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We report on the discovery, structural analysis, and the physical properties of Nb$_4$SiSb$_2$ -- a hitherto unknown compound crystallizing in the V$_4$SiSb$_2$-type structure with the tetragonal space group $I4/mcm$ and unit cell parameters $a$ = 10.3638(2) $\mathring{\mathrm{A}}$ and $c$ = 4.9151(2) $\mathring{\mathrm{A}}$. We find Nb$_4$SiSb$_2$ to be a metal undergoing a transition to a superconducting state at a critical temperature of $T_{\rm c} \approx$ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of $\Delta C(T_{\rm c})/\gamma T_{\rm c} = 1.33\, {\rm mJ}\, {\rm mol}^{-1}\, {\rm K}^{-2}$. We find that for Nb$_4$SiSb$_2$, the unoccupied sites on the $4b$ Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at $T_{\rm c} \approx\, 1.2\, {\rm K}$ for Nb$_4$Cu$_{0.2}$SiSb$_2$, and $T_{\rm c} \approx\, 0.8\, {\rm K}$ for Nb$_4$Pd$_{0.2}$SiSb$_2$ as well as for Nb$_4$Pt$_{0.14}$SiSb$_2$. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.

The superior electronic, optical and magnetic properties of the one-dimensional carbyne are of high technological relevance for the development of future optoelectronic and magnetoelectronic applications. However, the production of a monoatomic chain with more than 6000 carbon atoms is an enormous technological challenge. To predict the exact growth conditions and in particular the optimal chain length of a carbyne fibre in different molecular environments, we have developed a Monte Carlo model in which a finite-length carbyne with a size of 4000-15000 atoms is encapsulated by a carbon nanotube at finite temperature. Our simulation shows that the stability of the carbyne-nanotube is strongly influenced by the nature and porosity of the carbon nanotube, the external pressure, the temperature and the chain length. When the exact geometric structure of the carbon nanotube and the environmental parameters are provided as input, our simulation can predict the optimal length of the encapsulated carbyne. We have found four mechanisms of triggering chain breakage and have predicted the probable size of the carbyne fragments in excited states. Our work provides much needed input for optimising the carbyne length to produce carbon chains much longer than 6000 atoms at room temperature.

We present measurements of the magnetic torque, specific heat and thermal expansion of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields, with its layer structure aligned strictly parallel to the field using a piezo rotary positioner. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust to the Pauli paramagnetic effect and can therefore exist beyond the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2 for a precisely parallel field alignment. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit. © 2021, The Author(s). We thank U. Lampe for technical support. This work was supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (GRF-16302018, GRF-16303820, C6025-19G-A, SBI17SC14). M.A.-H. acknowledges the financial support from the Swedish Research Council (VR) under the project no. 2018-05393 and by the Arab-German Young Academy of Sciences and Humanities (AGYA). M.A.-H and D.C. acknowledge the financial support by the Megagrant 2020-220-08-6358 of the Government Council on Grants of the Russian Federation.

High-entropy alloys (HEA) form through the random arrangement of five or more chemical elements on a crystalline lattice. Despite the significant amount of resulting compositional disorder, a subset of HEAs enters a superconducting state below critical temperatures, $$T_{\text{c}} T c < 10 K. The superconducting properties of the known HEAs seem to suffice a Bardeen–Cooper–Schrieffer (BCS) description, but little is known about their superconducting order parameter and the microscopic role of disorder. We report on magnetic susceptibility measurements on films of the superconducting HEA (TaNb)$$_{1-x}$$ 1 - x (ZrHfTi)$$_{x}$$ x for characterizing the lower and upper critical fields $$H_{\text{c,1}}(T)$$ H c,1 ( T ) and $$H_{\text{c,2}}(T)$$ H c,2 ( T ) , respectively as a function of temperature T. Our resulting analysis of the Ginzburg–Landau coherence length and penetration depth demonstrates that HEAs of this type are single-band isotropic s-wave superconductors in the dirty limit. Despite a significant difference in the elemental composition between the $$x=0.35$$ x = 0.35 and $$x=0.71$$ x = 0.71 films, we find that the observed $$T_{\text{c}}$$ T c variations cannot be explained by disorder effects.

