arXiv subjects

Guo-qing Zheng

Publications and source records attributed to Guo-qing Zheng.

At least 37 records · Page 2Linked to original sources

arXiv2021

Microscopic evidence for the intra-unit-cell electronic nematicity inside the pseudogap phase in YBa$_2$Cu$_4$O$_8$

Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-$T_c$ superconductors. Here, we report $^{17}$O nuclear magnetic resonance (NMR) studies on two planar oxygen sites in stoichiometric cuprate YBa$_2$Cu$_4$O$_8$ to investigate the symmetry breaking inside the pseudogap phase. We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below $T_{\rm nem} \sim$ 185 K, which is close to the pseudogap temperature $T^{\ast}$. Our result provides a microscopic evidence for intra-unit-cell electronic nematicity. The difference in quadrupole resonance frequency between the two oxygen sites is unchanged below $T_{\rm nem}$, which suggests that the observed nematicity does not directly stem from the local charge density modulation. Furthermore, a short-range charge density wave (CDW) order is observed below $T \simeq$ 150 K. The additional broadening in the $^{17}$O-NMR spectra because of this CDW order is determined to be inequivalent for the two oxygen sites, which is similar to that observed in case of nematicity. These results suggest a possible connection between nematicity, CDW order, and pseudogap.

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arXiv2020

Localized-to-itinerant transition preceding antiferromagnetic quantum critical point and gapless superconductivity in CeRh$_{0.5}$Ir$_{0.5}$In$_5$

A fundamental problem posed from the study of correlated electron compounds, of which heavy-fermion systems are prototypes, is the need to understand the physics of states near a quantum critical point (QCP). At a QCP, magnetic order is suppressed continuously to zero temperature and unconventional superconductivity often appears. Here, we report pressure ($P$) -dependent $^{115}$In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh$_{0.5}$Ir$_{0.5}$In$_5$. These experiments reveal an antiferromagnetic (AF) QCP at $P_{\rm c}^{\rm AF}$ = 1.2 GPa where a dome of superconductivity reaches a maximum transition temperature $T_{\rm c}$. Preceding $P_{\rm c}^{\rm AF}$, however, the NQR frequency $ν_{\rm Q}$ undergoes an abrupt increase at $P_{\rm c}^{\rm *}$ = 0.8 GPa in the zero-temperature limit, indicating a change from localized to itinerant character of cerium's $f$-electron and associated small-to-large change in the Fermi surface. At $P_{\rm c}^{\rm AF}$ where $T_{\rm c}$ is optimized, there is an unusually large fraction of gapless excitations well below $T_{\rm c}$ that implicates spin-singlet, odd-frequency pairing symmetry.

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arXiv2020

Exotic Landau Diamagnetism and Weyl-Fermions Excitations in TaAs Revealed by $^{75}$As NMR and NQR

The electronic and superconducting properties associated with the topologically non-trivial bands in Weyl semimetals have recently attracted much attention. We report the microscopic properties of the type-I Weyl semimetal TaAs measured by $^{75}$As nuclear magnetic (quadrupole) resonance under zero and elevated magnetic fields over a wide temperature range up to 500 K. The magnetic susceptibility measured by the Knight shift $K$ is found to be negative at low magnetic fields and have a strong field ($B$) dependence as ln$B$ at $T$ = 1.56 K. Such nonlinear field-dependent magnetization can be well accounted for by Landau diamagnetism arising from the 3D linearly dispersed bands, and thus is a fingerprint of topological semimetals. We further study the low-energy excitations by the spin-lattice relaxation rate 1/$T_{1}$. At zero field and 30 K $\leq T\leq$ 250 K, 1/$T_{1}T$ shows a $T^{2}$ variation due to Weyl nodes excitations. At $B \sim$ 13 T, $1/T_1T$ exhibits the same $T$-dependence but with a smaller value, scaling with $K^2\propto T^2$, which indicates that the Korringa relation also holds for a Weyl semimetal. Analysis of the Korringa ratio reveals that the energy range of the linear bands is about 250 K in TaAs.

