Coexistence of bulk superconductivity and charge density wave in CuxZrTe3

Superconductivity in single-crystalline ZrTe3-x (x ≤ 0.5) nanoplates

Superconductivity with an unusual filamented character below 2 K has been reported in bulk ZrTe₃ crystals, a well-known charge density wave (CDW) material, but still lacks in its nanostructures. Here, we systemically investigated the transport properties of controllable chemical vapor transport synthesized ZrTe_3-x nanoplates. Intriguingly, superconducting behavior is found at T _c = 3.4 K and can be understood by the suppression of CDW due to the atomic disorder formed by Te vacancies. Magnetic field and angle dependent upper critical field revealed that the superconductivity in the nanoplates exhibits a large anisotropy and two-dimensional character. This two-dimensional nature of superconductivity was further satisfactorily described using the Berezinsky-Kosterlitz-Thouless transition. Our results not only demonstrate the critical role of Te vacancies for superconductivity in ZrTe_3-x nanoplates, but also provide a promising platform to explore the exotic physics in the nanostructure devices.

Possible Charge Density Wave and Enhancement of Thermoelectric Properties at Mild-Temperature Range in n-Type CuI-Doped Bi2Te2.1Se0.9 Compounds

Bi₂Te₃-based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi₂Te_2.1Se_0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI)_xBi₂Te_2.1Se_0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi₂Te₃-based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI)_0.003Bi₂Te_2.1Se_0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZT_avg (1.0) value than those of previously reported ones.

X-ray Assisted Scanning Tunneling Microscopy and Its Applications for Materials Science: The First Results on Cu Doped ZrTe3

Synchrotron X-ray Scanning Tunneling Microscopy (SX-STM) is a novel imaging technique capable of providing real space chemically specific mapping with a potential of reaching atomic resolution. Determination of chemical composition along with ultra-high resolution imaging by SX-STM can be realized through excitation of core electrons by incident X-rays when their energy is tuned to an absorption edge of a particular atom during raster scanning, as is done in the conventional STM experiments. In this work, we provide a brief summary and the current status of SX-STM and discuss its applications for material science. In particular, we discuss instrumentation challenges associated with the SX-STM technique and present early experiments on Cu doped ZrTe3 single crystals.

Pressure-induced enhancement in the superconductivity of ZrTe3

We report the superconductivity enhancement of ZrTe₃ on compression up to 33 GPa. The superconducting transition occurs above 4.1 GPa and the superconducting temperature (T _C) increases with pressure in further compression, reaching a maximum of 7.1 K at ~28 GPa. An anomalous change of superconducting temperature is seen in the compression above 21 GPa. No structural phase transition is observed in the whole compression up to 36 GPa, but a subtle change in structural parameter is seen between 17-19 GPa, which seems relevant to the anomalous increase in the superconducting temperature. First-principle calculations reveal that the density of states at the Fermi level increases with pressure, which explains the enhancement of T _C in ZrTe₃ under compression.

Critical properties of the quasi-two-dimensional metallic ferromagnet Fe2.85GeTe2

We have investigated the critical behavior of Fe_2.85GeTe₂ single crystals near the paramagnetic-ferromagnetic transition by bulk dc magnetization measurements. The critical exponents β, γ and δ, obtained from modified Arrott plot, Kouvel-Fisher method, and critical isothermal magnetization analysis, could fulfill the Widom scaling law. The self-consistency and reliability of these exponents are further verified by the magnetic state equations below and above the Curie temperature at high magnetic field. In addition, the exchange distance deduced from the susceptibility exponent is shown to decay as [Formula: see text]. Based on the observations, we suggest that the competition between direct magnetic exchange and Coulombic and/or RKKY interactions should be responsible for the intrinsic magnetism in this system.

Coexistence of superconductivity and charge-density wave in the quasi-one-dimensional material HfTe3

We present the first experimental evidence for metallicity, superconductivity (SC) and the co-existence of charge density waves (CDW) in the quasi-one-dimensional material HfTe₃. The existence of such phenomena is a typical characteristic of the transition metal chalcogenides however, without the application of hydrostatic pressure/chemical doping, it is rare for a material to exhibit the co-existence of both states. Materials such as HfTe₃ can therefore provide us with a unique insight into the relationship between these multiple ordered states. By improving on the original synthesis conditions, we have successfully synthesised single phase HfTe₃ and confirmed the resultant structure by performing Rietveld refinement. Using low temperature resistivity measurements, we provide the first experimental evidence of SC at ~1.4 K as well as a resistive anomaly indicative of a CDW formation at ~82 K. By the application of hydrostatic-pressure, the resistivity anomaly shifts to higher temperature. The results show that HfTe₃ is a promising new material to help study the relationship between SC and CDW.

Superconductivity and Charge Density Wave in ZrTe3-xSex

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 ZrTe₃ when the long range CDW order is gradually suppressed. Superconducting critical temperature T_c(x) in ZrTe_3−xSe_x (0 ≤ x ≤ 0.1) increases up to 4 K plateau for 0.04 ≤ x ≤ 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 ZrTe₃ CDW order in resistivity vanishes. The electronic-scattering for high T_c crystals is dominated by local CDW fluctuations at high temperatures, the resistivity is linear up to highest measured T = 300 K and contributes to substantial in-plane anisotropy.

The mechanism of charge density wave in Pt-based layered superconductors: SrPt2As2 and LaPt2Si2

The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt2As2 and LaPt2Si2 has been investigated based on the ab initio density functional theory band structure and phonon calculations. We have found that the CDW instabilities for both cases arise from the q-dependent electron-phonon coupling with quasi-nesting feature of the Fermi surface. The band structure obtained by the band-unfolding technique reveals the sizable q-dependent electron-phonon coupling responsible for the CDW instability. The local split distortions of Pt atoms in the [As-Pt-As] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 occur without split distortions of Pt atoms. The phonon calculations suggest that the CDW and the superconductivity coexist in [X-Pt-X] layers (X = As or Si) for both cases.

Superconductivity at 5.2 K in ZrTe3 polycrystals and the effect of Cu and Ag intercalation

We report the occurrence of superconductivity in polycrystalline samples of ZrTe(3) at temperature 5.2 K at ambient pressure. The superconducting state coexists with the charge density wave (CDW) phase, which sets in at 63 K. The intercalation of Cu or Ag does not have any bearing on the superconducting transition temperature but suppresses the CDW state. The feature of a CDW anomaly in these compounds is clearly seen in the DC magnetization data. Resistivity data are analyzed in order to estimate the relative loss of carriers and reduction in the nested Fermi surface area upon CDW formation in ZrTe(3) and the intercalated compounds.