Organic materials especially those with π-electrons have attracted widespread attention because t... more Organic materials especially those with π-electrons have attracted widespread attention because they are promising candidates to exhibit high temperature or even room temperature superconductivity. 2,2′-Bipyridine as a basic raw material is a simple molecule that only contains C, N, and H atoms, and is widely used in metal chelating ligands due to its ease of functionalization and robust redox stability. By doping sodium potassium alloy into 2,2′-bipyridine at approximately room temperature, we successfully detected superconductivity with a critical temperature around 7 K through both the dc and ac magnetic measurements together with the zero resistance state by resistance measurements. Furthermore, the superconducting parameters such as the critical fields, London penetration depth, and Ginzburg−Landau coherence length of the 7 K superconducting phase have been obtained. This finding not only broadens the applications of 2,2′bipyridine, but also opens an encouraging window for the search of superconductors in photoelectric materials such as pyridine compounds and their derivatives.
Organic materials especially those with π-electrons have attracted widespread attention because t... more Organic materials especially those with π-electrons have attracted widespread attention because they are promising candidates to exhibit high temperature or even room temperature superconductivity. 2,2′-Bipyridine as a basic raw material is a simple molecule that only contains C, N, and H atoms, and is widely used in metal chelating ligands due to its ease of functionalization and robust redox stability. By doping sodium potassium alloy into 2,2′-bipyridine at approximately room temperature, we successfully detected superconductivity with a critical temperature around 7 K through both the dc and ac magnetic measurements together with the zero resistance state by resistance measurements. Furthermore, the superconducting parameters such as the critical fields, London penetration depth, and Ginzburg-Landau coherence length of the 7 K superconducting phase have been obtained. This finding not only broadens the applications of 2,2′bipyridine, but also opens an encouraging window for the search of superconductors in photoelectric materials such as pyridine compounds and their derivatives.
Explorations of benzene-based organic superconductors and bismuth-based functional materials are ... more Explorations of benzene-based organic superconductors and bismuth-based functional materials are today's hottest topics in chemistry, physics, and materials science. Here, we show that by doping potassium into an organobismuth molecule, trip -tolylbismuthine, which is composed of one bismuth atom and three methylphenyl groups, all synthesized samples exhibit type-II superconductivity at 3.6 K at ambient pressure and one sample also shows superconductivity at 5.3 K. The common 3.6 K superconducting phase is identified to have a triclinic P1 structure, with a mole ratio of 3:1 between potassium and trip -tolylbismuthine. The calculated electronic structure indicates that superconductivity is produced by transferring an electron from K 4s to the C 2p orbital, which results in both red and blue shifts of the Raman spectra. Our study enriches the physical functionality of organobismuth compounds and illustrates a new route for the search of organic superconductors.
To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by ... more To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by an annealing or just a pestling process. The Meissner effect with critical temperatures ranging from 3.5 to 120 K is found by the magnetic susceptibility measurements in doped samples. The primary superconducting phase with a critical temperature of 7.2 K can be duplicated in the annealed and pestled samples. The charge transfer from metal to molecule is confirmed from the Raman scattering measurements. The X-ray diffraction analysis suggests that the lowtemperature superconducting phase is due to the two-electron doping, whereas the high-temperature one corresponds to the high doping content. The occurrence of superconductivity in potassium-doped p-quaterphenyl supports the chain link organic molecules as promising candidates for high-temperature superconductors. This work also provides a simple method for synthesizing organic superconductors by pestling without annealing.