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arXiv2020

NMR and NQR studies on transition-metal arsenide superconductors LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3

We report 75As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3. In the superconducting state of LaRu2As2, a Hebel- Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate-1/T1 just below Tc, which indicates that LaRu2As2 is a full-gap superperconducor. For KCa2Fe4As4F2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF = Tc1/Tab1 is small and temperature independent, implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa2Fe4As4F2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A2Cr3As3, we calculate the electric field gradient by first-principle method and assign the 75As-NQR peaks with two crystallographically different As sites, paving the way for further NMR investigation.

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arXiv2019

Direction and symmetry transition of the vector order parameter in topological superconductors Cu$_x$Bi$_2$Se$_3$

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu$_{0.3}$Bi$_2$Se$_3$ is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter $\textbf{d}$ and its pinning mechanism are still unclear. We have succeeded in growing Cu$_x$Bi$_2$Se$_3$ single crystals with unprecedented high doping levels. For samples with $x$ = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by Knight shift, the in-plane upper critical field $H_{\rm c2}$ shows a two-fold symmetry. However, the angle at which the $H_{\rm c2}$ becomes minimal is different by 90$^\circ$ among them, which indicates that the $\textbf{d}$-vector direction is different for each crystal likely due to a different local environment. The carrier density for $x$ = 0.46 and 0.54 increases substantially compared to $x\leq$ 0.37. Surprisingly, the in-plane $H_{\rm c2}$ anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases.

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arXiv2019

Tuning the distance to a possible ferromagnetic quantum critical point in A2Cr3As3

Although superconductivity in the vicinity of antiferromagnetic (AFM) instability has been extensively explored in the last three decades or so, superconductivity in compounds with a background of ferromagnetic (FM) spin fluctuations is still rare. We report 75As nuclear quadrupole resonance measurements on the A2Cr3As3 family, which is the first group of Cr-based superconductors at ambient pressure, with A being alkali elements. From the temperature dependence of the spin-lattice relaxation rate (1/T1), we find that by changing A in the order of A=Na, Na0.75K0.25, K, and Rb, the system is tuned to approach a FM quantum critical point (QCP). This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90 degrees, which enhances the FM interaction via the Cr2-As2-Cr2 path. Upon moving away from the QCP, the superconducting transition temperature Tsc increases progressively up to 8.0 K in Na2Cr3As3, which is in sharp contrast to the AFM case where Tsc usually shows a maximum around a QCP. The 1/T1 decreases rapidly below Tsc with no Hebel-Slichter peak, and ubiquitously follows a T5 variation below a characteristic temperature T*=0.6 Tsc, which indicates the existence of point nodes in the superconducting gap function commonly in the family. These results suggest that the A2Cr3As3 family is a possible solid-state analog of superfluid 3He.

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arXiv2018

Electron Mass Enhancement near a Nematic Quantum Critical Point in NaFe$_{1-x}$Co$_{x}$As

A magnetic order can be completely suppressed at zero temperature($T$), by doping carriers or applying pressure, at a quantum critical point(QCP), around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance(NMR) studies on NaFe$_{1-x}$Co$_{x}$As where magnetic and nematic transitions are well separated. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state, which is related to London penetration depth $λ_{\rm L}$ that measures the electron mass $m^*$. We discovered two peaks in the doping dependence of $λ_{\rm L}^2$($T\sim$0); one at $x_{\rm M}$=0.027 where the spin-lattice relaxation rate shows quantum critical behavior, and another at $x_{\rm c}$=0.032 around which the nematic transition temperature extrapolates to zero and the electrical resistivity shows a $T$-linear variation. Our results indicate that a nematic QCP lies beneath the superconducting dome at $x_{\rm c}$ where $m^*$ is enhanced. The impact of the nematic fluctuations on superconductivity is discussed.

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arXiv2018

Fully gapped spin-singlet superconductivity in noncentrosymmetric PbTaSe$_2$ : $^{207}$Pb-NMR

We report the $^{207}$Pb nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe$_2$ with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate $1/T_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\rm c}$ = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below $T$ = 0.7 $T_{\rm c}$. The Knight shift $K$ decreases below $T_{\rm c}$. These results suggest spin-singlet superconductivity with a fully-opened gap $2Δ= 3.5$ $k_{\rm B}T_{\rm c}$ in PbTaSe$_2$.

