Nanoscopic allotropes of tellurium, such as tellurene and tellurium nanowires, have attracted a lot of attention due to tellurium’s optical, electronic, and thermoelectric properties. This study introduces a new synthesis of nanofibered tellurium ropes. The fibers are only a few nanometers thick and they bundle up into ropes of lengths up to 10 μ m and thicknesses between 25 and 300 nm. The ratio of the rope diameter to length is never found to be below 0.02, which suggests that this is a natural stability limit. Our study reveals that the nanoropes are formed during a thermal treatment on a naturally oxidized Si (111) surface. When the same synthesis procedure is executed on GaAs (100) and Au on GaAs (100), we do not observe any formation of nanofibered ropes. Therefore, we conclude that the substrate plays a non-negligible role for the synthesis.

Abstract The miniaturisation of electronic building blocks is of uttermost importance. Efficient nanoscopic inductors and capacitors could pave the way to better energy economics and therewith more powerful processors. Topological materials which exhibit emergent electromagnetism are promising candidates for such future technologies. Due to its helical crystal structure and strong spin-orbit interaction, tellurium exhibits an experimentally verified internal inductance. By formulating the constitutive equations which describe electromagnetism in tellurium, we show how capacitive and inductive properties naturally emerge in the material. We demonstrate on the basis of AC-impedance measurements that elemental tellurium exhibits an unusually high virtual capacitance and inductance, far exceeding previously studied materials. The highest capacitance we detect in our samples is 130 mF and in one of our samples we measure an extremely impressive virtual inductance of 1.5 H. For low current densities a single piece of tellurium mimics the reactance of an RLC-circuit.

We investigate the topological phase transitions driven by band warping and a transverse magnetic field, for three-dimensional Weyl semimetals. First, we use the Chern number as a mathematical tool to derive the topological phase diagram. Next, we associate each of the topological sectors to a given angular momentum state of a rotating wave packet. Then we show how the position of the Weyl nodes can be manipulated by a transverse external magnetic field that ultimately quenches the wave packet rotation, first partially and then completely, thus resulting in a sequence of field-induced topological phase transitions. Finally, we calculate the current-induced magnetization and the anomalous Hall conductivity of a prototypical warped Weyl material. Both observables reflect the topological transitions associated with the wave packet rotation and can help to identify the elusive 3D quantum anomalous Hall effect in three-dimensional, warped Weyl materials.

Tellurium is a gyrotropic, p-type Weyl semiconductor with remarkable electronic, optical, and transport properties. It has been argued that some of these properties might stem from Weyl nodes at crossing points in the band structure, and their nontrivial topological textures. However, Weyl nodes in time-reversal invariant semiconductors are split up in energy, rather than in momentum, and located deep below (far above) the top (bottom) of the valence (conduction) band, challenging such an interpretation. Here, instead, we use a 4-band kp Hamiltonian for $p-$type tellurium to show how the k-dependent spin-orbit interaction mixes up the top two (Weyl node free) and bottom two (Weyl node containing) valence bands, generating a 3D hedgehog orbital magnetic texture at the uppermost valence band, accessible to transport already at the lowest doping. Hedgehog textures are important signatures of Weyl fermion physics in general and in the context of condensed matter physics arise form the carriers' wave packet rotation being locked to their propagation wavevector. For spatially dispersive media, such an induced hedgehog texture/carrier rotation stabilizes two novel, nonreciprocal and antisymmetric components to the Hall transport within different weak-localization (antilocalization) relaxation regimes: the anomalous and planar Hall effects, usually forbidden by time reversal symmetry. Our AC magnetotransport measurements on Sn-doped tellurium confirm the theoretical predictions and our work demonstrates how Weyl signatures generally appear in transport on enantiomorphic materials with natural optical activity.

Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition.

Elemental tellurium is a semiconductor with a small band gap of around 330 meV. Under the application of hydrostatic pressure, the band gap narrows, and it has been suggested that below its crystal phase transition at 40 kbar, the gap might close, leading to a transition into a topologically non-trivial state. Here, we present a pressure dependent study of the gap size of tellurium up to 22kbar, studied by temperature dependent electrical transport. We identify the gap size to be still well above 100 meV at our maximum pressure. Extrapolation of our data, assuming a non-linear gap narrowing (in agreement with previous studies), leaves us to assume that the gap does not close within the ambient pressure crystal phase. However, when assuming a linear narrowing of the gap, we leave a small possibility that there exists a minor pressure window at which a topological phase transition could occur. Furthermore, as the second systematic study of the gap size via transport, we see that historical inconsistencies between the measured gap size via transport and the measured optical gap size seem to be systematic and probably reflect some deeper physics of the material.

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The electronic structure of ultrathin ZnO (000 (1) over bar) films grown on Au (111) was investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). Our results show evidence of O-terminated films and formation of bulk-like ZnO (2 x 2) surface reconstruction for films with > 4 monolayers. The measurements indicate that the metal substrate plays a decisive role in the electronic structure of films since p-type doping is obtained as observed from the valence band energy shifts. Moreover, finite-size effects appear to significantly modify the Zn and O core-level energy positions. These electronic effects may account for the role of ZnO catalytic performance in ZnO/metal systems, as well as for their nanostructure optoelectronic properties.