Superconductivity has been predicted or measured for most alkali metals under high pressure, but ... more Superconductivity has been predicted or measured for most alkali metals under high pressure, but the computed critical temperature (Tc) of sodium (Na) at the face-centered cubic (fcc) phase is vanishingly low. Here we report a thorough, first-principles investigation of superconductivity in Na under pressures up to 260 GPa, where the metal-to-insulator transition occurs. Linear-response calculations and density functional perturbation theory were employed to evaluate phonon distributions and the electron-phonon coupling for bcc, fcc, cI16, and tI19 Na. Our results indicate that the maximum electron-phonon coupling parameter, λ, is 0.5 for the cI16 phase, corresponding to a theoretical peak in the critical temperature at Tc≈1.2 K. When pressure decreases or increases from 130 GPa, Tc drops quickly. This is mainly due to the lack of p-d hybridization in Na even at 260 GPa. Since current methods based on the Eliashberg and McMillian formalisms tend to overestimate the Tc (especially th...
Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for estab... more Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.
In the original version of this Letter published online, the x axis of Fig. 2a was labelled incor... more In the original version of this Letter published online, the x axis of Fig. 2a was labelled incorrectly. In addition, tick labels have been added to the x axes of Fig. 2b and Fig. 2d. This has been corrected in all versions of the Letter.
The boundary condition of the initial universe proposed by Vilenkin (1988) was studied for Einste... more The boundary condition of the initial universe proposed by Vilenkin (1988) was studied for Einstein gravitation with a positive cosmological constant in the minisuperspace model based on the Bianchi-III manifold. The classical equations of motion can be reduced to the motion equations of free particle in Minkowski spacetime by the special choice of local coordinate and lapse function. The Wheeler-DeWitt
First-principles computations based on the density functional theory have been performed to searc... more First-principles computations based on the density functional theory have been performed to search for the most stable structure of phase-III solid hydrogen. Specifically the phase diagram at zero temperature is predicted by calculating the static total energy and the zero-point motion. Our results suggest that solid hydrogen under pressures in a range of ˜150-300 GPa could have lower symmetry than
The structural stability of methane hydrate under pressure at room temperature was examined by bo... more The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil cells. The diffraction data for types II (sII) and H (sH) were refined to the known structures with space groups Fd3m and P6 3 /mmc, respectively. Upon compression, sI methane hydrate transforms to the sII phase at 120 MPa, and then to the sH phase at 600 MPa. The sII methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm3m structure and ice VII (Pn3m). The results highlight the role of guest (CH 4)-host (H 2 O) interactions in the stabilization of the hydrate structures under pressure.
The structural, vibrational, and magnetic properties of well prepared Bi 1−x Ho x FeO 3 (x = 0-0.... more The structural, vibrational, and magnetic properties of well prepared Bi 1−x Ho x FeO 3 (x = 0-0.175) powders are investigated by combining X-ray diffraction, Raman scattering, and magnetometry measurements. A structural symmetry breaking from the rhombohedral R3c to orthorhombic Pnma between x = 0.10 and 0.125 is identified from the X-ray and Raman measurements, accompanying a ferroelectric-paraelectric phase transition. The remnant magnetization of Bi 1−x Ho x FeO 3 is enhanced before approaching the ferroelectric-paraelectric phase boundary, and then it slightly decreases until x = 0.175. Such enhancement (0 ≤ x ≤ 0.10) is suggested to result from the destruction of the spin cycloid structure. The decrease in the remnant magnetization with higher substitution concentration is due to the further destruction in the space modulated spin structure allowing a more perfect antiferromagnetic ordering.
We study the electronic and lattice dynamical properties of compressed solid germane in the press... more We study the electronic and lattice dynamical properties of compressed solid germane in the pressure range up to 200 GPa with density functional theory. A stable metallic structure, Aba2, with a base-centered orthorhombic symmetry was found to be the lowest enthalpy phase for pressure from 23 to 177 GPa, suggesting an insulator to metal phase transition around 23 GPa. The Aba2 structure is predicted to have higher superconducting transition temperature than SiH4 reported recently, thus presenting new possibilities for exploring high temperature superconductivity in this hydrogen-rich system.
