Publications by authors named "Zhu-An Xu"

36 Publications

Possible Evidence for Berezinskii-Kosterlitz-Thouless Transition in Ba(FeCo)As Crystals.

Materials (Basel) 2021 Oct 22;14(21). Epub 2021 Oct 22.

Interdisciplinary Center for Quantum Information, Zhejiang Province Key Laboratory of Quantum Technology and Devices, Department of Physics, Zhejiang University, Hangzhou 310027, China.

In this study, we measure the in-plane transport properties of high-quality Ba(FeCo)As single crystals. Signatures of vortex unbinding Berezinskii-Kosterlitz-Thouless (BKT) transition are shown from both the conventional approach and the Fisher-Fisher-Huse dynamic scaling analysis, in which a characteristic Nelson-Kosterlitz jump is demonstrated. We also observe a non-Hall transverse signal exactly at the superconducting transition, which is explained in terms of guided motion of unbound vortices.
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http://dx.doi.org/10.3390/ma14216294DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8585216PMC
October 2021

Discovery of segmented Fermi surface induced by Cooper pair momentum.

Science 2021 Oct 28:eabf1077. Epub 2021 Oct 28.

School of Physics and Astronomy, Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National Laboratory for Materials Science, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China.

[Figure: see text].
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http://dx.doi.org/10.1126/science.abf1077DOI Listing
October 2021

Rashba valleys and quantum Hall states in few-layer black arsenic.

Nature 2021 05 5;593(7857):56-60. Epub 2021 May 5.

Zhejiang Province Key Laboratory of Quantum Technology and Device, Department of Physics, Zhejiang University, Hangzhou, People's Republic of China.

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin-orbit coupling (SOC) interplays dynamically with Coulomb interactions, band topology and external modulating forces. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle-hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with p symmetry, coexisting with doubly degenerate D valleys of p origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the p bands, which produces spin-valley-flavoured D valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the p symmetry. Intriguingly, the giant Stark effect shows the same p-orbital selectiveness, collectively shifting the valence band maximum of the D Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.
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http://dx.doi.org/10.1038/s41586-021-03449-8DOI Listing
May 2021

Coexistence of Ferroelectricity and Ferromagnetism in One-Dimensional SbN and BiN Nanowires.

ACS Appl Mater Interfaces 2021 Mar 9;13(11):13517-13523. Epub 2021 Mar 9.

School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China.

Ferroelectricity exists in a variety of three- and two-dimensional materials and is of great significance for the development of electronic devices. However, the presence of ferroelectricity in one-dimensional materials is extremely rare. Here, we predict ferroelectricity in one-dimensional SbN and BiN nanowires. Their polarization strengths are 1 order of magnitude higher than ever reported values in one-dimensional structures. Moreover, we find that spontaneous spin polarization can be generated in SbN and BiN nanowires by moderate hole doping. This is the first time the coexistence of both ferroelectricity and ferromagnetism in a one-dimensional system has been reported. Our finding not only broadens the family of one-dimensional ferroelectric materials but also offers a promising platform for novel electronic and spintronic applications.
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http://dx.doi.org/10.1021/acsami.0c20570DOI Listing
March 2021

Tunable Topological Energy Bands in 2D Dialkali-Metal Monoxides.

Adv Sci (Weinh) 2020 Feb 7;7(4):1901939. Epub 2020 Jan 7.

Zhejiang Province Key Laboratory of Quantum Technology and Device and Department of Physics in Zhejiang University State Key Lab of Silicon Materials School of Materials Science and Engineering in Zhejiang University Hangzhou 310027 P. R. China.