Hydrodynamic fluidity in condensed matter physics has been experimentally demonstrated only in a limited number of compounds because of the stringent conditions that must be satisfied. Herein, we demonstrate the existence of hydrodynamic-like properties driven by the collective excitation of the Dirac fluid in the three-dimensional topological semimetal ZrTe5. By measuring the electrical and thermal properties in a wide temperature range, we find a regime satisfying phononic hydrodynamic-like characteristics with two representative experimental evidences: a faster evolution of the thermal conductivity than in the ballistic regime and the existence of a local maximum of the effective mean free path. In contrast to phononic hydrodynamics, the Wiedemann-Franz law is violated by about a factor of 100. Moreover, phonon-dragged anomalies are observed, which serve as a signature of the Dirac fluidity in this system.

We present magnetic torque, specific heat and thermal expansion measurements combined with a piezo rotary positioner of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields applied strictly parallel to its layer structure. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust against the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit.

We present measurements of the magnetic torque, specific heat and thermal expansion of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields, with its layer structure aligned strictly parallel to the field using a piezo rotary positioner. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust to the Pauli paramagnetic effect and can therefore exist beyond the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2 for a precisely parallel field alignment. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit., Superconductivity is often destroyed under magnetic field larger than a critical value called Pauli limit. Here, the authors report superconductivity beyond the Pauli limit in bulk single crystals of NbS2, suggesting the development of a Fulde-Ferrell-Larkin-Ovchinnikov state.

A state of matter with a multi-component order parameter can give rise to vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. Vestigial states due to primary spin and charge-density-wave order have been discussed in iron-based and cuprate materials. Here we present the observation of a partially melted superconductivity in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z3, i.e., three-state Potts-model symmetry. Thermal expansion, specific heat and magnetization measurements of the doped topological insulators NbxBi2Se3 and CuxBi2Se3 reveal that this symmetry breaking occurs at $${{T}}_{\mathrm{nem}} \simeq 3.8\,K$$ T nem ≃ 3.8 K above $${T}_{{\mathrm{c}}} \simeq 3.25\,K$$ T c ≃ 3.25 K , along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish long-range coherence at Tc, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a crystalline distortion.

The authors thank Walter Ho and Ulf Lampe for their help. This project is supported by the Research Grants Council of Hong Kong grant RGC 16304314 and the collaborative grant KAUST18SC01 and URF/1/3435. R. L. wishes to acknowledge the support of grants SBI17SC14 and IEG16SC03.

We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron-based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We interpret our data in the context of a pressure-induced enhanced coupling of the Fe1+yTe interfacial layer with the Bi2Te3 surface state, which modifies the electronic properties of the interface layer in a way that superconductivity emerges and becomes further enhanced under pressure. This demonstrates the important role of the TI in the interfacial superconducting mechanism.

A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes. The corresponding physical quantity then directly reflects the symmetry of the order parameter. We present a study on the superconducting upper critical field of the Nb-doped topological insulator NbxBi2Se3 for various magnetic field orientations parallel and perpendicular to the basal plane of the Bi2Se3 layers. The data were obtained by two complementary experimental techniques, magnetoresistance and DC magnetization, on three different single crystalline samples of the same batch. Both methods and all samples show with perfect agreement that the in-plane upper critical fields clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. The two-fold symmetry is also found in the absolute value of the magnetization of the initial zero-field-cooled branch of the hysteresis loop and in the value of the thermodynamic contribution above the irreversibility field, but also in the irreversible properties such as the value of the characteristic irreversibility field and in the width of the hysteresis loop. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3. The superconducting phase of Nb-doped Bi2Se3 is nematic, that is, it has a 2-fold symmetry that breaks the 3-fold symmetry of the crystal. Topological insulators, such as Bi2Se3, can become superconductive when doped, which is interesting because topological superconductors host Majorana fermions, which hold potential for quantum computation. However, the nature of the superconductive pairing in these materials is debated. To settle the question for Nb-doped Bi2Se3, a team led by Rolf Lortz at the Hong Kong University of Science and Technology measured the dependence of the superconducting upper critical field on the orientation of an applied magnetic field. These measurements demonstrate that Nb-doped Bi2Se3 has an odd-parity nematic superconducting phase, as further supported by the fact that the data can be fitted with a model for nematic superconductivity.