Using two different experimental techniques, we studied single crystals of the 122-FeAs family wi... more Using two different experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field Hc1(T) of a Ca0.32Na0.68Fe2As2 (Tc ≈ 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gaps model, while a d-wave approach cannot be used to fit the London penetration depth data. The intrinsic multiple Andreev reflection effect (IMARE) spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same Tc. We estimate the range of the large gap value ∆L = (6-8) meV (depending on small variation of Tc) and its k-space anisotropy about 30%, and the small gap is about ∆S ≈ (1.7±0.3) meV. This clearly indicates that the gap structure of our investigated systems is more likely to be of the two-gap nodeless s-wave type.
A phenomenological analysis for the pressure-induced change of the hole concentration in the CuO2... more A phenomenological analysis for the pressure-induced change of the hole concentration in the CuO2 layers (nH) is developed. This effect along with the pressure-induced change of the maximum value of critical temperature (Tc) enables us to take account of the external pressure dependence of Tc on different oxygen doping and different ambient Tc on the basis of the inverted parabolic
The CuO2 layer dependence of the superconducting transition temperature Tc at ambient pressure on... more The CuO2 layer dependence of the superconducting transition temperature Tc at ambient pressure on the intrinsic transition temperature Tc(i) of the type-I CuO2 plane in which the copper atom has fivefold pyramid coordination of oxygen and the fourfold square coordinated type-II plane is studied in terms of the generalized Lawrence-Doniach theory. Calculations show that the increase of Tc with the number of CuO2 layers benefits from the difference of the intrinsic Tc(i) of the two types of CuO2 layers and that interlayer coupling between the neighboring CuO2 layers can enhance Tc for the multilayer cuprates. The upper limit of Tc is predicted to be 146 K for the bilayer thallium-based series. We present an extended pressure-induced charge transfer model for layered cuprate superconductors, assuming that the charge distribution among the crystallographically inequivalent CuO2 layers is nonhomogeneous, which enables us to investigate the pressure effect on the intrinsic Tc(i). The intrinsic Tc(i) of the two types of CuO2 layers is predicted to behave with pressure in a paraboliclike manner. For the optimally doped single, double, and triple CuO2 sheets compounds, the saturation values of Tc(I) of the type-I CuO2 plane of 91.1, 119.6, and 133.9 K are obtained when P=2.3, 2.9, and 6.0 GPa, respectively. For the underdoped Tl-2234 compound with Tc=113 K, the calculated Tc(I) of 118.5 K is obtained at P=6.0 GPa. Under the application of pressure, the intrinsic Tc(II) of the type-II CuO2 plane in Tl-2234 increases strongly compared with a modest increase of Tc(II) in Tl-2223, possibly resulting from its underdoped nature. We suggest that at low pressure the Tc is the intrinsic Tc(i) of the type-I CuO2 plane, and at relatively high pressures the intrinsic effect of the type-II plane dominates. Our theoretical results are in agreement with experiments.
Pressure has an essential role in the production and control of superconductivity in iron-based s... more Pressure has an essential role in the production and control of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature T(c) to a record high of 55 K in these materials. In the same way as T(c) exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned T(c) typically drops monotonically after passing the optimal pressure. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the T(c) drops from the first maximum of 32 K at 1 GPa. The T(c) of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0-48.7 K for Tl(0.6)Rb(0.4)Fe(1.67)Se(2), K(0.8)Fe(1.7)Se(2) and K(0.8)Fe(1.78)Se(2).