2D materials with nontrivial energy bands are highly desirable for exploring various topological phases of matter, as low dimensionality opens unprecedented opportunities for manipulating the quantum states. Here, it is reported that monolayer (ML) dialkali-metal monoxides, in the well-known 2H-MoS type lattice, host multiple symmetry-protected topological phases with emergent fermions, which can be effectively tuned by strain engineering. Based on first-principles calculations, it is found that in the equilibrium state, ML NaO is a 2D double Weyl semimetal, while ML KO is a 2D pseudospin-1 metal. These exotic topological states exhibit a range of fascinating effects, including universal optical absorbance, super Klein tunneling, and super collimation effect. By introducing biaxial or uniaxial strain, a series of quantum phase transitions between 2D double Weyl semimetal, 2D Dirac semimetal, 2D pseudospin-1 metal, and semiconductor phases can be realized. The results suggest monolayer dialkali-metal monoxides as a promising platform to explore fascinating physical phenomena associated with novel 2D emergent fermions.
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http://dx.doi.org/10.1002/advs.201901939DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7029633PMC
February 2020

Magnetic and transport properties of low-carrier-density Kondo semimetal CeSbTe.

J Phys Condens Matter 2019 Sep 24;31(35):355601. Epub 2019 May 24.

Zhejiang Province Key Laboratory of Quantum Technology and Device, Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

Single crystals of CeSbTe with a ZrSiS-type structure were synthesized using vapor transport method. The stoichiometry is deviated from the nominal composition, which may cause some disorder in this compound. The physical properties were characterized by measuring the magnetic susceptibility, electrical resistivity, Hall resistivity and specific heat. One antiferromagnetic (AFM) transition related to Ce ions was found at [Formula: see text] K, and a field-induced metamagnetic transition was observed below [Formula: see text]. The moderately enhanced Sommerfeld coefficient [Formula: see text] mJ mol · K and the estimated Kondo temperature [Formula: see text] K, indicate that CeSbTe is a moderately correlated AFM Kondo lattice compound with crystalline electric field effect. The carrier concentration of CeSbTe derived from the Hall coefficient is in the order of 10 cm, lower than most Kondo metals, which indicates that CeSbTe is a low-carrier-density Kondo semimetal.
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http://dx.doi.org/10.1088/1361-648X/ab2498DOI Listing
September 2019

Dialkali-Metal Monochalcogenide Semiconductors with High Mobility and Tunable Magnetism.

J Phys Chem Lett 2018 Dec 12;9(23):6695-6701. Epub 2018 Nov 12.

Zhejiang Province Key Laboratory of Quantum Technology and Device, Department of Physics , Zhejiang University , Hangzhou 310027 , P. R. China.

The discovery of archetypal two-dimensional (2D) materials provides enormous opportunities in both fundamental breakthroughs and device applications, as evident by the research booming in graphene, transition-metal chalcogenides, and black phosphorus. Here, we report a new, large family of semiconducting dialkali-metal monochalcogenides (DMMCs) with an inherent AX monolayer (ML) structure, in which two alkali sub-MLs form hexagonal close packing and sandwich the triangular chalcogen atomic plane. Such a unique lattice leads to extraordinary physical properties, such as good dynamical and thermal stability, visible to near-infrared energy gap, and high electron mobility. Most strikingly, DMMC MLs host extended van Hove singularities near the valence band (VB) edge, readily accessible by moderate hole doping within 1.0 × 10 cm. Upon critical doping, DMMC MLs undergo spontaneous ferromagnetic transition when the top VBs become fully spin-polarized by strong exchange interactions. Such 2D gate tunable magnetism are promising for exploring novel device concepts in spintronics, electronics and optoelectronics.
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http://dx.doi.org/10.1021/acs.jpclett.8b02859DOI Listing
December 2018

Superconductivity in a misfit layered compound (SnSe)(NbSe).

J Phys Condens Matter 2018 Sep 24;30(35):355701. Epub 2018 Jul 24.

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

The large size single crystals of (SnSe)(NbSe) misfit layered compound were grown and superconductivity with T of 3.4 K was first discovered in this system. Powder x-ray diffraction and high resolution transmission electron microscopy clearly display the misfit feature between SnSe and NbSe subsystems. The Sommerfeld coefficient γ inferred from specific-heat measurements is 16.73 mJ mol K, slightly larger than the usual misfit compounds. The normalized specific heat jump [Formula: see text] is about 0.98, and the electron-phonon coupling constant [Formula: see text] is estimated to be 0.80. The estimated value of the in-plane upper critical magnetic field, [Formula: see text](0), is about 7.82 T, exceeding the Pauli paramagnetic limit slightly. Both the specific-heat and H data suggest that (SnSe)(NbSe) is a multi-band superconductor.
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http://dx.doi.org/10.1088/1361-648X/aad575DOI Listing
September 2018

Pressure induced superconductivity bordering a charge-density-wave state in NbTe with strong spin-orbit coupling.

Sci Rep 2018 Apr 19;8(1):6298. Epub 2018 Apr 19.

State Key Laboratory of Silicon Materials and Department of Physics, Zhejiang University, Hangzhou, 310027, China.

Transition-metal chalcogenides host various phases of matter, such as charge-density wave (CDW), superconductors, and topological insulators or semimetals. Superconductivity and its competition with CDW in low-dimensional compounds have attracted much interest and stimulated considerable research. Here we report pressure induced superconductivity in a strong spin-orbit (SO) coupled quasi-one-dimensional (1D) transition-metal chalcogenide NbTe, which is a CDW material under ambient pressure. With increasing pressure, the CDW transition temperature is gradually suppressed, and superconducting transition, which is fingerprinted by a steep resistivity drop, emerges at pressures above 12.4 GPa. Under pressure p = 69 GPa, zero resistance is detected with a transition temperature T  = 2.2 K and an upper critical field μH = 2 T. We also find large magnetoresistance (MR) up to 102% at low temperatures, which is a distinct feature differentiating NbTe from other conventional CDW materials.
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http://dx.doi.org/10.1038/s41598-018-24572-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5908920PMC
April 2018

Unique [MnBi] Nanowires in KMnBi: A Quasi-One-Dimensional Antiferromagnetic Metal.

J Am Chem Soc 2018 03 19;140(12):4391-4400. Epub 2018 Mar 19.

Materials Science Division , Argonne National Laboratory , Argonne , Illinois 60439 , United States.

We report a new quasi-one-dimensional compound KMnBi composed of parallel nanowires crystallizing in a monoclinic space group C2/ m with a = 22.994(2) Å, b = 4.6128(3) Å, c = 13.3830(13) Å and β = 124.578(6)°. The nanowires are infinite [MnBi] columns each of which is composed of a nanotube of Bi atoms acting as the cladding with a nanorod of Mn atoms located in the central axis of the nanotubes. The nanorods of Mn atoms inside the Bi cladding are stabilized by Mn-Mn bonding and are defined by distorted Mn-centered cluster icosahedra of Mn sharing their vertices along the b axis. The [MnBi] nanowires are linked with weak internanowire Bi-Bi bonds and charge balanced with K ions. The [MnBi] nanowires were directly imaged by high-resolution transmission electron microscopy and scanning transmission electron microscopy. Magnetic susceptibility studies show one-dimensional characteristics with an antiferromagnetic transition at ∼75 K and a small average effective magnetic moment (1.56 μ/Mn for H ∥ b and 1.37 μ/Mn for H ⊥ b) of Mn from Curie-Weiss fits above 150 K. Specific heat measurements reveal an electronic specific heat coefficient γ of 6.5(2) mJ K(mol-Mn) and a small magnetic entropy change Δ S ≈ 1.6 J K (mol-Mn) across the antiferromagnetic transition. In contrast to a metallic resistivity along the column, the resistivity perpendicular to the column shows a change from a semiconducting behavior at high temperatures to a metallic one at low temperatures, indicating an incoherent-to-coherent crossover of the intercolumn tunneling of electrons.
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http://dx.doi.org/10.1021/jacs.8b00465DOI Listing
March 2018

Superconductivity in tantalum self-intercalated 4Ha-TaSe.

J Phys Condens Matter 2018 03 7;30(9):095703. Epub 2018 Mar 7.

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

TaSe has several different polytypes and abundant physical properties, such as superconductivity and charge density waves (CDW), which have been investigated in the past few decades. However, there has been no report on the physical properties of the 4Ha polytype up to now. Here we report the crystal growth and discovery of superconductivity in the tantalum self-intercalated 4Ha-TaSe single crystal with a superconducting transition onset temperature of [Formula: see text] K, which is the first observation of superconductivity in the 4Ha polytype of TaSe. A slightly suppressed CDW transition is found around 106 K. A large [Formula: see text] value of about 4.48 is found when a magnetic field is applied in the ab-plane, which probably results from the enhanced spin-orbit coupling. 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 remains the same, indicating that the stack mode of the 4Ha polytype may be beneficial to superconductivity in TaSe.
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http://dx.doi.org/10.1088/1361-648X/aaaa98DOI Listing
March 2018

Defects controlled hole doping and multivalley transport in SnSe single crystals.

Nat Commun 2018 01 3;9(1):47. Epub 2018 Jan 3.

Department of Physics, Zhejiang University, Hangzhou, 310027, China.

SnSe is a promising thermoelectric material with record-breaking figure of merit. However, to date a comprehensive understanding of the electronic structure and most critically, the self-hole-doping mechanism in SnSe is still absent. Here we report the highly anisotropic electronic structure of SnSe investigated by angle-resolved photoemission spectroscopy, in which a unique pudding-mould-shaped valence band with quasi-linear energy dispersion is revealed. We prove that p-type doping in SnSe is extrinsically controlled by local phase segregation of SnSe microdomains via interfacial charge transferring. The multivalley nature of the pudding-mould band is manifested in quantum transport by crystallographic axis-dependent weak localisation and exotic non-saturating negative magnetoresistance. Strikingly, quantum oscillations also reveal 3D Fermi surface with unusual interlayer coupling strength in p-SnSe, in which individual monolayers are interwoven by peculiar point dislocation defects. Our results suggest that defect engineering may provide versatile routes in improving the thermoelectric performance of the SnSe family.
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http://dx.doi.org/10.1038/s41467-017-02566-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5752673PMC
January 2018

Coexistence of Topological Edge State and Superconductivity in Bismuth Ultrathin Film.

Nano Lett 2017 05 20;17(5):3035-3039. Epub 2017 Apr 20.

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University , Shanghai 100140, China.

Ultrathin freestanding bismuth film is theoretically predicted to be one kind of two-dimensional topological insulators. Experimentally, the topological nature of bismuth strongly depends on the situations of the Bi films. Film thickness and interaction with the substrate often change the topological properties of Bi films. Using angle-resolved photoemission spectroscopy, scanning tunneling microscopy or spectroscopy and first-principle calculation, the properties of Bi(111) ultrathin film grown on the NbSe superconducting substrate have been studied. We find the band structures of the ultrathin film is quasi-freestanding, and one-dimensional edge state exists on Bi(111) film as thin as three bilayers. Superconductivity is also detected on different layers of the film and the pairing potential exhibits an exponential decay with the layer thicknesses. Thus, the topological edge state can coexist with superconductivity, which makes the system a promising platform for exploring Majorana Fermions.
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http://dx.doi.org/10.1021/acs.nanolett.7b00365DOI Listing
May 2017

Superconductivity at 35 K by self doping in RbGdFeAsO.

J Phys Condens Matter 2017 Mar 7;29(11):11LT01. Epub 2017 Feb 7.

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

We report synthesis, crystal structure and physical properties of a novel quinary compound RbGdFeAsO. The new iron oxyarsenide is isostructural to the fluo-arsenide KCaFeAsF, both of which contain separate double FeAs layers that are self hole-doped in the stoichiometric composition. Bulk superconductivity at [Formula: see text] K is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility and heat capacity. An exceptionally high value of the initial slope of the upper critical field ([Formula: see text]d[Formula: see text]/d[Formula: see text] [Formula: see text] T K) is measured for the polycrystalline sample.
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http://dx.doi.org/10.1088/1361-648X/aa58d2DOI Listing
March 2017

Majorana Zero Mode Detected with Spin Selective Andreev Reflection in the Vortex of a Topological Superconductor.

Phys Rev Lett 2016 Jun 21;116(25):257003. Epub 2016 Jun 21.

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.

Recently, theory has predicted a Majorana zero mode (MZM) to induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZM. Here, spin-polarized scanning tunneling microscopy or spectroscopy has been applied to probe SSAR of MZMs in a topological superconductor of the Bi_{2}Te_{3}/NbSe_{2} heterostructure. The zero-bias peak of the tunneling differential conductance at the vortex center is observed substantially higher when the tip polarization and the external magnetic field are parallel rather than antiparallel to each other. This spin dependent tunneling effect provides direct evidence of MZM and reveals its magnetic property in addition to the zero energy modes. Our work will stimulate MZM research on these novel physical properties and, hence, is a step towards experimental study of their statistics and application in quantum computing.
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http://dx.doi.org/10.1103/PhysRevLett.116.257003DOI Listing
June 2016

Superconductivity in Ta3Pd3Te14 with quasi-one-dimensional PdTe2 chains.

Sci Rep 2016 Feb 15;6:21628. Epub 2016 Feb 15.

Department of Physics, Zhejiang University, Hangzhou 310027, China.

We report bulk superconductivity at 1.0 K in a low-dimensional ternary telluride Ta3Pd3Te14 containing edge-sharing PdTe2 chains along crystallographic b axis, similar to the recently discovered superconductor Ta4Pd3Te16. The electronic heat capacity data show an obvious anomaly at the transition temperature, which indicates bulk superconductivity. The specific-heat jump is ΔC/(γ(n)T(c)) ≈ 1.35, suggesting a weak coupling scenario. By measuring the low-temperature thermal conductivity, we conclude that Ta3Pd3Te14 is very likely a dirty s-wave superconductor. The emergence of superconductivity in Ta3Pd3Te14 with a lower T(c), compared to that of Ta4Pd3Te16, may be attributed to the lower density of states.
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http://dx.doi.org/10.1038/srep21628DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4753496PMC
February 2016

A New ZrCuSiAs-Type Superconductor: ThFeAsN.

J Am Chem Soc 2016 Feb 12;138(7):2170-3. Epub 2016 Feb 12.

Department of Physics and State Key Lab of Silicon Materials, Zhejiang University , Hangzhou 310027, China.

We report the first nitrogen-containing iron-pnictide superconductor ThFeAsN, which is synthesized by a solid-state reaction in an evacuated container. The compound crystallizes in a ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters a = 4.0367(1) Å and c = 8.5262(2) Å at 300 K. The electrical resistivity and dc magnetic susceptibility measurements indicate superconductivity at 30 K for the nominally undoped ThFeAsN.
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http://dx.doi.org/10.1021/jacs.6b00236DOI Listing
February 2016

Coexistence of superconductivity and complex 4 f magnetism in Eu0.5Ce0.5BiS2F.

J Phys Condens Matter 2015 Sep 8;27(38):385701. Epub 2015 Sep 8.

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

EuBiS2F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4 f moment, superconductivity at 2.2 K in the BiS2 layers and possibly antiferromagnetic ordering at 2.1 K for the Eu-4 f spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6p orbitals that are responsible for the superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4 f spins is most likely due to a magnetic dipole-dipole interaction.
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http://dx.doi.org/10.1088/0953-8984/27/38/385701DOI Listing
September 2015

Multiband superconductivity in Ta4Pd3Te16 with anisotropic gap structure.

J Phys Condens Matter 2015 Aug 27;27(32):325701. Epub 2015 Jul 27.

Department of Physics, Zhejiang University of Science and Technology, Hangzhou 310023, People's Republic of China.

We carried out measurements of the magnetoresistance, magnetic susceptibility and specific heat on crystals of the low-dimensional transition metal telluride Ta4Pd3Te16. Our results indicate that Ta4Pd3Te16 is an anisotropic type-II superconductor in the clean limit with the extracted Ginzburg-Landau parameter KGL = 84. The upper critical field Hc2(T) shows an anomalous temperature dependence at low temperatures and the anisotropy of Hc2(T) is strongly T-dependent, both of which indicate a multiband scenario. The electronic specific heat Cel(T) can be consistently described by a two-gap (s   +   d waves) model from the base temperature T/Tc ~ 0.12 up to Tc. Our results suggest nodal and multiband superconductivity in Ta4Pd3Te16.
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http://dx.doi.org/10.1088/0953-8984/27/32/325701DOI Listing
August 2015

Experimental detection of a Majorana mode in the core of a magnetic vortex inside a topological insulator-superconductor Bi(2)Te(3)/NbSe(2) heterostructure.

Phys Rev Lett 2015 Jan 7;114(1):017001. Epub 2015 Jan 7.

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.

Majorana fermions have been intensively studied in recent years for their importance to both fundamental science and potential applications in topological quantum computing. They are predicted to exist in a vortex core of superconducting topological insulators. However, it is extremely difficult to distinguish them experimentally from other quasiparticle states for the tiny energy difference between Majorana fermions and these states, which is beyond the energy resolution of most available techniques. Here, we circumvent the problem by systematically investigating the spatial profile of the Majorana mode and the bound quasiparticle states within a vortex in Bi(2)Te(3) films grown on a superconductor NbSe(2). While the zero bias peak in local conductance splits right off the vortex center in conventional superconductors, it splits off at a finite distance ∼20  nm away from the vortex center in Bi(2)Te(3). This unusual splitting behavior has never been observed before and could be possibly due to the Majorana fermion zero mode. While the Majorana mode is destroyed by the interaction between vortices, the zero bias peak splits as a conventional superconductor again. This work provides self-consistent evidences of Majorana fermions and also suggests a possible route to manipulating them.
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http://dx.doi.org/10.1103/PhysRevLett.114.017001DOI Listing
January 2015

Anomalous Eu valence state and superconductivity in undoped Eu3Bi2S4F4.

J Am Chem Soc 2014 Oct 14;136(43):15386-93. Epub 2014 Oct 14.

Department of Physics, ¶Center of Electron Microscope, and ∥State Key Lab of Silicon Materials, Zhejiang University , Hangzhou 310027, China.

We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) Å, c = 32.4330(6) Å, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond valence sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, while Eu(2) has an average valence of ∼ +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mössbauer and magnetization measurements. The Eu(3+) components donate electrons into the conduction bands that are mainly composed of Bi 6px and 6py states. Consequently, the material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping.
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http://dx.doi.org/10.1021/ja508564sDOI Listing
October 2014

Possible spin frustration in Nd2Ti2O7 probed by muon spin relaxation.

J Phys Condens Matter 2014 Oct 9;26(43):436002. Epub 2014 Oct 9.

Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China.

Muon spin relaxation on Nd2Ti2O7 (NTO) and NdLaTi2O7 (NLTO) compounds are presented. The time spectra for both compounds are as expected for the paramagnetic state at high temperatures, but deviate from the exponential function below around 100 K. Firstly, the muon spin relaxation rate increases with decreasing temperature and then levels off below around 10 K, which is reminiscent of the frustrated systems. An enhancement of the relaxation rate by a longitudinal field in the paramagnetic state is observed for NTO and eliminated by a magnetic dilution for the NLTO sample. This suggests that the spectral density is modified by a magnetic dilution and thus indicates that the spins behave cooperatively rather than individually. The zero-field measurement at 0.3 K indicates that the magnetic ground state for NTO is ferromagnetic.
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http://dx.doi.org/10.1088/0953-8984/26/43/436002DOI Listing
October 2014

Kramers non-magnetic superconductivity in LnNiAsO superconductors.

J Phys Condens Matter 2014 Oct 24;26(42):425701. Epub 2014 Sep 24.

Department of Physics and Hangzhou Key Laboratory of Quantum Matters, Hangzhou Normal University, Hangzhou 310036, People's Republic of China.

We investigated a series of nickel-based oxyarsenides LnNiAsO (Ln=La, Ce, Pr, Nd, Sm) compounds. CeNiAsO undergoes two successive anti-ferromagnetic transitions at TN1=9.3 K and TN2=7.3 K; SmNiAsO becomes an anti-ferromagnet below TN≃3.5 K; NdNiAsO keeps paramagnetic down to 2 K but orders anti-ferromagnetically below TN≃1.3 K. Superconductivity was observed only in Kramers non-magnetic LaNiAsO and PrNiAsO with Tc=2.7 K and 0.93 K, respectively. The superconductivity of PrNiAsO is further studied by upper critical field and specific heat measurements, which reveal that PrNiAsO is a weakly coupled Kramers non-magnetic superconductor. Our work confirms that the nickel-based oxyarsenide superconductors are substantially different in mechanism to iron-based ones, and are likely to be described by the conventional superconductivity theory.
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http://dx.doi.org/10.1088/0953-8984/26/42/425701DOI Listing
October 2014

A Cu(2+) ion-F center complex view on the photoluminescence quenching and correlating ferrimagnetism in (Cu2(+)/Cu1(2+))(0.044)Zn(0.956)O electrospun nanobelts.

ACS Appl Mater Interfaces 2014 Mar 17;6(6):4490-7. Epub 2014 Mar 17.

Department of Physics and ‡State Key Laboratory of Silicon Materials, Zhejiang University , Hangzhou 310027, People's Republic of China.

Unlike to the most previous reports, mixed-cation Cu(+)/Cu(2+) doping-induced novel nanoscale phenomena, including photoluminescence quenching and a correlating ferrimagnetism with Néel temperature ≈ 14 K, were found in the as-calcined (Cu2(+)/Cu1(2+))0.044Zn0.956O electrospun nanobelts (NBs). There is also high strain (up to 1.98%) and shrunk lattice distortion (ΔV/V0 ∼ 0.127%) in the (Cu2(+)/Cu1(2+))0.044Zn0.956O NBs, leading to broken lattice symmetry in conjunction with nonstoichiometry (i.e., oxygen vacancies or accurate F centers), which could be possible origins of ferrimagnetism in the Cu-doped ZnO NBs. Electron paramagnetic resonance spectra reveal that there are giant and anisotropic g factors, suggesting that there is strong anisotropic spin-orbit interaction between the Cu(2+) ion and F center (i.e., forming Cu(2+)-F(+) complexes) in the (Cu2(+)/Cu1(2+))0.044Zn0.956O NBs. The above correlation enables the potential application of tuning of the optical and ferrimagnetic properties through strain and F-center engineering.
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http://dx.doi.org/10.1021/am5001135DOI Listing
March 2014

Superconductivity in a layered Ta4Pd3Te16 with PdTe2 chains.

J Am Chem Soc 2014 Jan 15;136(4):1284-7. Epub 2014 Jan 15.

Department of Physics, ‡State Key Lab of Silicon Materials and §Center of Electron Microscope, Zhejiang University , Hangzhou 310027, China.

Superconductivity in low-dimensional compounds has long attracted much interest. Here we report superconductivity in a low-dimensional ternary telluride Ta4Pd3Te16 in which the repeating layers contain edge-sharing octahedrally coordinated PdTe2 chains along the crystallographic b axis. Measurements of electrical resistivity, magnetic susceptibility and specific heat on the Ta4Pd3Te16 crystals, grown via a self-flux method, consistently demonstrate bulk superconductivity at 4.6 K. Further analyses of the data indicate significant electron-electron interaction, which allows electronic Cooper pairing in the present system.
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http://dx.doi.org/10.1021/ja412094nDOI Listing
January 2014

Variable range hopping conductivity and spin glass behavior in spin-ladder Ba0.6K0.4Fe2Se3 single crystals.

J Phys Condens Matter 2014 Jan 9;26(2):026002. Epub 2013 Dec 9.

Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

Ba0.6K0.4Fe2Se3 (BKFS) single crystals were investigated by means of measurements of powder x-ray diffraction, temperature-dependent resistivity, anisotropic dc magnetization, ac magnetic susceptibility and specific heat. The powder x-ray diffraction indicates staggered iron displacements along the ladders with short and long Fe-Fe bond lengths (2.64(2) and 2.91(2) Å) variation. The resistivity of BKFS exhibits variable range hopping behavior with ln(ρ) ~ T(-1/2) at low temperature. The magnetic susceptibility χ(T) exhibits a sharp cusp at around 20 K in a zero-field-cooled process. The frequency-dependent ac magnetic susceptibility reveals that the cusp feature is attributable to spin glass behavior. The anisotropic ac magnetic susceptibility indicates that BKFS is probably an anisotropic Heisenberg-like spin glass with its easy magnetization plane perpendicular to the chain direction. The specific heat also supports an insulating and spin glass ground state. Extended Curie-Weiss behavior above 40 K was observed with a reduced effective moment (μ(eff) = 1.66 μ(B)/Fe for H is perpendicular to b and μ(eff) = 1.82 μB/Fe for H is parallel to b) in BKFS, which is close to the spin-only magnetism with S=1/2.
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http://dx.doi.org/10.1088/0953-8984/26/2/026002DOI Listing
January 2014