The dimensional crossover from a 1D fluctuating state at high temperatures to a 3D phase coherent state in the low temperature regime in two coaxial weakly-coupled cylindrical surfaces formed by two-dimensional arrays of parallel nanowires is studied via an 8-state 3D-XY model. This system serves as a model for quasi-one-dimensional superconductors in the form of bundles of weakly-coupled superconducting nanowires. A periodic variation of the dimensional crossover temperature TDC is observed when the inner superconducting cylindrical surface is rotated in the angular plane. TDC reaches a maximum when the relative angle between the cylinders is 2.81°, which corresponds to the maximum separation of nanowires between the two cylindrical surfaces. We demonstrate that the relative strength of phase fluctuations in this system is controllable by the rotational angle between the two surfaces with a strong suppression of the fluctuation strength at 2.81°. The phase fluctuations are suppressed gradually upon cooling, before they abruptly vanish below TDC. Our model thus allows us to study how phase fluctuations can be suppressed in quasi-one-dimensional superconductors in order to achieve a global phase coherent state throughout the nanowire array with zero electric resistance.

International audience; A combined experimental and theoretical investigation on the cerium(IV) oxo complex [(L-OEt)(2)Ce(=O)(H2O)].MeC(O)NH2 (1; L-OEt(-)=[Co(eta(5)-C5H5){P(O)(OEt)(2)}(3)](-)) demonstrates that the intermediate spin-state nature of the ground state of the cerium complex is responsible for the versatility of its reactivity towards small molecules such as CO, CO2, SO2, and NO. CASSCF calculations together with magnetic susceptibility measurements indicate that the ground state of the cerium complex is of multiconfigurational character and comprised of 74 % of Ce-IV and 26 % of Ce-III. The latter is found to be responsible for its reductive addition behavior towards CO, SO2, and NO. This is the first report to date on the influence of the multiconfigurational ground state on the reactivity of a metal-oxo complex.

A state of matter with a multi-component order parameter can give rise to vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. Vestigial states due to primary spin and charge-density-wave order have been discussed in iron-based and cuprate materials. Here we present the observation of a partially melted superconductivity in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z3, i.e., three-state Potts-model symmetry. Thermal expansion, specific heat and magnetization measurements of the doped topological insulators NbxBi2Se3 and CuxBi2Se3 reveal that this symmetry breaking occurs at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{T}}_{\mathrm{nem}} \simeq 3.8\,K$$\end{document}Tnem≃3.8K above \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{{\mathrm{c}}} \simeq 3.25\,K$$\end{document}Tc≃3.25K, along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish long-range coherence at Tc, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a crystalline distortion., When an order parameter has multiple components, fluctuations can suppress the ordering partially, leaving behind vestigial order. Here the authors show that nematic superconductivity in electron-doped Bi2Se3 gives way to a vestigial nematic phase driven by fluctuating Cooper pairs.

We demonstrate that a field-effect transistor (FET) made of few-layer black phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum exhibits a room-temperature hole mobility of 5200 cm2/(Vs), being limited just by the phonon scattering. At cryogenic temperatures, the FET mobility increases up to 45 000 cm2/(Vs), which is five times higher compared to the mobility obtained in earlier reports. The unprecedentedly clean h-BN–BP–h-BN heterostructure exhibits Shubnikov–de Haas oscillations and a quantum Hall effect with Landau level (LL) filling factors down to v = 2 in conventional laboratory magnetic fields. Moreover, carrier density independent effective mass of m* = 0.26 m0 is measured, and a Lande g-factor of g = 2.47 is reported. Furthermore, an indication for a distinct hole transport behavior with up- and down-spin orientations is found.