Hydrogen-rich materials have fascinating physical and chemical properties such as various structu... more Hydrogen-rich materials have fascinating physical and chemical properties such as various structures and superconductivity under high-pressure. In this study, structural, electronic, dynamical, and superconducting properties of GeH 4 (H 2 ) 2 are investigated based on the first-principles calculations. We first predict several phase transitions of GeH 4 (H 2 ) 2 under pressure. Below 28 GPa, two degenerated structures with I4̅ m2 and Pmn2 1 symmetries are preferred, which can be viewed as the distortion of the experimentally observed fcc structure. Then, the GeH 4 (H 2 ) 2 , via a triclinic phase that stabilizes in the pressure range of 28-48 GPa, transforms into a metallic orthorhombic phase in which appears the metallization induced by pressure. Another metallic phase with P2 1 /c symmetry enters the phase diagram at around 220 GPa, which is more stable than the case of a decomposed material, and its stability is also confirmed by including the zero point energy correction. In the high-pressure P2 1 /c phase, the superconductivity is found, and the superconducting transition temperature is predicted to be as high as 76-90 K at 250 GPa. This superconductivity mainly results from the local vibrations of more H 2 units, though the vibration of Ge in an H 2 -formed grid also contributes to the electron-phonon interaction. This study is helpful for understanding the superconducting mechanism on hydrogen-rich compounds.
Proceedings of the National Academy of Sciences, 2007
High-temperature superconductivity in cuprates was discovered almost exactly 20 years ago, but a ... more High-temperature superconductivity in cuprates was discovered almost exactly 20 years ago, but a satisfactory theoretical explanation for this phenomenon is still lacking. The isotope effect has played an important role in establishing electron–phonon interaction as the dominant interaction in conventional superconductors. Here we present a unified picture of the oxygen isotope effect in cuprate superconductors based on a phonon-mediated d -wave pairing model within the Bardeen–Cooper–Schrieffer theory. We show that this model accounts for the magnitude of the isotope exponent as functions of the doping level as well as the variation between different cuprate superconductors. The isotope effect on the superconducting transition is also found to resemble the effect of pressure on the transition. These results indicate that the role of phonons should not be overlooked for explaining the superconductivity in cuprates.
Proceedings of the National Academy of Sciences, 2008
There is a great interest in electronic transitions in hydrogen-rich materials under extreme cond... more There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (CH 4 ), silane (SiH 4 ), and germane (GeH 4 ) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that SiH 4 undergoes three phase transitions before becoming opaque at 27–30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization.
Organic materials especially those with π-electrons have attracted widespread attention because t... more Organic materials especially those with π-electrons have attracted widespread attention because they are promising candidates to exhibit high temperature or even room temperature superconductivity. 2,2′-Bipyridine as a basic raw material is a simple molecule that only contains C, N, and H atoms, and is widely used in metal chelating ligands due to its ease of functionalization and robust redox stability. By doping sodium potassium alloy into 2,2′-bipyridine at approximately room temperature, we successfully detected superconductivity with a critical temperature around 7 K through both the dc and ac magnetic measurements together with the zero resistance state by resistance measurements. Furthermore, the superconducting parameters such as the critical fields, London penetration depth, and Ginzburg−Landau coherence length of the 7 K superconducting phase have been obtained. This finding not only broadens the applications of 2,2′bipyridine, but also opens an encouraging window for the search of superconductors in photoelectric materials such as pyridine compounds and their derivatives.
Organic materials especially those with π-electrons have attracted widespread attention because t... more Organic materials especially those with π-electrons have attracted widespread attention because they are promising candidates to exhibit high temperature or even room temperature superconductivity. 2,2′-Bipyridine as a basic raw material is a simple molecule that only contains C, N, and H atoms, and is widely used in metal chelating ligands due to its ease of functionalization and robust redox stability. By doping sodium potassium alloy into 2,2′-bipyridine at approximately room temperature, we successfully detected superconductivity with a critical temperature around 7 K through both the dc and ac magnetic measurements together with the zero resistance state by resistance measurements. Furthermore, the superconducting parameters such as the critical fields, London penetration depth, and Ginzburg-Landau coherence length of the 7 K superconducting phase have been obtained. This finding not only broadens the applications of 2,2′bipyridine, but also opens an encouraging window for the search of superconductors in photoelectric materials such as pyridine compounds and their derivatives.
