arXiv subjects

Yupeng Li

Publications and source records attributed to Yupeng Li.

64 records · Page 4Linked to original sources

arXiv2018

Superconductivity in tantalum self-intercalated 4$Ha$-Ta$_{1.03}$Se$_2$

TaSe$_2$ has several different polytypes and abundant physical properties such as superconductivity and charge density waves (CDW), which had been investigated in the past few decades. However, there is no report on the physical properties of 4$Ha$ polytype up to now. Here we report the crystal growth and discovery of superconductivity in the tantalum self-intercalated 4$Ha$-Ta$_{1.03}$Se$_2$ single crystal with a superconducting transition onset temperature $T_{\rm c}$ $\approx$ 2.7 K, which is the first observation of superconductivity in 4$Ha$ polytype of TaSe$_2$. A slightly suppressed CDW transition is found around 106 K. A large $μ_0H_{\rm c2}/T_{\rm c}$ value of about 4.48 is found when magnetic field is applied in the $ab$ plane, which probably results from the enhanced spin-orbit coupling(SOC). Special stacking faults are observed, which further enhance the anisotropy. Although the density of states at the Fermi level is lower than that of other polytypes, $T_{\rm c}$ remains the same, indicating the stack mode of 4$Ha$ polytype may be beneficial to superconductivity in TaSe$_2$.

cond-mat.supr-con↗

arXiv2017

Heavy fermion quantum criticality at dilute carrier limit in CeNi$_{2-δ}$(As$_{1-x}$P$_{x}$)$_{2}$

We study the quantum phase transitions in the nickel pnctides, CeNi$_{2-δ}$(As$_{1-x}$P$_{x}$)$_{2}$ ($δ$ $\approx$ 0.07-0.22). This series displays the distinct heavy fermion behavior in the rarely studied parameter regime of dilute carrier limit. We systematically investigate the magnetization, specific heat and electrical transport down to low temperatures. Upon increasing the P-content, the antiferromagnetic order of the Ce-4$f$ moment is suppressed continuously and vanishes at $x_c \sim$ 0.55. At this doping, the temperature dependences of the specific heat and longitudinal resistivity display non-Fermi liquid behavior. Both the residual resistivity $ρ_0$ and the Sommerfeld coefficient $γ_0$ are sharply peaked around $x_c$. When the P-content reaches close to 100\%, we observe a clear low-temperature crossover into the Fermi liquid regime. In contrast to what happens in the parent compound $x$ = 0.0 as a function of pressure, we find a surprising result that the non-Fermi liquid behavior persists over a nonzero range of doping concentration, $x_c<x<0.9$. In this doping range, at the lowest measured temperatures, the temperature dependence of the specific-heat coefficient is logarithmically divergent and that of the electrical resistivity is linear. We discuss the properties of CeNi$_{2-δ}$(As$_{1-x}$P$_{x}$)$_{2}$ in comparison with those of its 1111 counterpart, CeNi(As$_{1-x}$P$_{x}$)O. Our results indicate a non-Fermi liquid phase in the global phase diagram of heavy fermion metals.

cond-mat.str-el↗

arXiv2016

Negative Magnetoresistance in Weyl semimetals NbAs and NbP: Intrinsic Chiral Anomaly and Extrinsic Effects

Chiral anomaly induced negative magnetoresistance (NMR) has been widely used as a critical transport evidence on the existence of Weyl fermions in topological semimetals. In this mini review, we discuss the general observation of the NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can be contributed by intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as superimposition of Hall signals, field-dependent inhomogeneous current flow in the bulk, i.e. current jetting, and weak localization (WL) of coexistent trivial carriers. Such WL controlled NMR is heavily dependent on sample quality, and is characterized by pronounced crossover from positive to negative MR growth at elevated temperatures, as a result of the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation of NMR and chiral anomaly needs to be scrutinized, without the support of other complimentary techniques. Due to the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping of magnetic and non-magnetic impurities are thus more convincing in probing the existence of Weyl fermions than the NMR method.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced superconductivity in hole-doped Nb$_{2}$PdS$_{5}$