We investigated the dissipative regime of the Bi2Te3/FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking.

With the recent discovery of the quantum anomalous Hall insulator (QAHI), which exhibits the conductive quantum Hall edge states without external magnetic field, it becomes possible to create a topological superconductor (SC) by introducing superconductivity into these edge states. In this case, 2 distinct topological superconducting phases with 1 or 2 chiral Majorana edge modes were theoretically predicted, characterized by Chern numbers ( N ) of 1 and 2, respectively. We present spectroscopic evidence from Andreev reflection experiments for the presence of chiral Majorana modes in an Nb/(Cr 0.12 Bi 0.26 Sb 0.62 ) 2 Te 3 heterostructure with distinct signatures attributed to 2 different topological superconducting phases. The results are in qualitatively good agreement with the theoretical predictions.

We investigated the superconducting state of the noncentrosymmetric superconductors Li2Pd x Pt3−x B with superconducting transition temperature T c=5.16(8) K (x=2.25), 3.56(8) K (x=1.5) and 2.60 K (x=0) by means of muon-spin rotation (μSR) and specific heat experiments. The μSR relaxation rate σ sc was found to be constant at low temperatures for all the compounds. Data taken at different magnetic fields show that the magnetic penetration depth λ is field-independent for Li2Pd2.25Pt0.75B and Li2Pt3B. The electronic contribution to the specific heat measured in Li2Pd1.5Pt1.5B and Li2Pt3B increases exponentially at the lowest temperatures. These features suggest that the whole family of Li2Pd x Pt3−x B comprises single-gap s-wave superconductors across the entire doping regime.

The study of magnetism without the involvement of transition metals or rare earth ions is considered the key to the fabrication of next generation spintronic devices. Several recent reports claim that optimizing the occupation number of the mixed p-orbitals is the correct way to reinforce p-orbital magnetism in bulk crystals. We provide experimental evidence that the kinked monoatomic carbon chains, the so-called linear-chained carbon, generate intrinsic ferromagnetism even above room temperature. According to our ab initio calculations, unconventional magnetism has its origin in the p-shells. In contrast, the linear monoatomic carbon chains are non-magnetic. Although the optimized differential spin density of states at the Fermi level (SDOS) of the kinked carbon chains is higher than that of bulk Fe, the magnetic moment is as low as 0.3μB. In order to enhance the magnetic response, we decided to tune the p-orbital magnetism by adding dopants from groups IV to VII of the periodic table. We observed that the strength of the p-orbital magnetism and the sign of the exchange interaction depend not only on the kink angle, but also on the concentration of lone pair electrons, free radical electrons, lateral chain spacing, internal electric dipole, dative covalent bonds and the Bohr radius of the dopants. Surprisingly, the V and VII-doped carbon chains show a strong non-zero SDOS, which has its origin in the p-shells. The VII-doped carbon chains give the SDOS of the opposite sign. Our best system, the arsenic-doped carbon chain, exhibits a strong local magnetic moment of 1.5μB, which is comparable to that of the bulk Fe of 2.2μB, with the mean exchange–correlation energy reaching a 63% ratio relative to that of the bulk Fe.

We investigated the superconductivity in a few-layer ${\mathrm{NbSe}}_{2}$-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in ${\mathrm{NbSe}}_{2}$ were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in ${\mathrm{NbSe}}_{2}$ followed the expected temperature behavior in the limit of weak interband scattering, with a gap to ${T}_{c}$ ratio suggesting a weak to moderately strong coupling in few-layer systems.