Explorations of benzene-based organic superconductors and bismuth-based functional materials are ... more Explorations of benzene-based organic superconductors and bismuth-based functional materials are today's hottest topics in chemistry, physics, and materials science. Here, we show that by doping potassium into an organobismuth molecule, trip -tolylbismuthine, which is composed of one bismuth atom and three methylphenyl groups, all synthesized samples exhibit type-II superconductivity at 3.6 K at ambient pressure and one sample also shows superconductivity at 5.3 K. The common 3.6 K superconducting phase is identified to have a triclinic P1 structure, with a mole ratio of 3:1 between potassium and trip -tolylbismuthine. The calculated electronic structure indicates that superconductivity is produced by transferring an electron from K 4s to the C 2p orbital, which results in both red and blue shifts of the Raman spectra. Our study enriches the physical functionality of organobismuth compounds and illustrates a new route for the search of organic superconductors.
To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by ... more To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by an annealing or just a pestling process. The Meissner effect with critical temperatures ranging from 3.5 to 120 K is found by the magnetic susceptibility measurements in doped samples. The primary superconducting phase with a critical temperature of 7.2 K can be duplicated in the annealed and pestled samples. The charge transfer from metal to molecule is confirmed from the Raman scattering measurements. The X-ray diffraction analysis suggests that the lowtemperature superconducting phase is due to the two-electron doping, whereas the high-temperature one corresponds to the high doping content. The occurrence of superconductivity in potassium-doped p-quaterphenyl supports the chain link organic molecules as promising candidates for high-temperature superconductors. This work also provides a simple method for synthesizing organic superconductors by pestling without annealing.
Superconductivity has been predicted or measured for most alkali metals under high pressure, but ... more Superconductivity has been predicted or measured for most alkali metals under high pressure, but the computed critical temperature (Tc) of sodium (Na) at the face-centered cubic (fcc) phase is vanishingly low. Here we report a thorough, first-principles investigation of superconductivity in Na under pressures up to 260 GPa, where the metal-to-insulator transition occurs. Linear-response calculations and density functional perturbation theory were employed to evaluate phonon distributions and the electron-phonon coupling for bcc, fcc, cI16, and tI19 Na. Our results indicate that the maximum electron-phonon coupling parameter, λ, is 0.5 for the cI16 phase, corresponding to a theoretical peak in the critical temperature at Tc≈1.2 K. When pressure decreases or increases from 130 GPa, Tc drops quickly. This is mainly due to the lack of p-d hybridization in Na even at 260 GPa. Since current methods based on the Eliashberg and McMillian formalisms tend to overestimate the Tc (especially th...
Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for estab... more Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.
In the original version of this Letter published online, the x axis of Fig. 2a was labelled incor... more In the original version of this Letter published online, the x axis of Fig. 2a was labelled incorrectly. In addition, tick labels have been added to the x axes of Fig. 2b and Fig. 2d. This has been corrected in all versions of the Letter.
The boundary condition of the initial universe proposed by Vilenkin (1988) was studied for Einste... more The boundary condition of the initial universe proposed by Vilenkin (1988) was studied for Einstein gravitation with a positive cosmological constant in the minisuperspace model based on the Bianchi-III manifold. The classical equations of motion can be reduced to the motion equations of free particle in Minkowski spacetime by the special choice of local coordinate and lapse function. The Wheeler-DeWitt
First-principles computations based on the density functional theory have been performed to searc... more First-principles computations based on the density functional theory have been performed to search for the most stable structure of phase-III solid hydrogen. Specifically the phase diagram at zero temperature is predicted by calculating the static total energy and the zero-point motion. Our results suggest that solid hydrogen under pressures in a range of ˜150-300 GPa could have lower symmetry than
The structural stability of methane hydrate under pressure at room temperature was examined by bo... more The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil cells. The diffraction data for types II (sII) and H (sH) were refined to the known structures with space groups Fd3m and P6 3 /mmc, respectively. Upon compression, sI methane hydrate transforms to the sII phase at 120 MPa, and then to the sH phase at 600 MPa. The sII methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm3m structure and ice VII (Pn3m). The results highlight the role of guest (CH 4)-host (H 2 O) interactions in the stabilization of the hydrate structures under pressure.