We synthesized a series of Nb$_{2}$Pd$_{1-x}$Ru$_{x}$S$_{5}$ polycrystalline samples by a solid-state reaction method and systematically investigated the Ru-doping effect on superconductivity by transport and magnetic measurements. It is found that superconductivity is enhanced with Ru doping and is quite robust upon disorder. Hall coefficient measurements indicate that the charge transport is dominated by hole-type charge carriers similar to the case of Ir doping, suggesting multi-band superconductivity. Upon Ru or Ir doping, \emph H$_{c2}$/$\emph T_c$ exhibits a significant enhancement, exceeding the Pauli paramagnetic limit value by a factor of approximately 4. A comparison of $T_c$ and the upper critical field ($H_{c2}$) amongst the different doping elements on Pd site, reveals a significant role of spin--orbit coupling.

cond-mat.supr-con↗

arXiv2016

Giant Linear Magneto-resistance in Nonmagnetic PtBi2

We synthesized nonmagnetic PtBi$_2$ single crystals and observed a giant linear magneto-resistance (MR) up to 684\% under a magnetic field $μ_0H$ = 15 T at $T$ = 2 K. The linear MR decreases with increasing temperature, but it is still as large as 61\% under $μ_0H$ of 15 T at room temperature. Such a giant linear MR is unlikely to be described by the quantum model as the quantum condition is not satisfied. Instead, we found that the slope of MR scales with the Hall mobility, and it can be well explained by a classical disorder model.

cond-mat.mtrl-sci↗

arXiv2016

Negative Magnetoresistance in Topological Semimetals of Transition-Metal Dipnictides with Nontrivial Z2 Indices

Negative magnetoresistance (NMR) induced by the Adler-Bell-Jackiw anomaly is regarded as the most prominent quantum signature of Weyl semimetals when electrical field $E$ is collinear with the external magnetic field $B$. In this article, we report universal NMR in nonmagnetic, centrosymmetric transition metal dipnictides MPn$_{2}$ (M=Nb and Ta; Pn=As and Sb), in which the existence of Weyl fermions can be explicitly excluded. Using temperature-dependent magnetoresistance, Hall and thermoelectric coefficients of Nernst and Seebeck effects, we determine that the emergence of the NMR phenomena in MPn$_{2}$ is coincident with a Lifshitz transition, corresponding to the formation of unique electron-hole-electron ($e$-$h$-$e$) pockets along the $I-L-I'$ direction. First-principles calculations reveal that, along the $I-L-I'$ line, the $d_{xy}$ and $d_{x^{2}-y^{2}}$ orbitals of the transition metal form tilted nodal rings of band crossing well below the Fermi level. Strong spin-orbital coupling gaps all the crossing points and creates the characteristic $e$-$h$-$e$ structure, making MPn$_{2}$ a topological semimetal with $\mathbb{Z}_2$ indices of [0;(111)]. By excluding the weak localization contribution of the bulk states, we conclude that the universal NMR in MPn$_{2}$ may have an exotic origin in topological surface states, which appears in pairs with opposite spin-momentum locking on nontrivial surfaces.

