arXiv subjects

Li Pi

Publications and source records attributed to Li Pi.

At least 37 records · Page 2Linked to original sources

arXiv2021

Multiple Weyl fermions in the noncentrosymmetric semimetal LaAlSi

The noncentrosymmetric RAlPn (R = rare earth, Pn = Si, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provide an excellent platform for investigating the relation between magnetism and Weyl physics. By using high field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a careful comparison between experimental results and theoretical calculations, nontrivial Berry phases associated with the Shubnikov-de Haas oscillations are ascribed to the electron Fermi pockets related to both types of Weyl points located ~ 0.1 eV above and exactly on the Fermi level, respectively. Under high magnetic field, signatures of Zeeman splitting are also observed. These results indicate that, in addition to the importance for exploring intriguing physics of multiple Weyl fermions, LaAlSi as a comparison with magnetic Weyl semimetals in the RAlPn family would also yield valuable insights into the relation between magnetism and Weyl physics.

cond-mat.str-el↗

arXiv2020

NMR study of the spin correlations in the $S=1$ armchair chain Ni$_2$NbBO$_6$

We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni$_2$NbBO$_6$ with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by our NMR spectral analysis. For the spin dynamics, a prominent peak at $T\sim35$ K well above the Néel temperature ($T_N\sim20$ K at $μ_0H=10$ T) is observed from the spin-lattice relaxation data. As compared with the dc-susceptibility, this behavior indicates a antiferromagnetic coupling with the typical energy scale of $\sim3$ meV. Thus, the Ni$_2$NbBO$_6$ compound can be viewed as strongly ferromagnetically coupled armchair spin chains along the crystalline $b$-axis. These facts place strong constraints to the theoretical model for this compound.

cond-mat.str-el↗

arXiv2020

NMR study of the spin excitations in the frustrated antiferromagnet Yb(BaBO$_3$)$_3$ with a triangular lattice

In this paper, we study the spin excitation properties of the frustrated triangular-lattice antiferromagnet Yb(BaBO$_3$)$_3$ with nuclear magnetic resonance. From the spectral analysis, neither magnetic ordering nor spin freezing is observed with temperature down to $T=0.26$ K, far below its Curie-Weiss temperature $|θ_w|\sim2.3$ K. From the nuclear relaxation measurement, precise temperature-independent spin-lattice relaxation rates are observed at low temperatures under a weak magnetic field, indicating the gapless spin excitations. Further increasing the field intensity, we observe a spin excitation gap with the gap size proportional to the field intensity. These phenomena suggest a very unusual strongly correlated quantum disordered phase, and the implications for the quantum spin liquid state are further discussed.

cond-mat.str-el↗

arXiv2020

Magnetism-induced topological transition in EuAs3

The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We present $ab$ $initio$ band calculations, electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements on the magnetic semimetal EuAs$_3$, demonstrating a magnetism-induced topological transition from a topological nodal-line semimetal in the paramagnetic or the spin-polarized state to a topological massive Dirac metal in the antiferromagnetic (AFM) ground state at low temperature, featuring a pair of massive Dirac points, inverted bands and topological surface states on the (010) surface. Shubnikov-de Haas (SdH) oscillations in the AFM state identify nonzero Berry phase and a negative longitudinal magnetoresistance ($n$-LMR) induced by the chiral anomaly, confirming the topological nature predicted by band calculations. When magnetic moments are fully polarized by an external magnetic field, an unsaturated and extremely large magnetoresistance (XMR) of $\sim$ 2$\times10^5$ % at 1.8 K and 28.3 T is observed, likely arising from topological protection. Consistent with band calculations for the spin-polarized state, four new bands in quantum oscillations different from those in the AFM state are discerned, of which two are topologically protected. Nodal-line structures at the $Y$ point in the Brillouin zone (BZ) are proposed in both the spin-polarized and paramagnetic states, and the latter is proven by ARPES. Moreover, a temperature-induced Lifshitz transition accompanied by the emergence of a new band below 3 K is revealed. These results indicate that magnetic EuAs$_3$ provides a rich platform to explore exotic physics arising from the interaction of magnetism with topology.

cond-mat.mtrl-sci↗

arXiv2019

Giant enhancement of critical current density at high field in superconducting (Li,Fe)OHFeSe films by Mn doping

Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors. Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m^3, which is the highest record so far among all iron-based superconductors. Our results demonstrate that Mn incorporation is an effective method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising candidate for high-field applications.

