The phase fraction of Rb$_{3}$C$_{60}$ is found to behave with the doping in a similar manor as the superconducting shielding fraction. These rigorously established correlations among the superconducting transition temperature along with the structural and phonon vibrational properties allow us to single out Rb$_{3}$C$_{60}$ as the only superconducting phase with the nearly constant transition temperature regardless the doping level. These findings provide an experimental constraint on the theory developments for the superconductivity in fullerides.

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arXiv2019

Order-disorder transition in $p$-oligophenyls

Poly($para$-phenylene) has been recognized as one important family of conducting polymers upon doping with donors or acceptors. This system possesses a chain-like structure with infinite benzene rings linked with the single C-C bond. Oligophenyls as models of poly($para$-phenylene) with short chains in the \emph{para} position were found to exhibit superconductivity at transition temperatures ranging from 3 K to 123 K upon dopant. Structural studies have revealed that there exist the order-disorder transitions in many \emph{p}-oligophenyls with almost doubled lattice constants in the \emph{b} and \emph{c} directions at low temperatures, seemingly supporting the formation of the charge-density-wave order. Such a transition is of relevance to the understanding of the emergence of novel quantum functionality where unconventional polaronic interactions are relevant for the new emerging theory of superconductivity in this system. However, the accurate temperatures for the order-disorder transitions amongst these \emph{p}-oligophenyls are still needed to be determined. The chain length effects on the order-disorder transitions in this system remain unknown. Here we report the systematic investigation of the evolution of vibrational properties of the crystalline \emph{p}-oligophenyls over a wide temperature range. The order-disorder transition is identified by three indicators, the lowest energy peak together with the intensity ratios between the 1280 cm$^{-1}$ and 1220 cm$^{-1}$ modes and between the two modes at around 1600 cm$^{-1}$. A phase diagram of the order-disorder transition temperature as well as the melting curve for \emph{p}-oligophenyls is thus established. The former is found to increase with the chain length and saturates at around 350 K for poly($para$-phenylene).

cond-mat.mtrl-sci↗

arXiv2019

Superconductivity in an organometallic compound

Organometallic compounds constitute a very large group of substances that contain at least one metal-to-carbon bond in which the carbon is part of an organic group. They have played a major role in the development of the science of chemistry. These compounds are used to a large extent as catalysts (substances that increase the rate of reactions without themselves being consumed) and as intermediates in the laboratory and in industry. Recently, novel quantum phenormena such as topological insulators and superconductors were also suggested in these materials. However, there has been no report on the experimental exploration for the topological state. Evidence for superconductivity from the zero-resistivity state in any organometallic compound has not been achieved yet, though much efforts have been devoted. Here we report the experimental realization of superconductivity with the critical temperature of 3.6 K in a potassium-doped organometallic compound, $ i.e.$ tri-$o$-tolylbismuthine with the evidence of both the Meissner effect and the zero-resistivity state through the $dc$ and $ac$ magnetic susceptibility and resistivity measurements. The obtained superconducting parameters classify this compound as a type-II superconductor. The benzene ring is identified to be the essential superconducting unit in such a phenyl organometallic compound. The superconducting phase and its composition are determined by the combined studies of the X-ray diffraction and theoretical calculations as well as the Raman spectroscopy measurements. These findings enrich the applications of organometallic compounds in superconductivity and add a new electron-acceptor family for organic superconductors. This work also points to a large pool for finding superconductors from organometallic compounds.

cond-mat.supr-con↗

arXiv2018

Unusual pressure-induced periodic lattice distortion in SnSe$_2$

We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}=(1/3,1/3,1)$. In contrast, similar PLD transitions associated with charge density wave (CDW) orderings in transition metal dichalcogenides (TMDs) do not involve significant Fermi surface nesting. The discovered pressure-induced PLD is quite remarkable, as pressure usually suppresses CDW phases in related materials. Our findings, therefore, provide new playgrounds to study the intricate mechanisms governing the emergence of PLD in TMD-related materials.

cond-mat.mtrl-sci↗

arXiv2018

Superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductor

We report on the Andreev spectroscopy and specific heat of high-quality single crystals BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps $\Delta_L \approx 3.2 \textendash 4.5$\,meV, $\Delta_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_xk_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps $\Delta_L \approx 3.2$\,meV and $\Delta_S \approx 1.6$\,meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers a further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

cond-mat.supr-con↗

arXiv2018

Superconductivity from Meissner Effect and Zero Resistivity in a Phenyl Molecule

Recently, phenyl molecules have been reported to exhibit Meissner effect mainly from magnetization measurements. Realizing zero-resistivity state in these materials seems a challenge due to many practical difficulties but is required to characterize the existence of superconductivity. By choosing potassium-doped tris(2-methylphenyl)bismuthine as an example, we perform temperature-dependent magnetic susceptibility and resistivity measurements at different magnetic fields and pressures. The solid evidence for supporting superconductivity is achieved from the obtained Meissner effect and zero resistivity with the critical temperature ($T_c$) of 3.6 K at atmosphere pressure. Upon compression, we observe the gradual evolution of superconductivity from its initial phase with a parabolic behavior of $T_{c}$ to the second one with almost constant value of $T_{c}$ of 7 K. The 7 K phase seems a common feature for these newly discovered phenyl-based superconductors.

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arXiv2018

Highly reproducible superconductivity in potassium-doped triphenylbismuth

Using a new two-step synthesis method - ultrasound treatment and low temperature annealing, we explore superconductivity in potassium-doped triphenylbismuth, which is composed of one bismuth atom and three phenyl rings. The combination of dc and ac magnetic measurements reveals that one hundred percent of synthesized samples exhibit superconductivity at 3.5 K and/or 7.2 K at ambient pressure. The magnetization hysteresis loops provide a strong evidence of type-II superconductor, with the upper critical magnetic field up to 1.0 Tesla. Both calculated electronic structure and measured Raman spectra indicate that superconductivity is realized by transferring electron from potassium to carbon atom. Our study opens an encouraging window for the search of organic superconductors in organometallic molecules.

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arXiv2018

Thermoelectric properties of polycrystalline palladium sulfide

A suite measurements of the electrical, thermal, and vibrational properties are conducted on palladium sulfide (PdS) in order to investigate its thermoelectric performance. The tetragonal structure with the space group $P$42/$m$ for PdS is determined from X-ray diffraction measurement. The unique temperature dependence of mobility suggests that acoustic phonons and ion impurity scattering are two dominant scattering mechanisms within the compound. The obtained power factor of $27$ $\mu$Wcm$^{-1}$K$^{-2}$ at 800 K is the largest value in the remaining transition-metal sulfides studied so far. The maximum value of the dimensionless figure of merit is 0.33 at 800 K. The observed phonon softening with temperature indicates that the reduction of the lattice thermal conductivity is mainly controlled by the enhanced lattice anharmonicity. These results indicate that the binary bulk PdS has promising potential to have good thermoelectrical performance.

cond-mat.mtrl-sci↗