arXiv subjects

Xiao-Jia Chen

Publications and source records attributed to Xiao-Jia Chen.

At least 37 records · Page 2Linked to original sources

arXiv2018

Chain length effects of $p$-oligophenyls with comparison of benzene by Raman scattering

Raman scattering measurements are performed on benzene and a number of $p$-oligophenyls including biphenyl, $p$-terphenyl, $p$-quaterphenyl, $p$-quinquephenyl, and $p$-sexiphenyl at ambient conditions. The vibrational modes of the intra- and intermolecular terms in these materials are analyzed and compared. Chain length effects on the vibrational properties are examined for the C-H in-plane bending mode and the inter-ring C-C stretching mode at around 1200 cm$^{-1}$ and 1280 cm$^{-1}$, respectively, and the C-C stretching modes at around 1600 cm$^{-1}$. The complex and fluctuating properties of these modes result in an imprecise estimation of the chain length of these molecules. Meanwhile, the obtained ratio of the intensities of the 1200 cm$^{-1}$ mode and 1280 cm$^{-1}$ mode is sensitive to the applied lasers. A librational motion mode with the lowest energy is found to have a monotonous change with the increase in the chain length. This mode is simply relevant to the $c$ axis of the unit cell. Such an obvious trend makes it a better indicator for determining the chain length effects on the physical and chemical properties in these molecules.

cond-mat.mtrl-sci↗

arXiv2018

Superconductivity in potassium-doped 2,2$'$-bipyridine

Organic compounds are always promising candidates of superconductors with high transition temperatures. We examine this proposal by choosing 2,2$'$-bipyridine solely composed by C, H, and N atoms. The presence of Meissner effect with a transition temperature of 7.2 K in this material upon potassium doping is demonstrated by the $dc$ magnetic susceptibility measurements. The real part of the $ac$ susceptibility exhibits the same transition temperature as that in $dc$ magnetization, and a sharp peak appeared in the imaginary part indicates the formation of the weakly linked superconducting vortex current. The occurence of superconductivity is further supported by the resistance drop at the transition together with its suppression by the applied magnetic fields. The superconducting phase is identified to be K$_3$-2,2$'$-bipyridine from the analysis of Raman scattering spectra. This work not only opens an encouraging window for finding superconductivity after optoelectronics in 2,2$'$-bipyridine-based materials but also offers an example to realize superconductivity from conducting polymers and their derivatives.

cond-mat.supr-con↗

arXiv2018

Observation of Meissner effect in potassium-doped \emph{p}-quinquephenyl}

The chain-like organic compounds with conjugated structure have the potential to become high temperature superconductors. We examine this idea by choosing p-quinquephenyl with five phenyl rings connected in para position. The dc magnetic susceptibility measurements provide solid evidence for the presence of Meissner effect when the compound is doped by potassium. The real part of the ac susceptibility shows exactly same transition temperature as that in dc magnetization, and the imaginary part of nearly zero value after transition implies the realization of zero-resistivity. All these features support the existence of superconductivity with a critical temperature of 7.3 K in this material. The occurrence of bipolarons revealed by Raman spectra guarantees potassium metal intercalated into p-quinquephenyl and suggests the important role of this elementary excitation played on superconductivity.

cond-mat.supr-con↗

arXiv2018

Observation of Meissner effect in potassium-doped $p$-quaterphenyl

Inspired by the discovery of high temperature superconductivity in potassium-doped $p$-terphenyl, we examine the possibility of superconductivity in $p$-quaterphenyl with one more phenyl ring than $p$-terphenyl. The Meissner effect with critical temperatures ranging from 3.5 K to 120 K is found by the magnetic susceptibility measurements in $p$-quaterphenyl upon doping potassium in the condition of annealing or just pestling. The primary superconducting phase with critical temperature of 7.2 K can be reduplicated in several superconducting samples. In both the annealed and pestled superconducting samples, the observation of bipolaronic character by the Raman scattering measurements reveals the close relationship between bipolarons and superconductivity. The occurrence of superconductivity in potassium-doped $p$-quaterphenyl provides an indication that chain link organic molecules are potential candidates for high temperature superconductors.