The structural, vibrational, and magnetic properties of well prepared Bi 1−x Ho x FeO 3 (x = 0-0.... more The structural, vibrational, and magnetic properties of well prepared Bi 1−x Ho x FeO 3 (x = 0-0.175) powders are investigated by combining X-ray diffraction, Raman scattering, and magnetometry measurements. A structural symmetry breaking from the rhombohedral R3c to orthorhombic Pnma between x = 0.10 and 0.125 is identified from the X-ray and Raman measurements, accompanying a ferroelectric-paraelectric phase transition. The remnant magnetization of Bi 1−x Ho x FeO 3 is enhanced before approaching the ferroelectric-paraelectric phase boundary, and then it slightly decreases until x = 0.175. Such enhancement (0 ≤ x ≤ 0.10) is suggested to result from the destruction of the spin cycloid structure. The decrease in the remnant magnetization with higher substitution concentration is due to the further destruction in the space modulated spin structure allowing a more perfect antiferromagnetic ordering.
We study the electronic and lattice dynamical properties of compressed solid germane in the press... more We study the electronic and lattice dynamical properties of compressed solid germane in the pressure range up to 200 GPa with density functional theory. A stable metallic structure, Aba2, with a base-centered orthorhombic symmetry was found to be the lowest enthalpy phase for pressure from 23 to 177 GPa, suggesting an insulator to metal phase transition around 23 GPa. The Aba2 structure is predicted to have higher superconducting transition temperature than SiH4 reported recently, thus presenting new possibilities for exploring high temperature superconductivity in this hydrogen-rich system.
Using two different experimental techniques, we studied single crystals of the 122-FeAs family wi... more Using two different experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field Hc1(T) of a Ca0.32Na0.68Fe2As2 (Tc ≈ 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gaps model, while a d-wave approach cannot be used to fit the London penetration depth data. The intrinsic multiple Andreev reflection effect (IMARE) spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same Tc. We estimate the range of the large gap value ∆L = (6-8) meV (depending on small variation of Tc) and its k-space anisotropy about 30%, and the small gap is about ∆S ≈ (1.7±0.3) meV. This clearly indicates that the gap structure of our investigated systems is more likely to be of the two-gap nodeless s-wave type.
A phenomenological analysis for the pressure-induced change of the hole concentration in the CuO2... more A phenomenological analysis for the pressure-induced change of the hole concentration in the CuO2 layers (nH) is developed. This effect along with the pressure-induced change of the maximum value of critical temperature (Tc) enables us to take account of the external pressure dependence of Tc on different oxygen doping and different ambient Tc on the basis of the inverted parabolic
The CuO2 layer dependence of the superconducting transition temperature Tc at ambient pressure on... more The CuO2 layer dependence of the superconducting transition temperature Tc at ambient pressure on the intrinsic transition temperature Tc(i) of the type-I CuO2 plane in which the copper atom has fivefold pyramid coordination of oxygen and the fourfold square coordinated type-II plane is studied in terms of the generalized Lawrence-Doniach theory. Calculations show that the increase of Tc with the number of CuO2 layers benefits from the difference of the intrinsic Tc(i) of the two types of CuO2 layers and that interlayer coupling between the neighboring CuO2 layers can enhance Tc for the multilayer cuprates. The upper limit of Tc is predicted to be 146 K for the bilayer thallium-based series. We present an extended pressure-induced charge transfer model for layered cuprate superconductors, assuming that the charge distribution among the crystallographically inequivalent CuO2 layers is nonhomogeneous, which enables us to investigate the pressure effect on the intrinsic Tc(i). The intrinsic Tc(i) of the two types of CuO2 layers is predicted to behave with pressure in a paraboliclike manner. For the optimally doped single, double, and triple CuO2 sheets compounds, the saturation values of Tc(I) of the type-I CuO2 plane of 91.1, 119.6, and 133.9 K are obtained when P=2.3, 2.9, and 6.0 GPa, respectively. For the underdoped Tl-2234 compound with Tc=113 K, the calculated Tc(I) of 118.5 K is obtained at P=6.0 GPa. Under the application of pressure, the intrinsic Tc(II) of the type-II CuO2 plane in Tl-2234 increases strongly compared with a modest increase of Tc(II) in Tl-2223, possibly resulting from its underdoped nature. We suggest that at low pressure the Tc is the intrinsic Tc(i) of the type-I CuO2 plane, and at relatively high pressures the intrinsic effect of the type-II plane dominates. Our theoretical results are in agreement with experiments.
