arXiv subjects

Guo-qing Zheng

Publications and source records attributed to Guo-qing Zheng.

At least 55 records · Page 3Linked to original sources

arXiv2015

Ferromagnetic Spin Fluctuation and Unconventional Superconductivity in Rb$_{2}$Cr$_{3}$As$_{3}$ revealed by $^{75}$As NMR and NQR

We report $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb$_{2}$Cr$_{3}$As$_{3}$ with a quasi one-dimensional crystal structure. Below $T\sim$ 100 K, the spin-lattice relaxation rate (1/$T_{1}$) divided by temperature, 1/$T_{1}T$, increases upon cooling down to $T_{\rm c}$ = 4.8 K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/$T_{1}$ decreases rapidly below $T_{\text{c}}$ without a Hebel-Slichter peak, and follows a $T^5$ variation below $T\sim$ 3 K, which point to unconventional superconductivity with point nodes in the gap function.

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arXiv2015

Superconductivity and the electronic phase diagram of LaPt$_{2-x}$Ge$_{2+x}$

In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. LaPt$_2$Ge$_2$ undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at $T_{\rm s}$ = 394 K, accompanying a double period modulation in the $a$-axis direction, and superconducts at $T_{\rm c}$ = 0.41 K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesting. A reduction in the density of states in the monoclinic phase was found in the calculation and confirmed by $^{195}$Pt-NMR. We suggest a CDW as a possible cause for the SPT. By changing the stoichiometry between Pt and Ge, we succeeded in suppressing $T_{\rm s}$ and increasing $T_{\rm c}$ in LaPt$_{2-x}$Ge$_{2+x}$. Comparison of $^{139}$La- and $^{195}$Pt-NMR data reveals moderate fluctuations associated with SPT. From $^{139}$La-NQR measurements at zero field, we found that an isotropic superconducting gap is realized in LaPt$_{2-x}$Ge$_{2+x}$ ($x$ = 0.20). We discuss the relationship between superconductivity and the SPT order/fluctuations.

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arXiv2015

Coexistence of multiple charge-density waves and superconductivity in SrPt2As2 revealed by 75As-NMR/NQR and 195Pt-NMR

The relationship between charge density wave (CDW) orders and superconductivity in arsenide superconductor SrPt$_2$As$_2$ with $T_c$ = 5.2 K which crystallizes in the CaBe$_2$Ge$_2$-type structure was studied by $^{75}$As nuclear magnetic resonance (NMR) measurements up to 520 K, and $^{75}$As nuclear quadrupole resonance (NQR) and $^{195}$Pt-NMR measurements down to 1.5 K. At high temperature, $^{75}$As-NMR spectrum and nuclear spin relaxation rate ($1/T_1$) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below $T_{\rm CDW}^{\rm As(1)}$ $=$ 410 K and the other in the Pt-As-Pt layer below $T_{\rm CDW}^{\rm As(2)}$ $=$ 255 K. The $1/T_1$ measured by $^{75}$As-NQR shows a clear Hebel-Slichter peak just below $T_c$ and decreases exponentially well below $T_c$. Concomitantly, $^{195}$Pt Knight shift decreases below $T_c$. Our results indicate that superconductivity in SrPt$_2$As$_2$ is in the spin-singlet state with an $s$-wave gap and is robust under the two distinct CDW orders in different layers.

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arXiv2014

Magnetism and superconductivity in Sr$_2$VFeAsO$_3$ revealed by $^{75}$As- and $^{51}$V-NMR under elevated pressures

We report $^{75}$As- and $^{51}$V-nuclear magnetic resonance (NMR) measurements on the iron-based superconductor Sr$_2$VFeAsO$_3$ with alternating stacks structure. We find that the $^{75}$As nuclear spin-spin relaxation rate ($1/T_2$) shows a pronounced peak at $T_N$ = 165 K, below which the resonance peak shifts to a higher frequency due to the onset of an internal magnetic field. The $^{51}$V spectrum does not shift, but is broadened below $T_N$. We conclude that the Fe electrons oder antiferromagnetically below $T_N$ with a magnetic moment $m_{Fe}$ $\sim$ 0.4 $μ_B$. Application of external pressure up to 2.4 GPa reduces $T_N$ in a rate of $-$40 K/GPa, and enhances the superconducting transition temperature $T_c$ in a rate of 2 K/GPa. The pressure-temperature phase diagram for Sr$_2$VFeAsO$_3$ shows that superconductivity coexists with antiferromagnetism over a wide pressure range with an unprecedented high $T_c$ up to 36.5 K.