cond-mat.supr-con↗

arXiv2018

Origin of efficient thermoelectric performance in half-Heusler FeNb$_{0.8}$Ti$_{0.2}$Sb

A half-Heusler material FeNb$_{0.8}$Ti$_{0.2}$Sb has been identified as a promising thermoelectric material due to its excellent thermoelectric performance at high temperatures. The origins of the efficient thermoelectric performance are investigated through a series of low-temperature (2 - 400 K) measurements. The high data coherence of the low and high temperatures is observed. An optimal and nearly temperature-independent carrier concentration is identified, which is ideal for the power factor. The obtained single type of hole carrier is also beneficial to the large Seebeck coefficient. The electronic thermal conductivity is found to be comparable to the lattice thermal conductivity and becomes the dominant component above 200 K. These findings again indicate that electron scattering plays a key role in the electrical and thermal transport properties. The dimensionless figure of merit is thus mainly governed by the electronic properties. These effects obtained at low temperatures with the avoidance of possible thermal fluctuations together offer the physical origin for the excellent thermoelectric performance in this material.

cond-mat.mtrl-sci↗

arXiv2018

Pressure-induced superconductivity in palladium sulfide

An extended study on PdS is carried out with the measurements of the resistivity, Hall coefficient, Raman scattering, and X-ray diffraction at high pressures up to 42.3 GPa. With increasing pressure, superconductivity is observed accompanying with a structural phase transition at around 19.5 GPa. The coexistence of semiconducting and metallic phases observed at normal state is examined by the Raman scattering and X-ray diffraction between 19.5 and 29.5 GPa. After that, only the metallic normal state maintains with an almost constant superconducting transition temperature. The similar evolution between the superconducting transition temperature and carrier concentration with pressure supports the phonon-mediated superconductivity in this material. These results highlight the important role of pressure played in inducing superconductivity from these narrow band-gap semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Nontrivial Metallic State of Molybdenum Disulfide

The electrical conductivity and Raman spectroscopy measurements have been performed on MoS$_2$ at high pressures up to 90 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a metallic 2H$_a$ phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a new Raman phonon mode appears in the metallic state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge density wave state, the presence of which is indicative for a possibility for an emergence of superconductivity at higher pressures.

cond-mat.mtrl-sci↗

arXiv2017

Structural and bonding character of potassium-doped p-terphenyl superconductors

Recently, there is a series of reports by Wang et al. on the superconductivity in K-doped p-terphenyl (KxC18H14) with the transition temperatures range from 7 to 123 Kelvin. Identifying the structural and bonding character is the key to understand the superconducting phases and the related properties. Therefore we carried out an extensive study on the crystal structures with different doping levels and investigate the thermodynamic stability, structural, electronic, and magnetic properties by the first-principles calculations. Our calculated structures capture most features of the experimentally observed X-ray diffraction patterns. The K doping concentration is constrained to within the range of 2 and 3. The obtained formation energy indicates that the system at x = 2.5 is more stable. The strong ionic bonding interaction is found in between K atoms and organic molecules. The charge transfer accounts for the metallic feature of the doped materials. For a small amount of charge transferred, the tilting force between the two successive benzenes drives the system to stabilize at the antiferromagnetic ground state, while the system exhibits non-magnetic behavior with increasing charge transfer. The multiformity of band structures near the Fermi level indicates that the driving force for superconductivity is complicated.

cond-mat.supr-con↗

arXiv2017

Superconductivity in Solid Benzene Molecular Crystal

Light-element compounds hold great promise of high critical temperature superconductivity judging from the theoretical perspective. Hydrogen-rich material, benzene, is such a kind of candidate but also an organic compound. A series of first-principles calculations are performed on the electronic structures, dynamics properties, and electron-phonon interactions of solid benzene at high pressures. Benzene is found to be dynamically stable in the pressure range of 180 - 200 GPa and to exhibit superconductivity with a maximum transition temperature of 20 K at 195 GPa. The phonon modes of carbon atoms are identified to mainly contribute to the electron-phonon interactions driving this superconductivity. The predicted superconductivity in this simplest pristine hydrocarbon shows a common feature in aromatic hydrocarbons and also makes it a bridge to organic and hydrogen-rich superconductors.