cond-mat.supr-con↗

arXiv2019

The upper critical field and its anisotropy in RbCr$_{3}$As$_{3}$

The temperature dependence of the upper critical field ($H_{c2}$) in RbCr$% _{3}$As$_{3}$ single crystals ($T_{c}\approx $ 7.3 K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel ($H\parallel c $, $H_{c2}^{\parallel c}$) and perpendicular ($H\perp c$, $H_{c2}^{\perp c}$) to the Cr chains. The curves $H_{c2}^{\parallel c}(T)$ and $H_{c2}^{\perp c}(T)$ cross at $\sim $ 5.5 K. As a result, the anisotropy parameter $γ(T)=H_{c2}^{\perp c}/H_{c2}^{\parallel c}(T)$ increases from 0.5 near $T_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of $μ_0H_{c2}^{\parallel c}(0)\approx $ 27.2 T and $μ_0H_{c2}^{\perp c}(0)\approx $ 43.4 T, both exceeding the BCS weak-coupling Pauli limit $μ_0H_{p}=1.84T_{c}=13.4$ T. The results indicate that the paramagnetic pair breaking effect is strong for $H \parallel c$ but absent for $H \perp c$, which was further confirmed by the angle dependent magnetoresistance and $H_{c2}$ measurements.

cond-mat.supr-con↗

arXiv2019

Bulk Fermi surface of the layered superconductor TaSe3 with three-dimensional strong topological insulator state

High magnetic field transport measurements and ab initio calculations on the layered superconductor TaSe3 have provided compelling evidences for the existence of a three-dimensional strong topological insulator state. Longitudinal magnetotransport measurements up to ~ 33 T unveiled striking Shubnikov-de Hass oscillations with two fundamental frequencies at 100 T and 175 T corresponding to a nontrivial electron Fermi pocket at the B point and a nontrivial hole Fermi pocket at the Γ point respectively in the Brillouin zone. However, calculations revealed one more electron pocket at the B point, which was not detected by the magnetotransport measurements, presumably due to the limited carrier momentum relaxation time. Angle dependent quantum oscillations by rotating the sample with respect to the magnetic field revealed clear changes in the two fundamental frequencies, indicating anisotropic electronic Fermi pockets. The ab initio calculations gave the topological Z2 invariants of (1; 100) and revealed a single Dirac cone on the (1 0 -1) surface at the X point with helical spin texture at a constant-energy contour, suggesting a strong topological insulator state. The results demonstrate TaSe3 an excellent platform to study the interplay between topological phase and superconductivity and a promising system for the exploration of topological superconductivity.

cond-mat.supr-con↗

arXiv2019

Novel magnetic field tuning of quantum spin excitations in a weakly coupled $S=$1/2 Heisenberg spin chain as seen from NMR

We report our NMR study of the spin excitations in the quasi-one dimensional (1D) $S=1/2$ quantum magnet CH$_3$NH$_3$Cu(HCOO)$_3$ lying in the 1D-3D dimensional crossover regime with Dzyaloshinskii-Moriya(DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a power-law temperature dependence gradually, with the index increasing from zero to $\sim3$, and finally $\sim5$, which are the respective typical characteristic for the dominating two-magnon Raman process and three-magnon scattering contributions to the nuclear relaxation in a conventional 3D magnet. A possible physical mechanism for this novel magnetic field tuning of quantum spin excitations related with the enhanced effective staggered field created by the DM interactions under magnetic field is discussed.

cond-mat.str-el↗

arXiv2019

Magnetotransport in Al6Re

Since very few Type-I superconductors are known and most are elemental superconductors, there are very few experimental platforms where the interaction between Type-I superconductivity and topologically nontrivial band structure can be probed. The rhenium aluminide Al$_6$Re has recently been identified as a Type-I superconductor with a transition of 0.74\,K and a critical field of $\sim$50\,Oe. Here, we report its magnetotransport behavior including de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations. Angular dependence of the magnetoresistance reveals a highly anisotropic Fermi surface with dominant hole character. From the strong oscillatory component $ΔR_{xx}$ in high magnetic fields up to 33\,T, the Landau index infinite-field intercept in the case of a single oscillation frequency, and the phase factor $φ$ where multiple frequencies coexist, are both $\sim$1/4. This intermediate value is suggestive of possible nontrivial band topology but does not allow strong conclusions.

cond-mat.supr-con↗

arXiv2019

Anomalies of upper critical field in the spinel superconductor LiTi$_2$O$_{4-δ}$

High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-δ}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic $B_\mathrm{c2}$ up to $\sim$ 26 Tesla is observed in oxygen-rich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit. Such anomalies of $B_\mathrm{c2}$ were rarely reported in other three dimensional superconductors. Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of $B_\mathrm{c2}$ in oxygen-rich LiTi$_2$O$_{4-δ}$ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher $B_\mathrm{c2}$.

cond-mat.supr-con↗

arXiv2018

Phase transition and anomalous scaling in the quantum Hall transport of topological insulator Sn-Bi1.1Sb0.9Te2S devices

The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-driven plateau-to-plateau transition gives scaling relations of (dR$_{xy}$/dB)$^{max}$ \propto T$^{-κ}$ and \DeltaB$^{-1}$ \propto T$^{-κ}$ with a consistent exponent of κ~ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling, and reveals the dominance of electron-electron interaction of the topological surface states.

cond-mat.mes-hall↗

arXiv2018

Topological semimetal state and field-induced Fermi surface reconstruction in antiferromagnetic monopnictide NdSb