We present an extensive study of the ferromagnetic heavy fermion compound U$_4$Ru$_7$Ge$_6$. Measurements of electrical resistivity, specific heat and magnetic properties show that U$_4$Ru$_7$Ge$_6$ orders ferromagnetically at ambient pressure with a Curie temperature $T_{C} = 6.8 \pm 0.3$ K. The low temperature magnetic behavior of this soft ferromagnet is dominated by the excitation of gapless spin-wave modes. Our results on the transport properties of U$_4$Ru$_7$Ge$_6$ under pressures up to $2.49$ GPa suggest that U$_4$Ru$_7$Ge$_6$ has a putative ferromagnetic quantum critical point (QCP) at $P_c \approx 1.7 \pm 0.02$ GPa. In the ordered phase, ferromagnetic magnons scatter the conduction electrons and give rise to a well defined power law temperature dependence in the resistivity. The coefficient of this term is related to the spin-wave stiffness and measurements of the very low temperature resistivity allow to accompany the behavior of this quantity as the the ferromagnetic QCP is approached. We find that the spin-wave stiffness decreases with increasing pressure implying that the transition to the non-magnetic Fermi liquid state is driven by the softening of the magnons. The observed quantum critical behavior of the magnetic stiffness is consistent with the influence of disorder in our system. At quantum criticality ($P = P_c \approx 1.7 \pm 0.02$ GPa), the resistivity shows the behavior expected for an itinerant metallic system near a ferromagnetic QCP.

Superconductivity has been observed in doped SrTiO$_3$ at charge-carrier densities below 10$^{18}$ cm$^{-3}$, where the density of states at the Fermi level of the itinerant electrons is several orders of magnitude lower than that of conventional metals. In terms of the Bardeen-Cooper-Schrieffer description, this implies the existence of an extraordinarily strong interaction driving the formation of Cooper pairs, potentially comparable in order of magnitude to that in some high Tc superconductors. Under suitable conditions the interaction might remain effective at densities approaching metallic densities, leading to the possibility of pair formation at elevated temperatures. Here we investigate the pressure dependence of the resistivity and superconducting transition temperature, Tc, of SrTiO$_3$ at a carrier density near to optimal doping. Our experiments show that Tc collapses rapidly with pressure and hence with increasing frequency of the soft transverse-optical phonon mode connected to the ferroelectric quantum critical point. We show that the superconductivity phase diagram can be understood in terms of the coupling of electrons via two hybrid longitudinal polar modes, based on a model of dipolar fluctuations of the charge carrier-ion system. In particular, we predict that for carrier densities above the order of 10$^{18}$ cm$^{-3}$, Tc can be strongly enhanced on approaching the ferroelectric quantum critical point, as seen in our measurements of SrTiO$_3$ and as found in many electrically conducting magnetic analogues. However below this density we predict the reverse behaviour, namely that Tc is suppressed on approaching the ferroelectric quantum critical point. Our model is also relevant to superconductivity found in gated ferroelectric quantum critical systems such as KTaO$_3$ and can guide searches for new superconductors in a diversity of materials.

Here, we present a study on Si(111)/Ta(150 angstrom)/IrMn(150 angstrom)/NiFe(50 angstrom)/Nb(x)/NiFe(50 angstrom)/Ta( 50 angstrom) spin valves with x = 100-500 angstrom A. Samples with a Nb layer thickness of 300 angstrom or higher exhibit an unusual magnetization behavior below Tc. The superconducting Nb layer contributes strongly to the magnetization, and manifests itself in an anomalous hysteresis loop. While the hysteresis loop is seemingly similar to what is generally expected from hard superconductors and many superconductor/ferromagnet hybrid systems, its direction is invertedwhen compared towhat is generally observed. Thus, the samples show paramagnetic behavior for upsweeping fields and diamagnetic behavior for downsweeping fields. This means that the respective samples exhibit a magnetization, which is contrary to what Lenz's rule dictates.

The semimetal-to-semiconductor transition in fcc-Yb under modest pressure can be considered a picture book example of a metal–insulator transition of the Lifshitz type. We have performed transport measurements at low temperatures in the closest vicinity of the transition and related DFT calculations of the Fermi surface. Our resistivity measurements show a linear temperature dependence with an unusually low dρ/dT at low temperatures approaching the MIT. The calculations suggest fcc-ytterbium being an ultra-multi valley system with 24 electron and 6 hole pockets in the Brillouin zone. Such Fermi surface topology naturally supports the appearance of strongly correlated phases. An estimation of the quasiparticle-enhanced effective mass shows that the scattering rate is by at least two orders of magnitude lower than in other materials which exhibit linear-in-T behavior at a quantum critical point. However, we cannot exclude an excessive effective mass enhancement, when the van Hove singularity touches the Fermi level.

The Lifshitz-type semimetal-insulator transition, which is a transition of the electronic topology, has been considered as the most fundamental metal-insulator transition. Here, we present resistivity measurements under pressure in the vicinity of the quantum critical point of fcc Yb. We apply a previously suggested scaling for this type of transition and identify its universality class. Moreover, we observe an anomaly in the screening coefficient A of the T2 term in the resistivity at low temperatures in the metallic phase. We suggest an interpretation of this phenomenon as an effect of doping by Ca impurities unintentionally present in the Yb crystals. The observed behavior may very well be applicable to any doped system in the vicinity of such a transition.

In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer.