arXiv subjects

Z. Q. Mao

Publications and source records attributed to Z. Q. Mao.

At least 55 records · Page 3Linked to original sources

arXiv2016

Probing chiral superconductivity in Sr$_{2}$RuO$_{4}$ underneath the surface by point contact measurements

Sr$_{2}$RuO$_{4}$ (SRO) is the prime candidate for chiral $p$-wave superconductor with critical temperature $T_{c}(SRO)\sim$1.5 K. Chiral domains with opposite chiralities $p_{x}\pm ip_{y}$ were proposed, but yet to be confirmed. We measure the field dependence of the point contact (PC) resistance between a tungsten tip and the SRO-Ru eutectic crystal, where micrometer-sized Ru inclusions are embedded in SRO with atomic sharp interface. Ruthenium is an $s$-wave superconductor with $T_{c}(Ru)\sim$0.5 K, flux pinned near the Ru inclusions can suppress its superconductivity as reflected from the PC resistance and spectra. This flux pinning effect is originated from SRO \textit{underneath} the surface and is very strong. To fully remove it, one has to thermal cycle the sample above $T_{c}(SRO)$. This resembles the thermal demagnetization for a ferromagnet, where ferromagnetic domains are randomized above its Curie temperature. Another way is by applying alternating fields with decreasing amplitude, resembling field demagnetization for the ferromagnet. The observed hysteresis in magnetoresistance can be explained by domain dynamics, providing support for the existence of chiral domains. The origin of strong pinning \textit{underneath} the surface is also discussed.

cond-mat.supr-con↗

arXiv2016

Unusual interlayer quantum transport behavior caused by the zeroth Landau level in YbMnBi2

Relativistic fermions in topological quantum materials are characterized by linear energy-momentum dispersion near band crossing points. Under magnetic field, relativistic fermions acquire Berry phase of π in cyclotron motion, leading to a zeroth Landau level (LL) at the crossing point. Such field-independent zeroth LL, which distinguishes relativistic fermions from conventional electron systems, is hardly probed in transport measurements since the Fermi energy (EF) is usually not right at the band crossing points in most topological materials. Here we report the observation of exotic quantum transport behavior resulting from the zeroth LL in a multiband topological semimetal YbMnBi2 which possesses linear band crossings both at and away from the Fermi level (FL). We show that the Dirac bands with the crossing points being above or below the FL leads to Shubnikov de-Haas oscillations in the in-plane magnetoresistance, whereas the Dirac bands with the crossing points being at the FL results in unusual angular dependences of the out-of-plane magnetoresistance and in-plane Hall resistivity due to the dependence of the zeroth LL's degeneracy on field orientation. Our results shed light on the transport mechanism of the zeroth LL's relativistic fermions in layered materials.

cond-mat.mtrl-sci↗

arXiv2015

Origin of the turn-on temperature behavior in WTe$_2$

A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative 'turn-on' temperature behavior: when the applied magnetic field $H$ is above certain value, the resistivity versus temperature $ρ(T)$ curve shows a minimum at a field dependent temperature $T^*$, which has been interpreted as a magnetic-field-driven metal-insulator transition or attributed to an electronic structure change. Here, we demonstrate that $ρ(T)$ curves with turn-on behavior in the newly discovered XMR material WTe$_2$ can be scaled as MR $\sim(H/ρ_0)^m$ with $m\approx 2$ and $ρ_0$ being the resistivity at zero-field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature $T^* \sim (H-H_c)^ν$ with $ν\approx 1/2$, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity $ρ^* \approx 2 ρ_0$ at the onset of the XMR behavior, which fits the data remarkably well. These results exclude the possible existence of a magnetic-field-driven metal-insulator transition or significant contribution of an electronic structure change to the low-temperature XMR in WTe$_2$. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials.

cond-mat.mtrl-sci↗

arXiv2015

Spin Polarization Enhanced by Spin-Triplet Pairing in Sr$_2$RuO$_4$ Probed by NMR