Nanostructuring of superconducting materials to form dense arrays of thin parallel nanowires with significantly large transverse Josephson coupling has proven to be an effective way to increase the upper critical field of superconducting elements by as much as two orders of magnitude as compared to the corresponding bulk materials and, in addition, may cause considerable enhancements in their critical temperatures. Such materials have been realized in the linear pores of mesoporous substrates or exist intrinsically in the form of various quasi-1D crystalline materials. The transverse coupling between the superconducting nanowires is determined by the size-dependent coherence length ξ0. In order to obtain ξ0 over the Langer–Ambegaokar–McCumber–Halperin (LAMH) theory, extensive experimental fitting parameters have been required over the last 40 years. We propose a novel Monte Carlo algorithm for determining ξ0 of the multi-Cooper pair system in the 1D limit. The concepts of uncertainty principle, Pauli-limit, spin flip mechanism, electrostatic interaction, thermal perturbation and co-rotating of electrons are considered in the model. We use Pb nanowires as an example to monitor the size effect of ξ0 as a result of the modified electron-electron interaction without the need for experimental fitting parameters. We investigate how the coherence length determines the transverse coupling of nanowires in dense arrays. This determines whether or not a global phase-coherent state with zero resistance can be formed in such arrays. Our Monte Carlo results are in very good agreement with experimental data from various types of superconducting nanowire arrays. © 2018 Elsevier B.V.

Comprehensive microstructural, electronic and magnetic analyses have been carried out on ZnO:Cu thin films grown by pulsed laser deposition on c-plane sapphire under different oxygen partial pressures. Detailed X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) analyses reveal that increase in oxygen growth pressure degrades the epitaxy of ZnO:Cu thin films due to inclusion of nanosize CuO in the ZnO host lattice. HRTEM and magnetization studies suggest that thin film quality plays a less effective role in governing the magnetic properties of these samples. Instead, room temperature ferromagnetism (FM) of these ZnO:Cu thin film samples are highly tunable by the simultaneous presence of CuO nanophases and multivalent Cu and concentrations, which are in strong contest with each other. For low oxygen partial pressure grown sample, the effective network is the main contributor to the observed FM and is in competition with CuO nanophases only when there is a relatively low concentration with a dominant Cu2+ oxidation state. For vacuum grown samples containing high concentration and Cu1+ as dominant oxidation state, the network becomes less effective and a CuO nanophase (4–5 nm) is the dominent FM supplier. The extrinsic FM in the vacuum grown sample, which is the best epitaxial quality sample, is further confirmed by the zero field cooled (ZFC) and field cooled (FC) magnetization protocols.

We investigate the magnetic phase diagram near the upper critical field of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ by magnetic torque and specific heat experiments using a high-resolution piezorotary positioner to precisely control the parallel alignment of the magnetic field with respect to the FeAs layers. We observe a clear double transition when the field is strictly aligned in the plane and a characteristic upturn of the upper critical field line, which goes far beyond the Pauli limit at 4.8 T. This provides firm evidence that a Fulde-Ferrell-Larkin-Ovchinnikov state exists in this iron-based ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ superconductor.

High temperature superconductivity does not necessarily require correlated electron systems with complex competing or coexisting orders. Instead, it may be achieved in a phonon-mediated classical superconductor having a high Debye temperature and large electronic density of states at the Fermi level in a material with light atoms and strong covalent bonds. Quasi-1D conductors seem promising due to the Van Hove singularities in their electronic density of states. In this sense, quasi-1D carbon structures are good candidates. In thin carbon nanotubes, superconductivity at ~15 K has been reported, and it is likely the strong curvature of the graphene sheet which enhances the electron-phonon coupling. We use an ab-initio approach to optimize superconducting quasi-1D carbon structures. We start by calculating a Tc of 13.9 K for (4.2) carbon nanotubes (CNT) that agrees well with experiments. Then we reduce the CNT to a ring, open the ring to form chains, optimize bond length and kink structure, and finally form a new type of carbon ring that reaches a Tc value of 115 K.

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe

We investigate the re-entrant tetragonal phase in the iron-based superconductor Ba 0 .76 K 0.24 Fe 2 As 2 by DC magnetization and thermoelectrical measurements. The reversible magnetization confirms by a thermodynamic method that the spin alignment in the re-entrant C 4 phase is out-of-plane, in agreement with an itinerant double- Q magnetic order [Allred et al., Nat. Phys. 12 , 493 (2016)]. The Nernst coefficient shows the typical unusually large negative value in the stripe-type spin density wave (SDW) state owing to the Fermi surface reconstruction associated with SDW and nematic order. At the transition into the re-entrant C 4 tetragonal phase it hardly changes, which could indicate that instead of a complete vanishing of the associated charge order, the spin reorientation could trigger a redistribution of the charges to form a secondary charge order, e.g. in form of a chequerboard-like pattern that no longer breaks the rotational C 4 symmetry.

We fabricate high-mobility p-type few-layer WSe_{2} field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe_{2}, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe_{2} offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (Bi2Te3) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. Bi2Te3/FeTe also displays the anomalous oscillatory magnetoresistance phenomenon, which we had previously observed in cuprates.

With pulsed laser deposition, the Cu0.04Zn0.96O thin films are grown at 600 °C under three different oxygen pressures, namely PO2 = 0.00, 0.02, and 1.00 Pa. X-ray diffraction shows single-phase material for the samples grown under PO2 = 0.00 and 1.00 Pa and CuO secondary phase for the PO2 = 0.02 Pa grown sample. The observation of satellite structures in the Cu 2p core level X-ray photoelectron spectroscopy (XPS) spectra suggest the presence of Cu2+ and CuO secondary phases in the samples grown at PO2 = 0.02 and 1.00 Pa. The sample grown under vacuum (PO2 = 0.00 Pa) shows mixed Cu oxidation state of 1 + or 2 + . The sample grown without oxygen is n-type and those grown with oxygen are highly insulating. The insulating sample grown at PO2 = 0.02 Pa shows highest magnetization due to possible collective behavior of Cu2+ – O v – Cu2+ network in the form of bound magnetic polaron (BMP) and ferromagnetic superexchange interaction coming from uncompensated surface spins of the Cu ions in the CuO secondary phase. Both delocalized electrons (∼3.32 × 1018) due to oxygen deficient defects and reduced amount of effective Cu2+ ions discredit the BMP model for this vacuum grown sample, and magnetism is suggested due to O v and presence of possible CuO secondary phase.

This work focuses on the formation mechanism of a superconducting Fe1+xTe/Bi2Te3 bilayer fabricated through chemical reactions between a Bi2Te3 flux and an annealed Fe layer grown on a ZnSe(111)B buffer via the molecular beam epitaxy technique. The studies using energy-dispersive X-ray spectrometry and high-resolution transmission electron microscopy performed on a number of samples fabricated with different desired schemes provide evidence that several interfacial chemical reactions taking place at the Fe/ZnSe interface through the edges of voids of the annealed Fe layer contributed to the formation of the superconducting bilayer. We have also revealed that the bonding strength of the involved Fe compounds at the bottom interface of the annealed Fe layer seems to play an essential role.

Co ions with 100 keV energy with a fluence of 1 × 10(15) cm(-2) are implanted into ZnO(0001) single crystals at 300 °C under vacuum. The resulting Co-implanted ZnO single crystals and the subsequent 750 °C and 900 °C annealed samples are analysed with respect to their structural, optical, electronic, magnetic and ac electrical properties. Photoluminescence and X-ray photoelectron spectroscopy results indicate the signatures of the Co(2+) state and its substitution at the tetrahedrally coordinated Zn-sites. X-ray diffraction and X-ray photoelectron spectroscopy identify the presence of the ZnCo2O4 and Co3O4 phases in the 900 °C annealed sample. By comparing the resistance response of the identified phases towards different magnetic environments, the impedance spectroscopy results successfully identify two magnetic phases (ZnCo2O4 and Co3O4) and a paramagnetic (CoZn) phase for the 750 °C and 900 °C annealed samples, implying the extrinsic nature of room temperature ferromagnetism. The observed ferromagnetism in each sample is not of single origin, instead the mutual effects of the secondary phases embedded in the paramagnetic host matrix are in competition with each other.

