Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2Ying Xing,†,∥,# Kun Zhao,‡,# Pujia Shan,†,# Feipeng Zheng,†,# Yangwei Zhang,†,# Hailong Fu,†,# Yi Liu,†,#

Mingliang Tian,§ Chuanying Xi,§ Haiwen Liu,⊥ Ji Feng,†,# Xi Lin,†,# Shuaihua Ji,*,‡,# Xi Chen,‡,#

Qi-Kun Xue,‡,# and Jian Wang*,†,‡,#

†International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China‡State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China§High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China∥Beijing Key Laboratory of Optical Detection Technology for Oil and Gas, China University of Petroleum, Beijing 102249, China⊥Department of Physics, Beijing Normal University, Beijing 100875, China#Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

*S Supporting Information

ABSTRACT: Two-dimensional (2D) transition metal dichalco-genides (TMDs) have a range of unique physics properties andcould be used in the development of electronics, photonics,spintronics, and quantum computing devices. The mechanicalexfoliation technique of microsize TMD flakes has attractedparticular interest due to its simplicity and cost effectiveness.However, for most applications, large-area and high-quality filmsare preferred. Furthermore, when the thickness of crystalline filmsis down to the 2D limit (monolayer), exotic properties can beexpected due to the quantum confinement and symmetrybreaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe2 films on bilayer grapheneterminated surface of 6H-SiC(0001) substrates by a molecular beam epitaxy (MBE) method. The films exhibit an onsetsuperconducting critical transition temperature (Tc

onset) above 6 K and the zero resistance superconducting critical transitiontemperature (Tc

zero) up to 2.40 K. Simultaneously, the transport measurements at high magnetic fields and low temperaturesreveal that the parallel characteristic field Bc//(T = 0) is above 5 times of the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayerNbSe2 film shows the signature of quantum Griffiths singularity (QGS) when approaching the zero-temperature quantum criticalpoint.

KEYWORDS: NbSe2, transition-metal dichalcogenides, macro-size monolayer film,ultralow temperature and high magnetic field electrical transport, Ising superconductivity, quantum phase transition (QPT)

Q uasi-2D superconductors such as ultrathin films withthickness down to monolayer1−7 and compositeinterfaces8−10 have remained an active topic in recent yearsdue to fundamental research interests and potential applica-tions. Nevertheless, just a few monolayer crystalline super-conductors can be prepared successfully on special substratessince fluctuations can destroy the long-range correlation ofsuperconductivity in 2D systems. Recently, TMDs as naturallayered materials have provided a new platform to studysuperconductivity due to the tunable nature of the super-conducting properties coexistent with other collective elec-tronic excitations, as well as strong intrinsic spin−orbit coupling(SOC). The bulk crystals of TMDs are formed of monolayersbound to each other by van der Waals attraction, which makesit feasible to experimentally study monolayer TMDs.

2H-NbSe2, one kind of TMDs, is found to be super-conducting even in its freestanding monolayer.11−14 Moreinterestingly, Zeeman-protected Ising superconductivity isexpected in monolayer NbSe2 due to the noncentrosymmetricstructure with in-plane inversion symmetry breaking and strongSOC. Very recently, Ising superconductivity with theanomalous large in-plane critical magnetic field has becomeone important direction in crystalline 2D superconductors.7 Inrecent experiments on exfoliated micron-size NbSe2 mono-layers,11,12,14 the coexistence of charge density wave (CDW)and the superconducting phase was observed down to themonolayer limit, but the superconducting critical transition

Received: July 16, 2017Revised: September 29, 2017Published: October 2, 2017

Letter

pubs.acs.org/NanoLett

© 2017 American Chemical Society 6802 DOI: 10.1021/acs.nanolett.7b03026Nano Lett. 2017, 17, 6802−6807

Cite This: Nano Lett. 2017, 17, 6802-6807

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temperature of monolayer NbSe2 got significantly suppressed(Tc ∼ 3.0 K) compared with its bulk value (7.2 K).Superconductor−insulator (metal) transition (SIT/SMT), a

paradigm of QPT, is an important topic in condensed matterphysics.15−21 Despite of efforts over last few decades, there stillremain many open issues, such as different critical exponentssignifying different universality classes and various values ofcritical points found in different materials.22 Recent observa-tions of the QGS in thin Ga films23 and LaAlO3/SrTiO3(110)(LAO/STO) interface24 shed a new light on SIT/SMT22 andhave turned to one important topic in 2D superconductors.7

Verifying QGS in Ising superconductors would not onlydemonstrate the universal property of QGS but also help tounderstand the underlying mechanism of the 2D super-conductors where Ising superconductivity and QGS coexist.In this paper we successfully prepared atomically flat

monolayer NbSe2 films (∼0.6 nm thick) on bilayer grapheneterminated surface of 6H-SiC (0001) substrates by MBEmethod. The high-quality films are uniform and macro sizelarge (mm2). Here, monolayer means one Se−Nb−Sesequence, and each unit cell of NbSe2 consists of two Se−Nb−Se sequences. By low temperature and high magnetic fieldelectrical transport measurements, the Ising superconductivityis found in NbSe2 monolayers, with Tc

onset above 6 K (Tczero is

up to 2.40 K defined at the temperature showing zero resistancewithin instrumental resolution) and Bc// (T = 0) up to 32.43 T.The observed superconductivity anisotropy and Berezinski−Kosterlitz−Thouless (BKT)-like transition reveal the 2D natureof NbSe2 films. Through systematic ultralow temperaturetransport measurements, the monolayer NbSe2 films manifestmagnetic field induced SMT, and the critical exponent of SMTdiverges as T approaching zero, indicating QGS.Figure 1a shows the typical atomic-resolved scanning

tunneling microscope (STM) image (18 nm × 18 nm) on

NbSe2 monolayer, from which a 3 × 3 CDW superlattice can beclearly seen at 80 mK. A large-scale (1.9 μm × 1.9 μm) STMtopographic image with an atomically flat surface and regularsteps originating from the SiC (0001) surface is shown inFigure 1b. Very similar morphology images of NbSe2 areobserved by STM in different regions, which implies theuniformity of NbSe2 films in the macro size. The flat terracesare monolayer 2H-NbSe2 inherited from SiC substrate, and thesmall islands are bilayer NbSe2 with a random orientation (not2H phase) and in-plane size less than 100 nm, which couldsuppress the superconductivity in the islands. Furthermore, the

ratio of small triangular islands in the whole film is only 3.14%.Therefore, the superconducting features from ex situ transportmeasurement are mainly from monolayer NbSe2. Themonolayer NbSe2, bilayer graphene, and SiC (0001) substrateare all van der Waals coupling between each other. Due to thisweak coupling, the monolayer Se−Nb−Se is more like afreestanding state. Before being transferred out of MBE highvacuum chamber, an amorphous Se capping layer with thethickness of 20 nm was deposited on the monolayer NbSe2 at80 K to protect the film from degrading in ambient atmosphere.These formed a Se/NbSe2/bilayer graphene/SiC heterostruc-ture. The high quality nature of the macro-size NbSe2monolayer protected by the capping layer guarantees theobservation of superconductivity with zero resistance atrelatively high temperature by ex situ transport measurements,which could extend the investigation to a previously unexploredregime for the 2D limit NbSe2 monolayer in macro-size.Figure 2a exhibits the temperature dependence of sheet

resistance (Rs) of sample 1 at a zero magnetic field. With thetemperature decreasing, the Rs of monolayer NbSe2 rapidlyincreases at 120 K, saturates around 50 K, then begins todecrease at 6.61 K (Tc

onset), and drops to zero within theinstrumental resolution at 2.40 K (Tc

zero) (Figure 2b). Tcmiddle

determined at the temperature where the resistance dropreaches 50% of resistance at 8 K is 3.50 K. In the rest of thepaper, we use Tc as the Tc

middle. The superconductivity is betterthan that of mechanical exfoliated monolayer NbSe2 flakes(Tc

onset is ∼4.2 K, Tc ∼ 3.0 K)11 and MBE grown monolayer

NbSe2 films (Tconset ∼ 1.9 K, Tc ∼ 0.65 K, Tczero ∼ 0.46 K)