We report a novel phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr$_2$RuO$_4$ with nuclear magnetic resonance (NMR) that the spin susceptibility originating from the Ru-4$d$ electron slightly $increases$ by $\sim 2 $\% of total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted for superfluid $^3$He forty years ago, but had never been demonstrated in any superconductor.

cond-mat.supr-con↗

arXiv2015

Pi Berry phase and Zeeman splitting of TaP probed by high field magnetotransport measurements

The chiral anomaly-induced negative magnetoresistance and non-trivial Berry phase are two fundamental transport properties associated with the topological properties of Weyl fermions. In this work, we report the quantum transport of TaP single crystals in magnetic field up to 31T. Through the analyses of our magnetotransport data, we show TaP has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance. Furthermore, we have generalized the Lifshitz-Kosevich formula for Shubnikov-de Haas (SdH) oscillations with multi-frequencies, and determined the non-trivial Berry phase of Pi for multiple Fermi pockets in TaP through the direct fitting of the quantum oscillations. In high fields, we also probed signatures of Zeeman splitting, from which the Landé g-factor is extracted.

cond-mat.mtrl-sci↗

arXiv2015

Drastic pressure effect on the extremely large magnetoresistance in WTe2: quantum oscillation study

The quantum oscillations of the magnetoresistance under ambient and high pressure have been studied for WTe$_2$ single crystals, in which extremely large magnetoresistance was discovered recently. By analyzing the Shubnikov-de Haas oscillations, four Fermi surfaces are identified, and two of them are found to persist to high pressure. The sizes of these two pockets are comparable, but show increasing difference with pressure. At 0.3 K and in 14.5 T, the magnetoresistance decreases drastically from 1.25 $\times$ $10^5$\% under ambient pressure to 7.47 $\times$ $10^3$\% under 23.6 kbar, which is likely caused by the relative change of Fermi surfaces. These results support the scenario that the perfect balance between the electron and hole populations is the origin of the extremely large magnetoresistance in WTe$_2$.

cond-mat.mtrl-sci↗

arXiv2015

Lattice correlation of Hubbard excitons in a Mott insulator Sr2IrO4 and reconstruction of their hopping dynamics via time-dependent coherence analysis of the Bragg diffraction

In correlated oxides the coupling of quasiparticles to other degrees of freedom such as spin and lattice plays critical roles in the emergence of symmetry-breaking quantum ordered states such as high temperature superconductivity. We report a strong lattice coupling of photon induced Hubbard excitonic quasiparticles in spin-orbital coupling Mott insulator Sr2IrO4. Combining time-resolved optical spectroscopy techniques, we further reconstructed spatiotemporal map of the diffusion of quasiparticles via time-dependent coherence analysis of the x-ray Bragg diffraction peak. Due to the unique electronic configuration of the exciton, the strong lattice correlation is unexpected but extends the similarity between Sr2IrO4 and cuprates under highly non-equilibrium conditions. The coherence analysis method we developed may have important implications for characterizing the structure and carrier dynamics in a wider group of oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2015

Enhancing Electron Coherence via Quantum Phonon Confinement in Atomically Thin Nb3SiTe6

The extraordinary properties of two dimensional (2D) materials, such as the extremely high carrier mobility in graphene and the large direct band gaps in transition metal dichalcogenides MX2 (M = Mo or W, X = S, Se) monolayers, highlight the crucial role quantum confinement can have in producing a wide spectrum of technologically important electronic properties. Currently one of the highest priorities in the field is to search for new 2D crystalline systems with structural and electronic properties that can be exploited for device development. In this letter, we report on the unusual quantum transport properties of the 2D ternary transition metal chalcogenide - Nb3SiTe6. We show that the micaceous nature of Nb3SiTe6 allows it to be thinned down to one-unit-cell thick 2D crystals using microexfoliation technique. When the thickness of Nb3SiTe6 crystal is reduced below a few unit-cells thickness, we observed an unexpected, enhanced weak-antilocalization signature in magnetotransport. This finding provides solid evidence for the long-predicted suppression of electron-phonon interaction caused by the crossover of phonon spectrum from 3D to 2D.