cond-mat.supr-con↗

arXiv2017

Superconductivity above 120 kelvin in a chain link molecule

The search for new superconducting compounds with higher critical temperatures $T_{c}^{\prime}$s has long been the very heart of scientific research on superconductivity. It took 75 years for scientists to push the $T_{c}$ above liquid nitrogen boiling temperature since the discovery of superconductivity. So far, the record high $T_{c}$ of about 130 K at atmosphere pressure was reported in some multilayer Hg(Tl)-Ba-Ca-Cu-O compounds. Meanwhile, sulfur hydride system holds the highest $T_{c}$ of around 200 K at high pressure of about 150 GPa. While keeping these records for superconductivity, either the toxicity of these superconductors or the requirement of extreme pressure condition for superconductivity limits their technology applications. Here we show that doping a chain link molecule $-$ $p$-terphenyl by potassium can bring about superconductivity at 123 K at atmosphere pressure, which is comparable to the highest $T_{c}$ in cuprates. The easy processability, light weight, durability of plastics, and environmental friendliness of this kind of new superconductor have great potential for the fine-tuning of electrical properties. This study opens a window for exploring high temperature superconductivity in chain link organic molecules.

cond-mat.supr-con↗

arXiv2017

Superconductivity in $p$-Terphenyl

Motivated by the exploration of bipolaronic superconductivity in conducting polymers, we examine such a possibility in the starting member of $p$-oligophenyls $-$ $p$-terphenyl with three phenyl rings, belonging to the family of conducting polymer polyparaphenylene. The formation of bipolarons is identified from Raman scattering measurements. Both the dc and ac magnetic susceptibility measurements reveal that $p$-terphenyl is a type-II superconductor with a critical temperature of 7.2 K upon doping potassium. The electron-lattice interaction, manifested by pronounced bipolaronic bands, is suggested to account for the observed superconductivity. Conducting polymers are thus demonstrated to have the potential for the development of new superconducting technologies and devices.

cond-mat.supr-con↗

arXiv2017

Superconductivity at 43 K in a single C-C bond linked terphenyl

Organic compounds are promising candidates to exhibit high temperature or room temperature superconductivity. However, the critical temperatures of organic superconductors are bounded to 38 K. By doping potassium into $p$-terphenyl consisting of C and H elements with three phenyl rings connected by single C-C bond in para position, we find that this material can have a superconducting phase with the critical temperature of 43 K. The superconducting parameters such as the critical fields, coherent length, and penetration depth are obtained for this superconductor. These findings open an encouraging window for the search of high temperature superconductors in chain link organic molecules.

cond-mat.supr-con↗

arXiv2016

Hydrogen sulfide at high pressure: change in stoichiometry

Hydrogen-sulfide (H2S) was studied by x-ray synchrotron diffraction (XRD) and Raman spectroscopy up to 150 GPa at 180-295 K and by quantum-mechanical variable-composition evolutionary simulations. The experiments show that H2S becomes unstable with respect to formation of new compounds with different structure and composition, including Cccm and a body-centered-cubic (bcc) like (R3m or Im-3m) H3S, the latter one predicted previously to show a record-high superconducting transition temperature, Tc of 203 K. These experiments provide experimental ground for understanding of this record high Tc. The experimental results are supported by theoretical structure searches that suggest the stability of new H3S, H4S3, H5S8, H3S5, and HS2 compounds at elevated pressures.

cond-mat.mtrl-sci↗

arXiv2016

Crossover from Metal to Insulator in Dense Lithium-Rich Compound $\textrm{CLi}_{4}$