13 inprevious reports. The inset of Figure 2a schematically depictsthe diagram for transport measurements. The similar Rs(T)properties for sample 2 are displayed in Figure S1a with Tc

onset

up to 6.95 K and Tczero ∼ 1.50 K. The Se/bilayer graphene/SiC

heterostructure under the growth conditions identical to Se/NbSe2/bilayer graphene/SiC heterostructure exhibits insulatingbehavior (Figure S1b). This confirms that the superconductiv-ity only comes from the monolayer NbSe2. Theoretically, weinvestigate the electron−phonon coupling of freestandingmonolayer NbSe2 without charge density wave modulation bydensity-functional theory and density-functional perturbationtheory calculations within local density approximation. Thecritical temperature Tc calculated using the McMillan formula

25

falls in range between 3.8 and 4.1 K, which is close to ourexperimental result (Tc

onset ∼ 6.61 K and Tczero ∼ 2.40 K forsample 1). We also qualitatively investigated the influence ofbilayer graphene on the electronic band structure of themonolayer NbSe2 and found that the graphene has littleinfluence on the electronic band structure of the NbSe2 (FigureS4).A magnetic field up to 15.50 T perpendicular to the film was

applied (Figure 2b), and magnetoresistance isotherms(perpendicular magnetic field) were measured at temperaturefrom 0.50 to 10.00 K (Figure 2c, full data shown in Figure S2a).It is clearly evident that the increasing magnetic field graduallydestroys the superconductivity. When the magnetic field isparallel to the film (see Figure 2d), Bc(T) is apparently muchhigher than that in the perpendicular field. A field of 15 T canhardly destroy the superconductivity of monolayer NbSe2. Toobtain Bc at lower temperatures, we further measured themagnetoresistance of the same sample by applying steadyultrahigh magnetic field up to 35 T. Due to the degradation inatmosphere, Tc decreased from 3.50 to 2.69 K in ultrahighmagnetic field measurements. For T = 0.35 K, a 35 T high

Figure 1. (a) Atomic-resolved STM image on monolayer NbSe2 film(18 nm × 18 nm, sample bias Vs = 10 mV, tunneling current It = 100pA, temperature T = 80 mK). The 3 × 3 CDW superlattice is fully anduniformly developed for monolayer NbSe2. (b) Typical topography ofmonolayer NbSe2 on graphene/SiC(0001) (1.9 μm × 1.9 μm, Vs = 3.0V, It = 20 pA, T = 300 K).

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magnetic field still cannot destroy the superconductivity of themonolayer NbSe2 completely (Figure 2e). We define thecharacteristic field Bc(T) at a given temperature at the 50% ofnormal resistance for both perpendicular and parallel magneticfields, and summarize in Figure 2f. With T/Tc down to 0.13,Bc(T) can be fitted by Bc⊥(T) ∝ 1 − T/Tc and Bc//(T) ∝ (1 −T/Tc)

1/2, respectively,26,27 which yield Bc⊥(0) = 2.16 T andBc//(0) = 32.43 T. In the previous report on mechanicalexfoliation fabricated microsize monolayer NbSe2 flakes, T/Tcis down to 0.78.11 The anisotropic parameter ε = Bc//(0)/Bc⊥(0) is about 15 for NbSe2 monolayer but only 3.2 for bulkNbSe2.

28 A BKT-like transition has also been observed (FigureS3) in monolayer NbSe2. Such observations demonstrate the2D nature of the superconductivity.In the 2D limit regime, the orbital effect is restricted in the

parallel magnetic field. The Bc is only determined by Pauli-limiting field Bp, which originates from the Zeeman splitting. Ingeneral, the Pauli-limiting field is strong enough to break

Cooper pairs and destroy superconductivity. Bp is written as Bp= g−1/2Δ/μB, where g is the Lande ́ g-factor, μB is the Bohrmagneton, and Δ is the superconducting gap. For BCSsuperconductors, Bp can be simply rewritten as Bp = 1.84Tcwhen assuming a g-factor equals to 2. For our MBE grownmonolayer NbSe2 films, Bp is estimated to be 4.95 T (Tc ∼ 2.69K) by using g ∼ 2 in previous reports.11 If we use g factor valueof bulk NbSe2 (∼1.2),

29 then Bp should be 6.37 T. Surprisingly,as revealed in Bc − T/Tc phase diagram in the inset of Figure 2f,the monolayer NbSe2 could withstand an applied parallelmagnetic field as strong as 32.43 T (T = 0). Thus, Bc//(0) is5.09 times of Bp by using g ∼ 1.2 or 6.55 times of Bp by using g∼ 2. A similar phenomenon has ever been reported in gatedMoS2 flakes

30,31 and mechanical exfoliation fabricated microsizemonolayer NbSe2 flakes.

11 In noncentrosymmetric monolayerNbSe2 with considerable spin−orbit interactions, the spin−orbit interaction split the spin states and manifest as a strongeffective internal magnetic fields. The spin−orbit interactionexperienced by a moving electron with momentum k isproportional to k × E·σ, where E is the electric fieldexperienced by the electron and σ denoted the Pauli matrices.In monolayer NbSe2, the in-plane inversion symmetry is brokenand the electrons can experience effective in-plane electric field.Hence, the spins of the pairing electrons are strongly locked tothe out-of-plane orientation by an effective Zeeman field.Instead of damaging superconductivity, this special type ofinternal magnetic field due to strong spin−orbit interaction isable to protect the superconducting electron pairs under highexternal magnetic fields. This kind of superconductor is called“Ising superconductor”. Our measurement results at highmagnetic fields up to 35 T and the low temperature down to0.35 K as T/Tc down to 0.13 offers a more solid evidence ofIsing superconductivity in macro-size monolayer NbSe2approaching to lower temperature and higher magnetic field.We note that the high critical magnetic field in low

temperature regime has been observed in heavy Fermionsuperconductors32 and organic superconductors.33 The mech-anism was interpreted as the Fulde−Ferrell−Larkin−Ovchinni-kov (FFLO) state.34 For the FFLO phase to appear, thecompounds must have a Maki parameter35 α = √2Borb/Bplarger than 1.8 such that the upper critical field can easilyapproach the Pauli paramagnetic limit Bp. Simultaneously, thesystem must be in the clean limit ξ ≪ l, since the FFLO state isreadily destroyed by impurities.32 Calculations show that, inanisotropic superconductors, the FFLO state might lead to anenhancement of the upper critical field between 1.5 and 2.5times of the Pauli paramagnetic limit field.34,36 In perpendicularmagnetic field cases, the characteristic field Bc⊥(0) of NbSe2film is estimated to be 2.16 T, smaller than the Pauliparamagnetic limit field (6.37 T). Besides, the Maki parameterα⊥ = 0.48 is smaller than 1.8. In parallel magnetic field cases,the characteristic field Bc//(0) ∼ 32.43 T is above 5 times ofPauli paramagnetic limit field Bp, which exceeds the theoreticalpredictions of 1.5−2.5 times.33,36 Therefore, the chance of theexistence of FFLO state in monolayer NbSe2 is little.To further study the physical properties of monolayer NbSe2

in perpendicular magnetic field, the sample 3 (with Tconset ∼ 6

K, Figure S5a) was measured in ultralow temperatureenvironment by a dilution refrigerator (Figure 3, down to0.025 K). Figure S5b depicts the Rs(T) curves at variousperpendicular magnetic fields down to 2.00 K by a physicalproperty measurement system. With the increasing field, thesuperconducting film gradually turns to a metal, indicating a

Figure 2. Electrical transport measurements of monolayer NbSe2 film.(a) Temperature dependence of the sheet resistance of theheterostructure Se/NbSe2/BL Graphene/6H-SiC at zero magneticfield. The inset depicts the schematic diagram for heterostructure andstandard four-probe configuration for electrical measurements. (b)Rs(T) curves for various perpendicular magnetic fields up to 15.5 T(the magnetic fields from bottom to top are 0.00, 0.10, 0.20, 0.50, 0.80,1.00, 1.50, 2.00, 3.00, 5.00, 8.00, 10.00, 13.00, and 15.50 T). The Rs(T)curve at a zero magnetic field reveals Tc

zero = 2.40 K. The inset is thezoom-in image of Rs(T) curve from 4 to 8 K, showing Tc