cond-mat.mes-hall↗

arXiv2015

Enhanced superconductivity at the interface of W/Sr$_{2}$RuO$_{4}$ point contact

Differential resistance measurements are conducted for point contacts (PCs) between tungsten tip approaching along the $c$ axis direction and the $ab$ plane of Sr$_{2}$RuO$_{4}$ single crystal. Three key features are found. Firstly, within 0.2 mV there is a dome like conductance enhancement due to Andreev reflection at the normal-superconducting interface. By pushing the W tip further, the conductance enhancement increases from 3\% to more than 20\%, much larger than that was previously reported, probably due to the pressure exerted by the tip. Secondly, there are also superconducting like features at bias higher than 0.2 mV which persists up to 6.2 K, resembling the enhanced superconductivity under uniaxial pressure for bulk Sr$_{2}$RuO$_{4}$ crystals but more pronounced here. Third, the logarithmic background can be fitted with the Altshuler-Aronov theory of tunneling into quasi two dimensional electron system, consistent with the highly anisotropic electronic system in Sr$_{2}$RuO$_{4}$.

cond-mat.supr-con↗

arXiv2013

Spin-orbit coupling and weak antilocalization in thermoelectric material $β$-K$_{2}$Bi$_{8}$Se$_{13}$

We have studied the effect of spin-orbital coupling (SOC) on electronic transport properties of the thermoelectric material $β$-K$_{2}$Bi$_{8}$Se$_{13}$ via magnetoresistance (MR) measurements. We found that the strong SOC in this material results in weak antilocalization (WAL) effect, which can be well described by the three-dimensional weak localization model. The phase coherence length extracted from theoretical fitting exhibits a power-law temperature dependence with an exponent around 2.1, indicating that the electron phase dephasing is governed by electron - transverse phonon interactions. Like in topological insulators, the WAL effect in $β$-K$_{2}$Bi$_{8}$Se$_{13}$ can be quenched by magnetic impurities (Mn) but is robust against non-magnetic impurities (Te). Although our magnetotransport studies do not provide any evidences for topological surface states, our analyses suggest that SOC plays an important role in determining thermoelectric properties of $β$-K$_{2}$Bi$_{8}$Se$_{13}$.

cond-mat.mes-hall↗

arXiv2013

Coupling of electronic and magnetic properties in Fe1+y(Te1-xSex)

We have studied the coupling of electronic and magnetic properties in Fe1+y(Te1-xSex) via systematic specific heat, magnetoresistivity, and Hall coefficient measurements on two groups of samples with y = 0.02 and 0.1. In the y = 0.02 series, we find that the 0.09 < x < 0.3 composition region, where superconductivity is suppressed, has large Sommerfeld coefficient Gamma (~55-65 mJ/mol K^2), positive Hall coefficient R_H and negative magnetoresistance MR at low temperature, in sharp contrast with the x=0.4-0.5 region where Gamma drops to ~ 26 mJ/mol K^2 and R_H / MR becomes negative/positive at low temperature. Dramatic changes of Gamma, as well as sign reversal in low-temperature RH and MR, are also observed across the x~0.1 boundary where the long-range antiferromagnetic order is suppressed. However, for the system with rich interstitial excess Fe (y = 0.1), where bulk superconductivity is suppressed even for x=0.4-0.5, the variations of Gamma, R_H and MR with x are distinct from those seen in y = 0.02 system: Gamma is ~40 mJ/mol K^2 for 0.1 < x < 0.3, and drops to ~ 34 mJ/mol K^2 for x = 0.4-0.5; R_H and MR does not show any sign reversal as x is increased above 0.3. We will show that all these results can be understood in light of the evolution of the incoherent magnetic scattering by (pi,0) magnetic fluctuations with Se concentration. In addition, with the suppression of magnetic scattering by magnetic field, we observed the surprising effect of a remarkable increase in the superconducting volume fraction under moderate magnetic fields for x=0.3-0.4 samples in the y = 0.02 system.