On the origin of the two thermally driven relaxations in diluted spin ice Dy(1.6)Y(0.4)Ti2O7.

J Phys Condens Matter 2013 Jan 20;25(4):046005. Epub 2012 Dec 20.

Department of Physics, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA.

We report quantitative analysis of the ac susceptibility of the diluted spin ice compound Dy(1.6)Y(0.4)Ti(2)O(7), by fitting the frequency spectra of the two observed relaxation modes with a double Cole-Cole formula. Our results suggest that both modes are thermally activated, with the same intrinsic frequency, but different activation barriers. We propose that these thermally activated relaxation modes can be attributed to correlated spin clusters.
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http://dx.doi.org/10.1088/0953-8984/25/4/046005DOI Listing
January 2013

Ba2Ti2Fe2As4O: A new superconductor containing Fe2As2 layers and Ti2O sheets.

J Am Chem Soc 2012 Aug 26;134(31):12893-6. Epub 2012 Jul 26.

Department of Physics, ‡State Key Lab of Silicon Materials, and §Center of Electron Microscope, Zhejiang University , Hangzhou 310027, China.

We have synthesized a new oxypnictide, Ba2Ti2Fe2As4O, via a solid-state reaction under a vacuum. The compound crystallizes in a body-centered tetragonal lattice, which can be viewed as an intergrowth of BaFe2As2 and BaTi2As2O, thus containing Fe2As2 layers and Ti2O sheets. Bulk superconductivity at 21 K is observed after annealing the as-prepared sample at 773 K for 40 h. In addition, an anomaly in resistivity and magnetic susceptibility around 125 K is revealed, suggesting a charge- or spin-density wave transition in the Ti sublattice.
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http://dx.doi.org/10.1021/ja304315eDOI Listing
August 2012

The coexistence of superconductivity and topological order in the Bi₂Se₃ thin films.

Science 2012 Apr 15;336(6077):52-5. Epub 2012 Mar 15.

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China.

Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi(2)Se(3)) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi(2)Se(3) surface in the regime of Bi(2)Se(3) film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.
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http://dx.doi.org/10.1126/science.1216466DOI Listing
April 2012

Effect of Zn doping in hole-type 1111 phase (Pr, Sr)FeAsO.

J Phys Condens Matter 2011 Nov;23(46):464203

State Key Laboratory of Silicon Materials and Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.

There is an anomalous broad hump in the normal state resistivity in hole-type 1111 phase FeAs-based superconductors and its origin is an open issue. We study the effect of Zn doping on this anomaly in order to determine whether it is associated with the residual structural/antiferromagnetic (AFM) phase transition as in the parent compounds. A series of Zn doped Pr₀.₈Sr₀.₂FeAsO samples are prepared and their resistivity, magnetoresistance, Hall effect and specific heat are measured. Zn doping should not introduce extra charge carriers, and instead it can suppress the structural/AFM transition efficiently in the parent LaFeAsO system. The hump in resistivity remains unchanged with 6% Zn doping in Pr₀.₈Sr₀.₂FeAsO. The measurements of magnetoresistance reveal that the magnetoresistance is negligible in the Zn doped Pr₀.₈Sr₀.₂FeAsO samples, in contrast to the large positive magnetoresistance below the temperature of structure/AFM phase transition in the parent compound PrFeAsO. The results indicate that the anomalous broad hump in resistivity does not originate from the structural/AFM transition. The Hall effect and specific heat data are also consistent with this conclusion.
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November 2011
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