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Z. F. Ding

Publications and source records attributed to Z. F. Ding.

11 recordsLinked to original sources
arXiv2021

Muon spin rotation and relaxation study on topological noncentrosymmetric superconductor PbTaSe$_2$

Topological superconductivity is an exotic phenomenon due to the symmetry-protected topological surface state, in which a quantum system has an energy gap in the bulk but supports gapless excitations confined to its boundary. Symmetries including central and time-reversal (TRS), along with their relations with topology, are crucial for topological superconductivity. We report muon spin relaxation/rotation ($μ$SR) experiments on a topological noncentrosymmetric superconductor PbTaSe$_2$ to study its TRS and gap symmetry. Zero-field $μ$SR experiments indicate the absence of internal magnetic field in the superconducting state, consistent with previous $μ$SR results. Furthermore, transverse-field $μ$SR measurements reveals that the superconducting gap of PbTaSe$_2$ is an isotropic three-dimensional fully-gapped single-band. The fully-gapped results can help understand the pairing mechanism and further classify the topological superconductivity in this system.

cond-mat.supr-con↗
arXiv2020

Evidence for the topological order in a kagome antiferromagnet

A Z2 quantum spin liquid hosts one of the simplest topological orders and exhibits many exotic properties due to long-range quantum entanglements. Its elementary excitations are anyons such as spinons carrying fractionalized spin quantum number and visons carrying emergent Z2 gauge flux. However, experimental detection of these anyons remains elusive. The difficulties lie not only in the fact that there exists few candidates for Z2 quantum spin liquids but also in that visons are magnetically inert hence immune to available experimental techniques. Here we have studied the spin excitations and specific heats of kagome-lattice antiferromagnet Cu$_4$(OH)$_6$FBr and Cu$_3$Zn(OH)$_6$FBr, which consists of two-dimensional Cu$^{2+}$ kagome layers with either Cu$^{2+}$ or Zn$^{2+}$ ions in between. By combining the first principle calculations and inelastic neutron scattering data in the former, we show that the dominate couplings in Cu$_4$(OH)$_6$FBr are between the nearest neighbor spins within the kagome planes, and the kagome and interlayer spin systems are essentially decoupled above the antiferromagnetic transition temperature. The intrinsic spin excitations and specific heats of the kagome layers for Cu$_3$Zn(OH)$_6$FBr are thus derived by removing the contributions from the residual interlayer Cu$^{2+}$ magnetic impurities. Accordingly, the kagome spin system exhibits spin continuum with momentum-dependent spin gap and a large magnetic entropy at low temperature that is insensitive to magnetic field, which can be understood as the evidences of spinons and visons in this kagome quantum spin liquid candidate. Our results suggest the existence of the Z2 anyons in the material, and therefore provide a comprehensive set of evidences for the Z2 topological order in kagome quantum spin liquid and bring their choreographed entanglement dances to the stage of real materials.

cond-mat.str-el↗
arXiv2020

Distinct Kondo Screening Behaviors in Heavy Fermion Filled Skutterudites with 4f1 and 4f2 Configurations

Filled-skutterudite heavy fermion (HF) compounds host rich ground states depending on the f electron configurations. CeOs4Sb12 (COS) with Ce 4f1, and PrOs4Sb12 (POS) with Pr 4f2 configurations show distinct properties of Kondo insulating and HF superconductivity, respectivity. We unveiled the underlying microscopic origin by angle-resolved photoemission spectroscopy studies. Their eV-scale band structure matches well, representing the common characters of conduction electrons in ROs4Sb12 systems (R = rare earth). However, f electrons interact differently with conduction electrons in them. Strong hybridization between conduction electrons and f electrons is observed in COS with band dependent hybridization gaps, and the development of Kondo insulating state is directly revealed. Although the ground state of POS is a singlet, finite but incoherent hybridization exists due to Kondo scattering with the thermally excited triplet crystalline electric field (CEF) state. Our results help to understand the intriguing properties in COS and POS, and provide a clean demonstration of the microscopic differences in HF systems with 4f1 and 4f2 configurations.

cond-mat.str-el↗
arXiv2020

Unusual slow magnetic fluctuations and critical slowing down in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$

Hidden magnetic order of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$, $x = 0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero applied field and weak longitudinal fields ($μ_0H_L \lesssim 2$~mT), muon spin relaxation data can be well described by exponentially-damped static Lorentzian Kubo-Toyabe functions, indicating that static and dynamic local fields coexist at each muon site. For $μ_0H_L \gtrsim 2$~mT, the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. In both zero field and $μ_0H_L = 1$--2~mT, the temperature dependencies of the exponential muon spin relaxation rate exhibit maxima at 215~K for $x = 0.05$ and 175~K for $x = 0.1$, suggesting critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation. The correlation time of this electronic spin fluctuation is in the range of~2--5~ns for Sr$_2$Ir$_{0.9}$Rh$_{0.1}$O$_4$, and shorter than 2~ns for Sr$_2$Ir$_{0.95}$Rh$_{0.05}$O$_4$. The rms fluctuating field is on the order of 1 mT, which is consistent with the polarized neutron diffraction cross-section.

cond-mat.str-el↗
arXiv2018

Tracing crystal-field splittings in the rare earth-based intermetallic CeIrIn$_5$