We report the experimental realization of Dirac semimetal state in NdSb, a material with antiferromagnetic ground state. The occurrence of topological semimetal state has been well supported by our band structure calculations and the experimental observation of chiral anomaly induced negative magnetoresistance. A field-induced Fermi surface reconstruction is observed, in response to the change of spin polarization. The observation of topological semimetal state in a magnetic material provides an opportunity to investigate the magneto-topological phenomena.

cond-mat.mtrl-sci↗

arXiv2018

Non-saturating Quantum Magnetization in Weyl semimetal TaAs

Detecting the spectroscopic signatures of Dirac-like quasiparticles in emergent topological materials is crucial for searching their potential applications. Magnetometry is a powerful tool for fathoming electrons in solids, yet its ability for discerning Dirac-like quasiparticles has not been recognized. Adopting the probes of magnetic torque and parallel magnetization for the archetype Weyl semimetal TaAs in strong magnetic field, we observed a quasi-linear field dependent effective transverse magnetization and a strongly enhanced parallel magnetization when the system is in the quantum limit. Distinct from the saturating magnetic responses for massive carriers, the non-saturating signals of TaAs in strong field is consistent with our newly developed magnetization calculation for a Weyl fermion system in an arbitrary angle. Our results for the first time establish a thermodynamic criterion for detecting the unique magnetic response of 3D massless Weyl fermions in the quantum limit.

cond-mat.mes-hall↗

arXiv2017

Evolution of Weyl orbit and quantum Hall effect in Dirac semimetal Cd3As2

Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple- to single-frequency Shubnikov-de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the dominant role of surface transport. With the increase of magnetic field, the SdH oscillations further develop into quantum Hall state with non-vanishing longitudinal resistance. By tracking the oscillation frequency and Hall plateau, we observe a Zeeman-related splitting and extract the Landau level index as well as sub-band number. Different from conventional two-dimensional systems, this unique quantum Hall effect may be related to the quantized version of Weyl orbits. Our results call for further investigations into the exotic quantum Hall states in the low-dimensional structure of topological semimetals.

cond-mat.mtrl-sci↗

arXiv2017

Tricritical point and phase diagram based on critical scaling in monoaxial chiral helimagnet Cr1/3NbS2

In this work, the magnetism of the single crystal Cr$_{1/3}$NbS$_{2}$, which exhibits chiral magnetic soliton lattice (CSL) state, is investigated. The magnetization displays strong magnetic anisotropy when the field is applied perpendicularly and parallel to the $c$-axis in low field region ($H<H_{S}$, $H_{S}$ is the saturation field). The critical exponents of Cr$_{1/3}$NbS$_{2}$ are obtained as $β=$ 0.370(4), $γ=$ 1.380(2), and $δ=$ 4.853(6), which are close to the theoretical prediction of three-dimensional Heisenberg model. Based on the scaling equation and the critical exponents, the $H-T$ phase diagram in the vicinity of the phase transition is constructed, where two critical points are determined. One is a tricritical point which locates at the intersection between the CSL, forced ferromagnetic (FFM), and paramagnetic (PM) states. The other one is a critical point situated at the boundaries between CSL, helimagnetic (HM), and PM states.

cond-mat.other↗

arXiv2017

Manipulating Multiple Order Parameters via Oxygen Vacancies: The case of Eu0.5Ba0.5TiO3-δ

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (VO) is a key issue for the future development of transition metal oxides. Progress in this field is currently addressed through VO variations and their impact on mainly one order parameter. Here we reveal a new mechanism for tuning both magnetism and ferroelectricity simultaneously by using VO. Combined experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-δ, we demonstrate that oxygen vacancies create Ti3+ 3d1 defect states, mediating the ferromagnetic coupling between the localized Eu 4f7 spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-δ.

cond-mat.mtrl-sci↗

arXiv2017

Stepwise quantized surface states and delayed Landau level hybridization in Co cluster-decorated BiSbTeSe2 topological insulator devices

In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top and bottom surfaces. However, due to the no-go theorem, the two TSSs always appear as a pair and are expected to quantize synchronously. Quantized transport of a separate Dirac channel is still desirable, but has never been observed in graphene even after intense investigation over a period of 13 years, with the potential aim of half-QHE. By depositing Co atomic clusters, we achieved stepwise quantization of the top and bottom surfaces in BiSbTeSe2 (BSTS) TI devices. Renormalization group flow diagrams13, 22 (RGFDs) reveal two sets of converging points (CVPs) in the (Gxy, Gxx) space, where the top surface travels along an anomalous quantization trajectory while the bottom surface retains 1/2 e2/h. This results from delayed Landau-level (LL) hybridization (DLLH) due to coupling between Co clusters and TSS Fermions.

cond-mat.mes-hall↗

arXiv2016

Superconductivity and Charge Density Wave in ZrTe$_{3-x}$Se$_{x}$

Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0\leq}x\leq0.1$) increases up to 4 K plateau for $0.04$$\leq$$x$$\leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.

cond-mat.supr-con↗