onset = 6.61 K.(c) Magnetoresistance measured in perpendicular magnetic field atselected temperatures from 0.5 to 10 K. (d) Rs(T) and (e) Rs(B)characteristics at parallel magnetic fields (the magnetic field is parallelto the film up to 35 T). (f) The characteristic magnetic field Bc forboth perpendicular and parallel fields. Bc⊥(0) = 2.16 T, Bc//(0) = 32.43T. Bc⊥(T) and Bc//(T) are extracted from panels c and e as the fields atwhich the sample resistance drops to 50% of the normal stateresistance. The green line is the fitting curve using Bc//(T) ∝ (1 − T/Tc)

1/2. The black line is linear fitting by Bc⊥(T) ∝ 1 − T/Tc. Thedashed blue line indicates the Pauli limit field Bp = 6.37 T (g = 1.2).

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magnetic field induced SMT, which is further revealed by Rs(T)curves down to 25 mK (Figure 3a). The magnetoresistanceisothermals seem to cross a point from 2.00 to 4.50 K (FigureS5c). However, systematic ultralow temperature data display aseries of magnetoresistance crossing points between 0.025 and1.30 K, which form a continuous line of SMT “critical” points(Figure 3b). We summarize the SMT “critical” points Bc′ inFigure 3c. The black squares are crossing points of Rs(B) curvesat every two adjacent temperatures. The Rs plateaus extractedfrom Rs(T) curves in Figure 3a are shown as red dots, at whichthe dRs/dT changes sign for a given magnetic field. The blueline is the linear fitting curve. As we can see, the Bc′ slightlydiverges the linear tendency in the ultralow temperature region(T < 0.264 K). The linear extrapolation gives Bc′(0 K) = 2.940T, obviously below the actual experimental data (>3.200 T).For SIT, the sample critical resistance on phase transition is

the quantum resistance for pairs (h/4e2 ∼ 6450 Ω), and thecritical exponent remains a constant.37 This is not in agreementwith our experimental results. The critical resistance ofmonolayer NbSe2 film is much smaller than h/4e

2, indicatingthe unpaired normal electrons also contribute to theconductance. Such unpaired electrons can originate from thedissipation effect, which gives rise to the SMT with criticalresistance much smaller than h/4e2.38 Subsequent theoreticalinvestigations reveal that the quenched disorder dramaticallychanges the scaling behavior of SMT39 and results in activatedscaling identical to that of the random transverse field Isingmodel, in which the dynamical exponent z continuously varieswhen approaching the quantum critical point.40 This activescaling behavior can be regarded as the “quantum version” ofGriffiths singularity, which is called QGS.41 In previous reportson SITs and SMTs, the magnetic resistance isotherms cross inone or two points, which are considered as the quantum

criticality.15−21 However, in recent work on 2D super-conducting Ga films,23 the similar continuous line of SMT“critical” points and the dynamical critical exponent divergencewere detected, which experimentally revealed QGS of SMTs inGa films.7,22,23

We then analyze our data referring to the finite size analysismethod23 (See Supporting Information for details). Theresulting B dependence of effective “critical” exponent zv issummarized in Figure 3d. In relatively high temperature regime,zv increases slowly with magnetic field. In the ultralowtemperature regime, zv grows quickly and diverges when thecharacteristic magnetic field (Bc*) is approaching. We fit theexperimental values (zv > 1) as a function of B by the activatedscaling law zv = C|Bc* − B|−vψ,

42 where the correlation lengthexponent v ≈ 1.2, the tunneling critical exponent in twodimensions ψ ≈ 0.5 and C is a constant. The activated scalingbehavior can fit the experimental data very well (Figure 3d).This indicates the existence of infinite randomness QPT inmonolayer NbSe2, consistent with the QGS

40,42−46 behavior.Although the monolayer NbSe2 films are atomically flat andcrystalline in the macro size, there are still a small amount ofdefects. Besides, the amorphous protecting layer and substratecan provide scattering disorder to the monolayer NbSe2 film.These can be the origin of the quenched disorder at ultralowtemperatures, which physically causes the QGS. The interfaceeffect from the substrate and protection layer for MBE grownultrathin films may also explain why the quantum metallic state(resistance saturation under magnetic field below Tc) can befound in mechanical exfoliated bilayer NbSe2 flakes

47 and ion-gated 2D superconducting flakes10 (both are in the disorder-free limit) but absent in our MBE grown NbSe2 monolayerfilms.