Crystal structures of $\textrm{CLi}_4$ compounds are explored through \emph{ab} \emph{initio} evolutionary methodology. Phase transition from metal to semimetal and semiconductor, and eventually to insulator with increasing pressure are revealed under pressure. Pressure-induced evolution of anti-metallization has been described quantitatively by Fermi Surface Filling Ratio and electron energy band gap using \emph{ab} \emph{initio} \emph{GW} calculations. Anti-metallization is attributed to the hybrid valence electrons and their repulsion by core electrons into the lattice interstices. Very weak electron-phonon coupling interactions are found in the metallic phases, resulting in very low superconducting temperature.

cond-mat.mtrl-sci↗

arXiv2015

Formation of As-As Interlayer Bonding in the collapsed tetragonal phase of NaFe$_2$As$_2$ under pressure

NaFe$_2$As$_2$ is investigated experimentally using powder x-ray diffraction and Raman spectroscopy at pressures up to 23 GPa at room temperature and using ab-initio calculations. The results reveal a pressure-induced structural modification at 4 GPa from the starting tetragonal to a collapsed tetragonal phase. We determined the changes in interatomic distances under pressure that allowed us to connect the structural changes and superconductivity. The transition is related to the formation of interlayer As-As bonds at the expense of weakening of Fe-As bonds in agreement with recent theoretical predictions.

cond-mat.supr-con↗

arXiv2015

Crossover of the pairing symmetry from $s$- to $d$-wave in iron pnictide superconductors

One of the major themes in the physics of condensed matter is unconventional superconductivity in correlated electronic systems. The symmetry structure of Cooper pairs is thought to be the key to the understanding of the pairing mechanism of their superconductivity. In iron pnictide superconductors, the precise symmetry of the order parameter as well its evolution with doping remains highly controversial. There is no general consensus on the nature of pairing in iron-based superconductors leaving the perspectives ranging from $s^{++}$ wave, to $s^{\pm}$, and $d$ wave. Here we show that doping induced a sudden change of the pairing symmetry from the $s^{++}$- to $d$-wave in the BaFe$_{2-x}$Ni$_{x}$As$_{2}$ system with an $s^{\pm}$ state near the crossover boundary at $x=0.15$. The presence of the $s^{\pm}$ state is of particular scientific interest because an intermediate phase breaks the pairing symmetry when the $s$- and $d$-wave phases come together to interfere. For $x <$ 0.15, the temperature dependence of the London penetration depth, lower critical field, and specific heat in the superconducting (SC) state provides strong evidence for a two-band $s$-wave order parameter. Upon doping for $x >$ 0.15, we find the temperature and magnetic-field contributions to the specific heat in $T^{2}$ and $\sqrt{H}$, respectively, indicating line nodes in the SC gap. These findings thus clarify the symmetry nature as well as the evolution of the SC order parameter with doping in pnictide superconductors.

cond-mat.supr-con↗

arXiv2015

Pressure tuning of superconductivity independent of disorder in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+δ}$

Varying the superconducting transition temperature over a large scale of a cuprate superconductor is a necessary step for identifying the unsettled mechanism of superconductivity. Chemical doping or element substitution has been proven to be effective but also brings about lattice disorder. Such disorder can completely destroy superconductivity even at a fixed doping level. Pressure has been thought to be the most clean method for tuning superconductivity. However, pressure-induced increase of disorder was recognized from recent experiments. By choosing a disordered Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+δ}$ at the optimal doping, we perform single-crystal x-ray diffraction and magnetic susceptibility measurements at high pressures. The obtained structural data provides evidence for the robust feature for the disorder of this material in the pressure range studied. This feature ensures the pressure effects on superconductivity distinguishable from the disorder. The derived parabolic-like behavior of the transition temperature with pressure up to near 30 GPa, having a maximum around 7 GPa, offers a platform for testing any realistic theoretical models in a nearly constant disorder environment. Such a behavior can be understood when considering the carrier concentration and the pairing interaction strength as two pressure intrinsic variables.

cond-mat.supr-con↗

arXiv2015

Superconducting properties of sulfur-doped iron selenide

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_{c}$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth $λ_{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $λ_{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$-wave and single-gap BCS theory under the weak-coupling approach can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.

cond-mat.supr-con↗