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arXiv2017

Charge-density-wave order takes over antiferromagnetism in Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6}$ superconductors

Superconductivity appears in the cuprates when a spin order is destroyed, while the role of charge is less known. Recently, charge density wave (CDW) was found below the superconducting dome in YBa$_2$Cu$_3$O$_y$ when a high magnetic field is applied perpendicular to the CuO$_2$ plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped. Here, by $^{63}$Cu-nuclear magnetic resonance, we report the discovery of CDW induced by an in-plane field that does not create vortex cores in the plane, setting in above the dome in single-layered Bi$_2$Sr$_{2-x}$La$_x$CuO$_6$.The onset temperature $T_{\rm CDW}$ takes over the antiferromagnetic order temperature $T_{\rm N}$ beyond a critical doping level at which superconductivity starts to emerge, and scales with the psudogap temperature $T^{*}$. These results provide important insights into the relationship between spin order, CDW and the pseudogap, and their connections to high-temperature superconductivity.

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arXiv2017

Structural Phase Transition, Antiferromagnetism and Two Superconducting Domes in LaFeAsO$_{1-x}$F$_{x}$ (0 $< x \leq$ 0.75)

We report $^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO$_{1-x}$F$_{x}$. There are two superconducting domes in this material. The first one appears at 0.03 $\leq$ $x$ $\leq$ 0.2 with $T_{\rm c}$$^{max}$ = 27 K, and the second one at 0.25 $\leq$ $x$ $\leq$ 0.75 with $T_{\rm c}$$^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$ structural phase transition (SPT) takes place above both domes, with the transition temperature $T_{\rm s}$ varying strongly with $x$. In the first dome, the SPT is followed by an antiferromagnetic (AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. In LaFeAsO$_{0.97}$F$_{0.03}$, we find that AF order and superconductivity coexist microscopically via $^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$) measurements. In the coexisting region, 1/$T_1$ decreases at $T_{\rm c}$ but becomes to be proportional to $T$ below 0.6$T_{\rm c}$, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for $x \leq$ 0.2 is quite similar to the doped BaFe$_{2}$As$_{2}$ system. The electrical resistivity in the second dome can be fitted by $ρ= {{ρ}_{0}}+A{{T}^{n}}$ with $n$ = 1 and a maximal coefficient $A$ at around $x_{opt}$ = 0.5$\sim$0.55 where $T_{\rm s}$ extrapolates to zero and $T_{\rm c}$ is the maximal, which suggest the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO$_{1-x}$F$_{x}$, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.

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arXiv2017

Gapped spin-1/2 spinon excitations in a new kagome quantum spin liquid compound Cu$_3$Zn(OH)$_6$FBr

We report a new kagome quantum spin liquid candidate Cu$_3$Zn(OH)$_6$FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature ($\sim$ 200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from $^{19}$F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.

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arXiv2016

Full-gap superconductivity in non-centrosymmetric Re$_6$Zr, Re$_{27}$Zr$_{5}$ and Re$_{24}$Zr$_{5}$

Non-centrosymmetric superconductor Re$_6$Zr has attracted much interest, for its possible unconventional superconducting state with time reversal symmetry broken. Here we report the $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements on Re$_6$Zr ($T_c$ = 6.72 K) and the isostructural compounds Re$_{27}$Zr$_{5}$ ($T_c$ = 6.53 K) and Re$_{24}$Zr$_{5}$ ($T_c$ = 5.00 K). The nuclear spin-lattice relaxation rate $1/T_1$ shows a coherence peak below $T_c$ and decreases exponentially at low temperatures in all three samples. The superconducting gap $Δ$ derived from the $1/T_1$ data is $2Δ=3.58 $ $k_BT_c$, 3.55 $k_BT_c$, and 3.51 $k_BT_c$ for Re$_6$Zr ,Re$_{27}$Zr$_{5}$, and Re$_{24}$Zr$_{5}$, respectively, which is close to the value of 3.53 $k_BT_c$ expected for weak-coupling superconductivity. These data suggest conventional $s$-wave superconductivity with a fully-opened gap in this series of compounds.