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arXiv2014

Spin-singlet superconductivity with a full gap in locally non-centrosymmetric SrPtAs

We report $^{195}$Pt-NMR and $^{75}$As-NQR measurements for the locally non-centrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin lattice relaxation rate $1/T_1$ shows a well-defined coherence peak below $T_c$ and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below $T_c$ down to $T<T_c/6$. These data together with the penetration depth obtained from the NMR spectra can be consistently explained by assuming a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-breaking is not large enough to bring about an exotic superconducting state, or the inter-layer hopping interaction is larger than the spin-orbit coupling.

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arXiv2013

Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2

A quantum critical point (QCP) is a point in a system's phase diagram at which an order is completely suppressed at absolute zero temperature (T). The presence of a quantum critical point manifests itself in the finite-T physical properties, and often gives rise to new states of matter. Superconductivity in the cuprates and in heavy fermion materials is believed by many to be mediated by fluctuations associated with a quantum critical point. In the recently-discovered iron-pnictide high temperature superconductors, it is unknown whether a QCP exists or not in a carrier-doped system. Here we report transport and nuclear magnetic resonance (NMR) measurements on BaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} = 0.10 and x_{c2} = 0.14. The electrical resistivity follows ρ= ρ_0 + A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMR measurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2} is a new type of QCP associated with a nematic structural phase transition. Our results suggest that the superconductivity in carrier-doped pnictides is closely linked to the quantum criticality.

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arXiv2013

NMR and NQR Studies on Non-centrosymmetric Superconductors Re7B3, LaBiPt, and BiPd

We report the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements for non-centrosymmetric superconductors Re7B3, LaBiPt, and BiPd containing heavy elements. For all three compounds, the spin-lattice relaxation rate 1/T1 shows a coherence peak just below Tc and decreases exponentially at low temperatures, which indicates that an isotropic superconducting gap is dominant in these compounds. In BiPd, the height of the coherence peak just below Tc is much suppressed, which suggests that there exists a substantial component of gap with nodes in this compound. Our results indicate that heavy element is not the only factor, but the extent of inversion symmetry breaking is also important to induce a large spin-orbit coupling and an unconventional superconducting state.

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arXiv2013

Superconductivity in Tetragonal LaPt_{2-x}Ge_{2+x}

We find that a tetragonal CaBe_2Ge_2-type structure can be stabilized in non-stoichiometric LaPt_{2-x}Ge_{2+x}. We further discovered that tetragonal LaPt_{2-x}Ge_{2+x} with x=0.15 and 0.2 respectively superconduct at Tc=1.85 K and 1.95 K, which is about four time higher than that in monoclinic LaPt_2Ge_2.

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arXiv2012

The Mott State and Superconductivity in Face-Centred Cubic Structured Cs3C60: A 133Cs-Nuclear Magnetic Resonance Study under Pressure

Over the past 20 years, fullerides have been studied as the source of high-transition-temperature (Tc) superconductivity except for copper oxides. The recent finding of the Mott insulating state right beside superconductivity in Cs3C60 has suggested that magnetism helps raise Tc even in fullerides as in heavy-fermion compounds, high-Tc copper oxides, two-dimensional organic conductors, and iron pnictides. Namely, one tends to think that the link between Mott insulator and superconductivity takes place in fullerides, which can give rise to the mechanism beyond the Bardeen-Cooper-Schrieffer framework. However, the relationship between the Mott state and the superconductivity in Cs3C60 is still under debate. By nuclear magnetic resonance measurements under pressure, we find that the magnetism and superconductivity in Cs3C60 are competing orders. Different from previous reports, the phase separation of Cs3C60 crystals into the Mott and metallic states allows us to systematically study the evolution of the ground state under pressure. Our careful experiments have found that the prevention of a magnetic order is rather essential for the superconductivity in face-centred cubic Cs3C60, which presents a basic strategy for finding still higher Tc in this system.

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arXiv2012

Abrupt enhancement of non-centrosymmetry and appearance of the spin-triplet superconducting state in Li_2(Pd_{1-x}Pt_{x})_3B beyond x=0.8

We report synthesis, ^{195}Pt, ^{11}B and ^{7}Li NMR measurements, and first-principle band calculation for non-centrosymmetric superconductors Li_{2}(Pd_{1-x}Pt_{x})_{3}B (x=0, 0.2, 0.5, 0.8, 0.84, 0.9 and 1). For 0 \leq x \leq 0.8, the spin-lattice relaxation rate 1/T_1 shows a clear coherence peak just below T_c, decreasing exponentially at low temperature, and the Knight shift ^{195}K decreases below T_c. For x=0.9 and 1.0, in contrast, 1/T_1 shows no coherence peak but a T^3 variation and ^{195}K remains unchanged across T_c. These results indicate that the superconducting state changes drastically from a spin-singlet dominant to a spin-triplet dominant state at x=0.8. We find that the distortion of B(Pt,Pd)_6 increases abruptly above x=0.8, which leads to an abrupt enhancement of the asymmetric spin-orbit coupling as confirmed by band calculation. Such local structure distortion that enhances the extent of inversion-symmetry breaking is primarily responsible for the pairing symmetry evolution. The insight obtained here provides a new guideline for searching new NCS superconductors with large spin-triplet component.