Pressure has an essential role in the production and control of superconductivity in iron-based s... more Pressure has an essential role in the production and control of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature T(c) to a record high of 55 K in these materials. In the same way as T(c) exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned T(c) typically drops monotonically after passing the optimal pressure. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the T(c) drops from the first maximum of 32 K at 1 GPa. The T(c) of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0-48.7 K for Tl(0.6)Rb(0.4)Fe(1.67)Se(2), K(0.8)Fe(1.7)Se(2) and K(0.8)Fe(1.78)Se(2).
Hydrogen-rich materials have fascinating physical and chemical properties such as various structu... more Hydrogen-rich materials have fascinating physical and chemical properties such as various structures and superconductivity under high-pressure. In this study, structural, electronic, dynamical, and superconducting properties of GeH 4 (H 2 ) 2 are investigated based on the first-principles calculations. We first predict several phase transitions of GeH 4 (H 2 ) 2 under pressure. Below 28 GPa, two degenerated structures with I4̅ m2 and Pmn2 1 symmetries are preferred, which can be viewed as the distortion of the experimentally observed fcc structure. Then, the GeH 4 (H 2 ) 2 , via a triclinic phase that stabilizes in the pressure range of 28-48 GPa, transforms into a metallic orthorhombic phase in which appears the metallization induced by pressure. Another metallic phase with P2 1 /c symmetry enters the phase diagram at around 220 GPa, which is more stable than the case of a decomposed material, and its stability is also confirmed by including the zero point energy correction. In the high-pressure P2 1 /c phase, the superconductivity is found, and the superconducting transition temperature is predicted to be as high as 76-90 K at 250 GPa. This superconductivity mainly results from the local vibrations of more H 2 units, though the vibration of Ge in an H 2 -formed grid also contributes to the electron-phonon interaction. This study is helpful for understanding the superconducting mechanism on hydrogen-rich compounds.
Proceedings of the National Academy of Sciences, 2007
High-temperature superconductivity in cuprates was discovered almost exactly 20 years ago, but a ... more High-temperature superconductivity in cuprates was discovered almost exactly 20 years ago, but a satisfactory theoretical explanation for this phenomenon is still lacking. The isotope effect has played an important role in establishing electron–phonon interaction as the dominant interaction in conventional superconductors. Here we present a unified picture of the oxygen isotope effect in cuprate superconductors based on a phonon-mediated d -wave pairing model within the Bardeen–Cooper–Schrieffer theory. We show that this model accounts for the magnitude of the isotope exponent as functions of the doping level as well as the variation between different cuprate superconductors. The isotope effect on the superconducting transition is also found to resemble the effect of pressure on the transition. These results indicate that the role of phonons should not be overlooked for explaining the superconductivity in cuprates.
Proceedings of the National Academy of Sciences, 2008
There is a great interest in electronic transitions in hydrogen-rich materials under extreme cond... more There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (CH 4 ), silane (SiH 4 ), and germane (GeH 4 ) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that SiH 4 undergoes three phase transitions before becoming opaque at 27–30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization.
Uploads
Papers by Xiaojia Chen