cond-mat.str-el↗

arXiv2016

Helicity protected ultrahigh mobility Weyl fermions in NbP

Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exotic relativistic Weyl fermions. In this letter, we elucidate the physical origin of the unprecedented charge carrier mobility of NbP, which can reach $1\times10^{7}$ cm $^{2}$V$^{-1}$s$^{-1}$ at 1.5 K. Angle- and temperature-dependent quantum oscillations, supported by density function theory calculations, reveal that NbP has the coexistence of p- and n-type WSM pockets in the $k_{z}$=1.16$π$/c plane (W1-WSM) and in the $k_{z}$=0 plane near the high symmetry points $Σ$ (W2-WSM), respectively. Uniquely, each W2-WSM pocket forms a large dumbbell-shaped Fermi surface (FS) enclosing two neighboring Weyl nodes with the opposite chirality. The magneto-transport in NbP is dominated by these highly anisotropic W2-WSM pockets, in which Weyl fermions are well protected from defect backscattering by real spin conservation associated to the chiral nodes. However, with a minimal doping of $\sim$1\% Cr, the mobility of NbP is degraded by more than two order of magnitude, due to the invalid of helicity protection to magnetic impurities. Helicity protected Weyl fermion transport is also manifested in chiral anomaly induced negative magnetoresistance, controlled by the W1-WSM states. In the quantum regime below 10 K, the intervalley scattering time by impurities becomes a large constant, producing the sharp and nearly identical conductivity enhancement at low magnetic field.

cond-mat.mes-hall↗

arXiv2016

Topological phase transition induced extreme magnetoresistance in TaSb$_{2}$

We report extremely large positive magnetoresistance of 1.72 million percent in single crystal TaSb$_{2}$ at moderate conditions of 1.5 K and 15 T. The quadratic growth of magnetoresistance (MR $\propto\,B^{1.96}$) is not saturating up to 15 T, a manifestation of nearly perfect compensation with $<0.1\%$ mismatch between electron and hole pockets in this semimetal. The compensation mechanism is confirmed by temperature-dependent MR, Hall and thermoelectric coefficients of Nernst and Seebeck, revealing two pronounced Fermi surface reconstruction processes without spontaneous symmetry breaking, \textit{i.e.} Lifshitz transitions, at around 20 K and 60 K, respectively. Using quantum oscillations of magnetoresistance and magnetic susceptibility, supported by density-functional theory calculations, we determined that the main hole Fermi surface of TaSb$_{2}$ forms a unique shoulder structure along the $F-L$ line. The flat band top of this shoulder pocket is just a few meV above the Fermi level, leading to the observed topological phase transition at 20 K when the shoulder pocket disappears. Further increase in temperature pushes the Fermi level to the band top of the main hole pocket, induced the second Lifshitz transition at 60 K when hole pocket vanishes completely.

cond-mat.str-el↗

arXiv2015

Observation of Negative Magnetoresistance and nontrivial $π$ Berrys phase in 3D Weyl semi-metal NbAs

We report the electric transport properties of NbAs, which is a Weyl semimetal candidate proposed by recent theoretical calculations and confirmed by recent angle-resolved photoemission spectroscopy (ARPES) data. We detected the long-anticipated negative magneto-resistance generated by the chiral anomaly in NbAs. Clear Shubnikov de Haas (SdH) oscillations have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits to be $π$, indicating the existence of Weyl points.

cond-mat.str-el↗

arXiv2014

Sr$_{0.9}$K$_{0.1}$Zn$_{1.8}$Mn$_{0.2}$As$_{2}$: a ferromagnetic semiconductor with colossal magnetoresistance

A bulk diluted magnetic semiconductor (Sr,K)(Zn,Mn)$_{2}$As$_{2}$ was synthesized with decoupled charge and spin doping. It has a hexagonal CaAl$_{2}$Si$_{2}$-type structure with the (Zn,Mn)$_{2}$As$_{2}$ layer forming a honeycomb-like network. Magnetization measurements show that the sample undergoes a ferromagnetic transition with a Curie temperature of 12 K and \revision{magnetic moment reaches about 1.5 $μ_{B}$/Mn under $μ_0H$ = 5 T and $T$ = 2 K}. Surprisingly, a colossal negative magnetoresistance, defined as $[ρ(H)-ρ(0)]/ρ(0)$, up to $-$38\% under a low field of $μ_0H$ = 0.1 T and to $-$99.8\% under $μ_0H$ = 5 T, was observed at $T$ = 2 K. The colossal magnetoresistance can be explained based on the Anderson localization theory.

cond-mat.str-el↗