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arXiv2012

Microscopic coexistence of antiferromagnetic order and superconductivity in Ba$_{0.77}$K$_{0.23}$Fe$_{2}$As$_{2}$

We report ^{75}As nuclear magnetic resonance studies on an underdoped single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below T_N = 46 K, the NMR peaks for H//c split and those for H //a shift to higher frequencies, which indicates that an internal magnetic field along the c axis develops below T_N. The spin-lattice relaxation rate 1/T_1 measured at the shifted peak with H//a, which experiences the internal field, shows a distinct decrease below Tc(μ_0H = 12 T) = 16 K, following a T^3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

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arXiv2012

Multiple Superconducting Gaps, Anisotropic Spin Fluctuations and Spin-Orbit Coupling in Iron-Pnictides

This article reviews the NMR and NQR studies on iron-based high-temperature superconductors by the IOP/Okayama group. It was found that the electron pairs in the superconducting state are in the spin-singlet state with multiple fully-opened energy gaps. The antiferromagnetic spin fluctuations in the normal state are found to be closely correlated with the superconductivity. Also the antiferromagnetic spin fluctuations are anisotropic in the spin space, which is different from the case in copper oxide superconductors. This anisotropy originates from the spin-orbit coupling and is an important reflection of the multiple-bands nature of this new class of superconductors.

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arXiv2012

Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance

We report a systematic study by 75As nuclear-quadrupole resonance in LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime 0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of the superconducting transition temperature Tc is found, with the highest Tc = 27 K at x = 0.06 which is realized under significant AFSF. With increasing x further, the AFSF decreases, and so does Tc. These features resemble closely the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1) below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates that the energy gaps are fully-opened. The temperature variation of 1/T1 below Tc is rendered nonexponential for other x by impurity scattering.

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arXiv2011

Nodeless energy gaps of single-crystalline Ba0.68K0.32Fe2As2 as seen via 75As NMR

We report $^{75}$As nuclear magnetic resonance studies on a very clean hole-doped single-crystal Ba$_{0.68}$K$_{0.32}$Fe$_{2}$As$_{2}$ ($T_{\rm {c}}=38.5$ K). The spin-lattice relaxation rate $1/T_{1}$ shows an exponential decrease below $T \simeq 0.45 T_{\rm c}$ down to $T \simeq 0.11 T_{\rm c}$, which indicates a fully opened energy gap. From the ratio $(T_{1})_{c} / (T_{1})_{a}$, where $a$ and $c$ denote the crystal directions, we find that the antiferromagnetic spin fluctuation is anisotropic in the spin space above $T_{\rm c}$. The anisotropy decreases below $T_{\rm c}$ and disappears at $T \rightarrow 0$. We argue that the anisotropy stems from spin-orbit coupling whose effect vanishes when spin-singlet electron pairs form with a nodeless gap.

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arXiv2011

Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog

We present extensive 75As NMR and NQR data on the superconducting arsenides PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below Tc and shows a step-wise T variation as well. These results indicate spinsinglet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO0.9F0.1. We will discuss the implications of these results and highlight the importance of the Fermi surface topology.

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arXiv2010

Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi2Sr2-xLaxCuO6+delta Revealed by 63,65Cu-Nuclear Magnetic Resonance in Very High Magnetic Fields

We report the results of the Knight shift by 63,65Cu-nuclear-magnetic resonance (NMR) measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely and the pseudogap ground state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite density of states (DOS) at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x > 0.8, which suggests that the DOS plunges to zero upon approaching the Mott insulating phase.

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arXiv2010

$^{75}$As NQR and NMR studies of superconductivity and electron correlations in iron arsenide Li$_{x}$FeAs

We report the $^{75}$As-NQR and NMR studies on the iron arsenide superconductor Li$_{x}$FeAs with $T_{\rm c} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\rm c}$ without a coherence peak, and can be fitted by gaps with s$^{\pm}$-wave symmetry in the presence of impurity scattering. In the normal state, both $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50 K$. Estimate of the Korringa ratio shows that the spin correlations are weaker than that in other families of iron arsenides, which may account for the lower $T_{\rm c}$ in this material.

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arXiv2010

Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2

75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak just below Tc(P) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.

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