cond-mat.supr-con↗

arXiv2012

Inhomogeneous superconductivity induced by interstitial Fe deintercalation in oxidizing-agent-annealed and HNO3-treated Fe1+y(Te1-xSex)

We have systematically investigated the annealing effect on the superconductivity of iron chalcogenide Fe1+y(Te1-xSex). The atmospheres used for annealing include O2, N2, I2 vapor, air and vacuum. We observed that annealing in O2, I2 and air could enhance superconductivity for the underdoped samples, consistent with the results reported in literatures. Interestingly, we found that annealing in N2 also leads to superconductivity enhancement, similar to the annealing effects of O2, I2 and air. However, vacuum annealing does not enhance superconductivity, which indicates that the enhanced superconductivity in O2-, N2-, I2- and air-annealed samples is not due to improved homogeneity. In addition, we have treated the underdoped samples with nitric acid, which is found to enhance superconductivity as well. Our analyses of these results support the argument that the superconductivity enhancement, caused either by annealing or nitric acid treatment, originates from the variation of interstitial Fe. The interstitial Fe, which is destructive to superconducting pairing, can be reduced by annealing in oxidation agents or nitric acid treatment. We also find that although N2-, O2- and air-annealed samples exhibit strong superconducting diamagnetism with -4pichi ~1 (chi, dc magnetic susceptibility) for some samples, their actual superconducting volume fraction probed by specific heat is low, ranging from 10% to 30% for 0.09 < x < 0.3, indicating that the superconductivity suppression remains significant even in annealed samples. The strong diamagnetism is associated with the superconducting shielding effect on the non-superconducting phase. We have also established the phase diagram of the annealed samples and compared it with that of the as-grown samples. The effect of annealing on the interplay between magnetism and superconductivity is discussed.

cond-mat.supr-con↗

arXiv2012

Unconventional quantum oscillations in mesoscopic rings of spin-triplet superconductor Sr2RuO4

Odd-parity, spin-triplet superconductor Sr2RuO4 has been found to feature exotic vortex physics including half-flux quanta trapped in a doubly connected sample and the formation of vortex lattices at low fields. The consequences of these vortex states on the low-temperature magnetoresistive behavior of mesoscopic samples of Sr2RuO4 were investigated in this work using ring device fabricated on mechanically exfoliated single crystals of Sr2RuO4 by photolithography and focused ion beam. With the magnetic field applied perpendicular to the in-plane direction, thin-wall rings of Sr2RuO4 were found to exhibit pronounced quantum oscillations with a conventional period of the full-flux quantum even though the unexpectedly large amplitude and the number of oscillations suggest the observation of vortex-flow-dominated magnetoresistance oscillations rather than a conventional Little-Parks effect. For rings with a thick wall, two distinct periods of quantum oscillations were found in high and low field regimes, respectively, which we argue to be associated with the "lock-in" of a vortex lattice in these thick-wall rings. No evidence for half-flux-quantum resistance oscillations were identified in any sample measured so far without the presence of an in-plane field.

cond-mat.supr-con↗

arXiv2012

Effect of Cleaving Temperature on the Surface and Bulk Fermi Surface of Sr2RuO4 Investigated by High Resolution Angle-Resolved Photoemission