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arXiv2016

Quasi-localized Impurity State in Doped Topological Crystalline Insulator Sn$_{0.9}$In$_{0.1}$Te Probed by $^{125}$Te-NMR

The In-doped topological crystalline insulator Sn$_{1-x}$In$_x$Te is a promising candidate for a topological superconductor, where it is theoretically suggested that In creates an impurity state responsible for superconductivity. We synthesized high purity Sn$_{1-x}$In$_x$Te samples and performed $^{125}$Te-nuclear magnetic resonance (NMR) measurements. The NMR spectra under a magnetic field of $H_0$ = 5 T show a broadening characteristic due to a localized impurity state. The spin-lattice relaxation rate ($1/T_1$) divided by temperature shows a Curie-Weiss like temperature-dependence under $H_0$ = 0.1 T but is temperature-independent under $H_0$ = 5 T. These results indicate the existence of quasi-localized impurity states due to In doping.

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arXiv2016

Charge fluctuations and nodeless superconductivity in quasi-one-dimensional Ta$_{4}$Pd$_{3}$Te$_{16}$ revealed by $^{125}$Te-NMR and $^{181}$Ta-NQR

We report $^{125}$Te nuclear magnetic resonance and $^{181}$Ta nuclear quadrupole resonance studies on single-crystal Ta$_{4}$Pd$_{3}$Te$_{16}$, which has a quasi-one-dimensional structure and superconducts below $T_{\rm c}=4.3$ K. $^{181}$Ta with spin $I=7/2$ is sensitive to quadrupole interactions, while $^{125}$Te with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ( $1/T_{1}$) of $^{181}$Ta and $^{125}$Te, we found that electric-field-gradient (EFG) fluctuations develop below $80$ K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge density wave that sets in at $T_{\rm CDW}=20$ K, below which the spectra are broadened and $1/T_{1}T$ drops sharply. In the superconducting state, $1/T_{1}$ shows a Hebel-Slichter coherence peak just below $T_{\rm c}$ for $^{125}$Te, indicating that Ta$_{4}$Pd$_{3}$Te$_{16}$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/T_{1}$ of $^{181}$Ta due to the strong EFG fluctuations.

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arXiv2016

Orbital Order and Spin Nematicity in the Tetragonal Phase of Electron-doped Iron-Pnictides NaFe$_{1-x}$Co$_{x}$As

In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42 K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on NaFe$_{1-x}$Co$_x$As (0 $\leq x \leq$ 0.042) that revealed orbital and spin nematicity occurring at a temperature $T^{\rm *}$ far above $T_{\rm s}$ in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below $T^{\rm *}$ and becomes commensurate below $T_{\rm s}$, which brings about the observed spin nematicity.

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arXiv2015

Spin-Rotation Symmetry Breaking in the Superconducting State of CuxBi2Se3

Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break additional symmetries. In particular, spin rotational symmetry is expected to be broken in spin-triplet superconductors. However, experimental evidence for such symmetry breaking has not been conclusively obtained so far in any candidate compounds. Here, by 77Se nuclear magnetic resonance measurements, we show that spin rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc=3.4 K. Our results not only establish spin-triplet superconductivity in this compound, but may also serve to lay a foundation for the research of topological superconductivity.

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arXiv2015

Doping-Enhanced Antiferromagnetism in Ca1-xLaxFeAs2

In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here we show that antiferromagnetism in Ca$_{1-x}$La$_x$FeAs$_2$ is robust against and even enhanced by doping. Using $^{75}$As-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the Néel temperature $T_{\rm N}$ = 62 K at a high doping concentration ($x$ = 0.15) where superconductivity sets in at the transition temperature $T_{\rm c}$ = 35 K. Unexpectedly, $T_{\rm N}$ is enhanced with increasing doping, rising up to $T_{\rm N}$ = 70 K at $x$ = 0.24. The obtained phase diagram of this new system enriches the physics of iron-based high-$T_{\rm c}$ superconductors.

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arXiv2015

New Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by Structural Transition

High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \leq$ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\leq x \leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$\sim$0.55 is even higher than that at $x \leq$ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$. Our results point to a new paradigm of high temperature superconductivity.

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