High-resolution DC magnetization and AC-specific heat data of the cubic helimagnet FeGe have been measured as function of temperature and magnetic field. The magnetization data as well as the isothermal susceptibility data confirm the complexity of the magnetic phase diagram in the vicinity of the onset of long-rang magnetic order (Tc = 278.5 K) and the existence of a segmented A-phase region. Moreover, these data revealed independent and clear indications of phase boundaries and crossovers within the A-phase region. Together with the anomalies in the specific-heat data around Tc and at small magnetic fields (H < 600 Oe) a complex magnetic phase diagram of FeGe is obtained., Presented at the QCNP conference in Dreden, Germany, August 2012

We fabricate high-mobility p-type few-layer WSe_{2} field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe_{2}, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe_{2} offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

Sn is a classical superconductor on the border between type I and type II with critical temperature of 3.7 K. We show that its critical parameters can be dramatically increased if it is brought in the form of loosely bound bundles of thin nanowires. The specific heat displays a pronounced double phase transition at 3.7 K and 5.5 K, which we attribute to the inner ‘bulk’ contribution of the nanowires and to the surface contribution, respectively. The latter is visible only because of the large volume fraction of the surface layer in relation to the bulk volume. The upper transition coincides with the onset of the resistive transition, while zero resistance is gradually approached below the lower transition. In contrast to the low critical field Hc = 0.03 T of Sn in its bulk form, a magnetic field of more than 3 T is required to fully restore the normal state.

In the interfacial superconductor Bi2Te3/Fe1+yTe, two dimensional superconductivity occurs in direct vicinity to the surface state of a topological insulator. If this state were to become involved in superconductivity, under certain conditions a topological superconducting state could be formed, which is of high interest due to the possibility of creating Majorana fermionic states. We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show highly unconventional superconductivity, which appears as complex as in the cuprate high temperature superconductors. A very large superconducting twin-gap structure is replaced by a pseudogap above ~12 K which persists up to 40 K. While the larger gap shows unconventional order parameter symmetry and is attributed to a thin FeTe layer in proximity to the interface, the smaller gap is associated with superconductivity induced via the proximity effect in the topological insulator Bi2Te3.

We report measurements of the specific heat and electrical resistivity of $\mathrm{C}{\mathrm{a}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.91}\mathrm{C}{\mathrm{o}}_{0.09})}_{4}\mathrm{S}{\mathrm{n}}_{13}$ in comparison to $\mathrm{C}{\mathrm{a}}_{3}\mathrm{I}{\mathrm{r}}_{4}\mathrm{S}{\mathrm{n}}_{13}$. The Co substitution was chosen to suppress the structural anomaly ${T}^{*}=38\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ reported in $\mathrm{C}{\mathrm{a}}_{3}\mathrm{I}{\mathrm{r}}_{4}\mathrm{S}{\mathrm{n}}_{13}$. While the superconducting transition temperature of $\mathrm{C}{\mathrm{a}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.91}\mathrm{C}{\mathrm{o}}_{0.09})}_{4}\mathrm{S}{\mathrm{n}}_{13}$ is slightly higher than in the stoichiometric parent compound $\mathrm{C}{\mathrm{a}}_{3}\mathrm{I}{\mathrm{r}}_{4}\mathrm{S}{\mathrm{n}}_{13}$, and it displays a stronger superconducting coupling strength, the absence of a structural phase transition allows us to determine the spectral distribution of phonon states and its relation to superconductivity. The phonon density of states $F$(\ensuremath{\omega}) and the spectral electron-phonon scattering function ${{a}_{tr}}^{2}F(\ensuremath{\omega})$ are determined by deconvolution of the specific heat and electrical resistivity, respectively. A dominant mode appears in both histograms at $\ensuremath{\hbar}\ensuremath{\omega}=12\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which indicates that this phonon mode is responsible for the superconductivity in $\mathrm{C}{\mathrm{a}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.91}\mathrm{C}{\mathrm{o}}_{0.09})}_{4}\mathrm{S}{\mathrm{n}}_{13}$. The data suggest that superconductivity in this material class can be conclusively explained within a conventional strong -coupling phonon-mediated scenario.