arXiv subjects

Martin Bremholm

Publications and source records attributed to Martin Bremholm.

25 records · Page 2Linked to original sources

arXiv2017

Experimental evidence for pressure-induced first order transition in cerium nitride from B1 to B10 structure type

The crystal structure of CeN was investigated up to pressures of 82 GPa, using diamond anvil cell powder X-ray diffraction in two experiments with He and Si-oil as the pressure transmitting media. In contrast to previous reports, we do not observe the B2 (CsCl type) structure at high pressure. Instead, the structural phase transition, starting at 65 GPa, from the ambient rock salt B1 structure results in a distorted CsCl-like B10 structure, irrespective of the pressure medium. Our result unambiguously confirms two recent density functional theory (DFT) studies predicting the B10 phase to be stable at these pressures, rather than the B2 (CsCl type) phase previously reported. The B10 structure appears to approach the B2 structure as pressure is increased further, but DFT calculations indicate that an L1 0 structure (AuCu type) is energetically favored.

cond-mat.mtrl-sci↗

arXiv2016

Absence of superconductivity in ultra-thin layers of FeSe synthesized on a topological insulator

The structural and electronic properties of FeSe ultra-thin layers on Bi$_{2}$Se$_{3}$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multi-layers, which are predominantly 3-5 monolayers (ML) thick, exhibit a hole pocket-like electron band at \barΓ and a dumbbell-like feature at \bar{M}, similar to multi-layers of FeSe on SrTiO$_{3}$. Moreover, the topological state of the Bi2Se3 is preserved beneath the FeSe layer, as indicated by a heavily \it{n}-doped Dirac cone. Low temperature STS does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K.

cond-mat.supr-con↗

arXiv2016

Evidence for a two-fold symmetric superconducting gap in a monolayer of FeSe$_{0.5}$Te$_{0.5}$ on a topological insulator

We present our investigations on the superconducting properties of monolayers of FeSe$_{0.5}$Te$_{0.5}$ grown on the 3D topological insulator Bi$_{2}$Se$_{1.2}$Te$_{1.8}$ using low temperature scanning tunneling spectroscopy (STS). While the morphology and the overall transition temperature resemble those of similarly doped bulk crystals, the spatially resolved spectroscopic data at 1.1K shows a much larger spatial inhomogeneity in the superconducting energy gaps. Despite the gap inhomogeneity all the spectra can be fitted with a two-fold anisotropic s-wave gap function. The two-fold nature of the gap symmetry is evident from the Bogoliubov quasiparticle interference (QPI) pattern which shows distinct C$_{2}$ symmetric scattering intensities. We argue that the gap inhomogeneity emerges as a result of intrinsic disorder in our system similar to disordered conventional superconductors. Even though most of our findings clearly differ from the current understanding of the corresponding bulk system, it provides an ideal platform to study unconventional superconductivity in Fe chalcogenides thinned down to a single layer and in close proximity to a topological insulator.

cond-mat.supr-con↗

arXiv2016

Reorientation of the bicollinear antiferromagnetic structure at the surface of Fe$_{1+y}$Te bulk and thin films

Establishing the relation between the ubiquitous antiferromagnetism in the non-superconducting parent compounds of unconventional superconductors and their superconducting phase is believed to be important for the understanding of the complex physics in these materials. Going from the bulk systems to thin films strongly affects the phase diagram of unconventional superconductors. For Fe$_{1+y}$Te, the parent compound of the Fe$_{1+y}$Se$_{1-x}$Te$_x$ superconductors, bulk sensitive neutron diffraction has revealed an in-plane oriented bicollinear antiferromagnetic structure. Here, we show by spin-resolved scanning tunneling microscopy that on the surfaces of bulk Fe$_{1+y}$Te, as well as on thin films grown on the topological insulator Bi$_2$Te$_3$, the spin direction is canted both away from the surface plane and from the high-symmetry directions of the surface unit cell, while keeping the bicollinear magnetic structure. Our results demonstrate that the magnetism at the Fe-chalcogenide surface markedly deviates from a simple in-plane oriented bicollinear antiferromagnetic structure, which implies that the pairing at the surface of the related superconducting compounds might be different from that in the bulk.

cond-mat.str-el↗

arXiv2016

Evidence for interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator

The discovery of high-temperature superconductivity in Fe-based compounds [1,2] has triggered numerous investigations on the interplay between superconductivity and magnetism [3] and, more recently, on the enhancement of transition temperatures through interface effects [4]. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture of this relationship remains elusive [1] due to the lack of data with atomic spatial resolution [5-7]. Here, we present a spin-polarized scanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$ (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level indicating superconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers grown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk compound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists with bicollinear AFM order. This finding opens novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers.

cond-mat.supr-con↗

arXiv2014

Strongly anisotropic spin-orbit splitting in a two-dimensional electron gas

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a degree where a large splitting ($\approx 0.06$ Å$^{-1}$) can be found in the $\barΓ\bar{M}$ direction while the states are hardly split along $\barΓ\bar{K}$. The direction of the anisotropy is found to be qualitatively inconsistent with results expected for a third-order anisotropic Rashba Hamiltonian. However, a $\mathbf{k} \cdot \mathbf{p}$ model that includes the possibility of band structure anisotropy as well as both isotropic and anisotropic third order Rashba splitting can explain the results. The isotropic third order contribution to the Rashba Hamiltonian is found to be negative, reducing the energy splitting at high $k$. The interplay of band structure, higher order Rashba effect and tuneable doping offers the opportunity to engineer not only the size of the spin-orbit splitting but also its direction.

cond-mat.str-el↗

arXiv2013

Direct Measurement of Surface Transport on a Bulk Topological Insulator

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillations, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present the first direct measurement of surface-dominated conduction on an atomically clean surface of bulk-insulating Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm$^{2}$V$^{-1}$s$^{-1}$ at 30 K at a carrier concentration of 8.71(7)$\times 10^{12}$ cm$^{-2}$.

cond-mat.mtrl-sci↗