High resolution angle-resolved photoemission measurements are carried out to systematically investigate the effect of cleaving temperature on the electronic structure and Fermi surface of Sr$_2$RuO$_4$. Different from previous reports that high cleaving temperature can suppress surface Fermi surface, we find that the surface Fermi surface remains obvious and strong in Sr$_2$RuO$_4$ cleaved at high temperature, even at room temperature. This indicates that cleaving temperature is not a key effective factor in suppressing the surface bands. On the other hand, in the aged surface of Sr$_2$RuO$_4$ that is cleaved and held for a long time, the bulk bands can be enhanced. We have also carried out laser ARPES measurements on Sr$_2$RuO$_4$ by using vacuum ultra-violet laser (photon energy at 6.994 eV) and found an obvious enhancement of bulk bands even for samples cleaved at low temperature. These information are important in realizing an effective approach in manipulating and detecting the surface and bulk electronic structure of Sr$_2$RuO$_4$. In particular, the enhancement of bulk sensitivity, together with its super-high instrumental resolution of VUV laser ARPES, will be advantageous in investigating fine electronic structure and superconducting properties of Sr$_2$RuO$_4$ in the future.

cond-mat.supr-con↗

arXiv2012

Unusual Fermi Surface Sheet-Dependent Band Splitting in Sr2RuO4 Revealed by High Resolution Angle-Resolved Photoemission

High resolution angle-resolved photoemission measurements have been carried out on Sr2RuO4. We observe clearly two sets of Fermi surface sheets near the (π,0)-(0,π) line which are most likely attributed to the surface and bulk Fermi surface splitting of the βband. This is in strong contrast to the nearly null surface and bulk Fermi surface splitting of the αband although both have identical orbital components. Extensive band structure calculations are performed by considering various scenarios, including structural distortion, spin-orbit coupling and surface ferromagnetism. However, none of them can explain such a qualitative difference of the surface and bulk Fermi surface splitting between the αand βsheets. This unusual behavior points to an unknown order on the surface of Sr2RuO4 that remains to be uncovered. Its revelation will be important for studying and utilizing novel quantum phenomena associated with the surface of Sr2RuO4 as a result of its being a possible p-wave chiral superconductor and a topological superconductor.

cond-mat.supr-con↗

arXiv2012

Dislocations and the enhancement of superconductivity in odd-parity superconductor Sr$_2$RuO$_4$

We report observation of the enhancement of superconductivity near lattice dislocations and the absence of the strengthening of vortex pinning in odd-parity superconductor Sr$_2$RuO$_4$, both surprising results in direct contrast to the well known sensitivity of superconductivity in Sr$_2$RuO$_4$ to disorder. The enhanced superconductivity appears to be related fundamentally to the two-component nature of the superconducting order parameter, as revealed in our phenomenological theory taking into account the effect of symmetry reduction near a dislocation.

cond-mat.supr-con↗

arXiv2012

Magnetic structure of quasi-1D antiferromagnetic TaFe1+yTe3 with two-leg zigzag ladders

We report the magnetic structure of TaFe1+yTe3 single crystals by means of neutron diffraction measurements. TaFe1+yTe3 possesses a layered structure with a formation of two-leg zigzag ladders along the b-axis. We find that TaFe1+yTe3 undergoes an antiferromagnetic transition at 178 K with Fe1 spins of the intra-ladders ferromagnetically aligned while spins of the inter-ladders antiferromagneitcally coupled. Furthermore, spins of the neighboring interstitial Fe2 (y) ions order parallel to the Fe1 spins of each ladder. These findings are distinct from the magnetic structure of the recently-discovered spin-ladder compound BaFe2Se3. TaFe1+yTe3 may serve as an interesting quasi-one dimensional ferromagnetic system.

cond-mat.mtrl-sci↗

arXiv2011

Friedel-like Oscillations from Interstitial Iron in Superconducting Fe1+yTe0.62Se0.38

Using polarized and unpolarized neutron scattering we show that interstitial Fe in superconducting Fe_{1+y}Te_{1-x}Se_x induces a magnetic Friedel-like oscillation that diffracts at Q_(in-plane)=(1/2,0) and involves >50 neighboring Fe sites. The interstitial >2 mu_B moment is surrounded by compensating ferromagnetic four spin clusters that may seed double stripe ordering in Fe_{1+y}Te. A semi-metallic 5-band model with (1/2,1/2) Fermi surface nesting and four fold symmetric super-exchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.

cond-mat.supr-con↗