Crystal electric field states in rare earth intermetallics show an intricate entanglement with the many-body physics that occurs in these systems and that is known to lead to a plethora of electronic phases. Here, we attempt to trace different contributions to the crystal electric field (CEF) splittings in CeIrIn$_5$, a heavy-fermion compound and member of the Ce$M$In$_5$ ($M$= Co, Rh, Ir) family. To this end, we utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) and present a spectroscopic study of the electronic structure of this unconventional superconductor over a wide temperature range. As a result, we show how ARPES can be used in combination with thermodynamic measurements or neutron scattering to disentangle different contributions to the CEF splitting in rare earth intermetallics. We also find that the hybridization is stronger in CeIrIn$_5$ than CeCoIn$_5$ and the effects of the hybridization on the Fermi volume increase is much smaller than predicted. By providing the first experimental evidence for $4f_{7/2}^{1}$ splittings which, in CeIrIn$_5$, split the octet into four doublets, we clearly demonstrate the many-body origin of the so-called $4f_{7/2}^{1}$ state.

cond-mat.str-el↗
arXiv2017

Discovery of slow magnetic fluctuations and critical slowing down in the pseudogap phase of YBa$_2$Cu$_3$O$_y$

Evidence for intra-unit-cell (IUC) magnetic order in the pseudogap region of high-$T_c$ cuprates below a temperature $T^\ast$ is found in several studies, but NMR and $μ$SR experiments do not observe the expected static local magnetic fields. It has been noted, however, that such fields could be averaged by fluctuations. Our measurements of muon spin relaxation rates in single crystals of YBa$_2$Cu$_3$O$_y$ reveal magnetic fluctuations of the expected order of magnitude that exhibit critical slowing down at $T^\ast$. These results are strong evidence for fluctuating IUC magnetic order in the pseudogap phase.

cond-mat.supr-con↗
arXiv2016

Superconducting gap structure in ambient-pressure-grown $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$

We have performed transverse-field muon spin relaxation (TF-$μ$SR) measurements on ambient-pressure-grown polycrystalline $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$. From these measurements, no signature of magnetic order is found down to 25 mK. The value of the magnetic penetration depth extrapolated to 0 K is 0.89 (5) $μ$m. The temperature dependence of superconducting penetration depth is best described by either a multigap s + s-wave model with $Δ_{1}$ = 0.947 (7) meV and $Δ_{2}$ = 0.22 (4) meV or the ansiotropic s-wave model with $Δ(0)$ = 0.776 meV and anisotropic gap amplitude ratio $Δ_{min}/Δ_{max}$ = 0.34. Comparisons with other potentially multigap $\mathrm{BiS_{2}}$-based superconductors are discussed. We find that these $\mathrm{BiS_{2}}$-based superconductors, including $\mathrm{Bi_{4}O_{4}S_3}$ and the high-pressure synthesized $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$, generally conform to the Uemura relation.

cond-mat.supr-con↗
arXiv2016

Direct observation of how the heavy fermion state develops in CeCoIn5

Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spectroscopy measurements on CeCoIn5, a prototypical heavy fermion compound, and reveal the long-sought band hybridization and Fermi surface expansion. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized at the lowest temperature. Moreover, crystal field excitations likely play an important role in the anomalous temperature dependence. Our results paint an comprehensive unanticipated experimental picture of the heavy fermion formation in a periodic multi-level Anderson/Kondo lattice, and set the stage for understanding the emergent properties in related materials.

cond-mat.str-el↗
arXiv2016

Quantum Criticality and Inhomogeneous Magnetic Order in Fe-doped alpha-YbAlB4

The intermediate-valent polymorphs $α$- and $β$-YbAlB$_4$ exhibit quantum criticality and other novel properties not usually associated with intermediate valence. Iron doping induces quantum criticality in $α$-YbAlB$_4$ and magnetic order in both compounds. We report results of muon spin relaxation ($μ$SR) experiments in the intermediate-valent alloys $α$-YbAl$_{1-x}$Fe$_x$B$_4$, $x = 0.014$ and 0.25. For $x = 0.014$ we find no evidence for magnetic order down to 25 mK\@. The dynamic muon spin relaxation rate $λ_d$ exhibits a power-law temperature dependence $λ_d \propto T^{-a}$, $a = 0.40(4)$, in the temperature range 100 mK--2 K, in disagreement with predictions by theories of antiferromagnetic (AFM) or valence quantum critical behavior. For $x = 0.25$, where AFM order develops in the temperature range 7.5--10 K, where we find coexistence of meso- or macroscopically segregated paramagnetic and AFM phases, with considerable disorder in the latter down to 2 K.

cond-mat.str-el↗
arXiv2015

Broken Time Reversal Symmetry in Superconducting Pr1-xCexPt4Ge12

We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors~Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$, $x = 0$, 0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field~$B_s$ below the superconducting transition temperature. We find that $B_s$ decreases linearly with $x$ and $\to 0$ at $x \approx 0.4$, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt$_4$Ge$_{12}$ and isostructural (Pr,La)Os$_4$Sb$_{12}$ alloy series both exhibit superconductivity with broken TRS, and in both the decrease of $B_s$ is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS\@. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt$_4$Ge$_{12}$ pair-breaking by Ce doping suppresses superconductivity. For all $x$ the dynamic muon spin relaxation rate decreases somewhat in the superconducting state. This may be due to Korringa relaxation by conduction electrons, which is reduced by the opening of the superconducting energy gap.

cond-mat.supr-con↗