Figure 3. (a) Rs(T) curves at different magnetic fields. The temperatures at Rs maximum (dRs/dT = 0) in each curves are 0.136, 0.380, 0.69, 1.25,1.64, 1.92, 2.18, and 2.72 K, respectively. (b) Magnetic field dependence of Rs at various temperatures from 0.025 to 2.20 K. (c) The characteristicmagnetic field Bc′ at various temperatures, extracted from panels a and b. The black squares are crossing points of Rs(B) data at adjacent temperaturesin panel b; the red dots are the Rs maxima on Rs(T) curves in panel a. The crossing points are well consistent with Rs maxima. The blue line is thelinear fitting curve. (d) SMT exponent zv as a function of magnetic field. The red curve is a fitting based on the activated scaling law equation shownin panel d.

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We note that, in different 2D superconducting quantumGriffths singularity systems, the Bc′(T) in higher temperatureregime (T < Tc

onset) may exhibit different behaviors. Intransition metal dichalcogenides NbSe2 monolayers, Isingsuperconductivity is observed, originating from the strongspin orbit coupling effect. As a consequence, this Isingsuperconductivity gives rise to the deviation of phase boundaryBc′(T) from the Werthamer−Helfand−Hohenberg (WHH)theory48 in the high temperature regime (Figure 3c). On theother hand, in Gallium thin films the SOC can be neglected,and the phase boundary Bc′(T) follows the WHH formula inthe high temperature region.23 Remarkably, the result fromNbSe2 monolayers manifests that SOC effect does not changethe QGS at ultralow temperatures, which implies theuniversality of QGS of SMT in different 2D superconductors.In summary, several innovations are displayed in this paper.

(i) The high-quality macro-size atomically flat monolayerNbSe2 films are successfully grown on bilayer graphene/SiC bythe MBE method, and the film exhibits Tc

onset above 6 K (6.0−6.9 K) and Tc

zero up to 2.40 K by electronic transport study,higher than reported values on mechanically exfoliated NbSe2monolayers.11,12,14 (ii) By the measurements at high magneticfields up to 35 T, we undoubtedly reveal the superconductingsurvivability under a strong parallel magnetic field with Bc//(0)> 5Bp as T/Tc down to 0.13 and experimentally demonstrateIsing superconductivity in macro-size monolayer NbSe2 atlower temperatures and higher magnetic fields. (iii) Themagnetic field driven SMT is detected in monolayer NbSe2films, and the QPT exhibits the signature of QGS. Thecoexistence of Ising superconductivity and QGS at 2D limitcould be an important topic in future for further understanding2D superconductivity. Besides the innovations mentionedabove, monolayer NbSe2 has also been theoretically proposedto be a new platform to create topological superconductivityand Majorana fermions.49−51 Our findings will definitelystimulate more investigations on 2D TMD superconductors.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.7b03026.

Sample preparation, transport measurements and BKT-like transition, theoretical calculations, finite size scalinganalysis (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: jianwangphysics@pku.edu.cn.*E-mail: shji@mail.tsinghua.edu.cn.ORCIDMingliang Tian: 0000-0002-0870-995XJian Wang: 0000-0002-7212-0904Author ContributionsY.X. and K.Z. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Dingping Li for useful discussions. This work wassupported by the National Basic Research Program of China

(grants nos. 2013CB934600 and 2017YFA0303302), theResearch Fund for the Doctoral Program of Higher Education(RFDP) of China (grants no. 20130001110003), the OpenResearch Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University undergrant no. KF201501 and no. KF201703, the Open ProjectProgram of the Pulsed High Magnetic Field Facility (grant no.PHMFF2015002), Huazhong University of Science andTechnology, the National Natural Science Foundation ofChina (grant nos. 11774008, 11704414, 11622433, and11574175) and the Science Foundation of China Universityof Petroleum, Beijing (grant no. 2462017YJRC012).

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