arXiv subjects

Sergey Borisenko

Publications and source records attributed to Sergey Borisenko.

30 records · Page 2Linked to original sources

arXiv2018

Energy scale of nematic ordering in the parent iron-based superconductor:BaFe2As2

Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111 and 122 compounds: energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here we revisit the electronic structure of undoped parent BaFe2As2 (122 family). Our systematic ARPES study including the detailed temperature and photon energy dependencies points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity.

cond-mat.supr-con↗

arXiv2017

Non-Magnetic Half-Metals

Half-metals are a class of materials that are metallic only for one spin direction, and are essential for spintronics applications where one needs to read, write, store and transfer spin-data. This spin sensitivity appears to restrict them to be magnetic, and the known examples indeed are. The fabrication of real spintronic devices from such materials is often hampered, however, by stray magnetic fields, domain walls, short spin coherence times, scattering on magnetic atoms or magnetically active interfaces, and other characteristics that come along with the magnetism. The surfaces of topological insulators, or Dirac or Weyl semimetals, could be an alternative, but production of high-quality thin films without the presence of the bulk states at the Fermi level remains very challenging. Here we introduce non-magnetic half-metals and demonstrate that this state is realized in IrBiSe. Using angle-resolved photoemission spectroscopy and band structure calculations we find a record-high Dresselhaus spin-orbit splitting, fully spin-polarized remnant Fermi surfaces and a chiral 3D spin-texture, all with no magnetism present. Promising applications include using IrBiSe as a source of spin-polarized electrons, and lightly doped IrBiSe is expected to generate electric-field-controlled spin-polarized currents, free from back scattering, and could host triplet superconductivity.

cond-mat.mtrl-sci↗

arXiv2016

TaIrTe4 a ternary Type-II Weyl semi-metal

In metallic condensed matter systems two different types of Weyl fermions can in principle emerge, with either a vanishing (type-I) or with a finite (type-II) density of states at the Weyl node energy. So far only WTe2 and MoTe2 were predicted to be type-II Weyl semi-metals. Here we identify TaIrTe4 as a third member of this family of topological semi-metals. TaIrTe4 has the attractive feature that it hosts only four well-separated Weyl points, the minimum imposed by symmetry. Moreover, the resulting topological surface states - Fermi arcs connecting Weyl nodes of opposite chirality - extend to about 1/3 of the surface Brillouin zone. This large momentum-space separation is very favorable for detecting the Fermi arcs spectroscopically and in transport experiments.

cond-mat.str-el↗

arXiv2016

Time-Reversal Symmetry Breaking Type-II Weyl State in YbMnBi2

Detection of Dirac, Majorana and Weyl fermions in real materials may significantly strengthen the bridge between high-energy and condensed-matter physics. While the presence of Dirac fermions is well established in graphene and topological insulators, Majorana particles have been reported recently and evidence for Weyl fermions in non-centrosymmetric crystals has been found only a couple of months ago, the 'magnetic' Weyl fermions are still elusive despite numerous theoretical predictions and intense experimental search. In order to detect a time-reversal symmetry breaking Weyl state we designed two materials with Fermi velocities superior to that of graphene and present here the experimental evidence of the realization of such a state in one of them, YbMnBi2. We model the time reversal symmetry breaking observed by magnetization measurements by a canted antiferromagnetic state and find a number of Weyl points both above and below the Fermi level. Using angle-resolved photoemission, we directly observe these latter Weyl points and a hallmark of the exotic state - the arc of the surface states which connects these points. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.

cond-mat.mes-hall↗

arXiv2016

Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9

The compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)3I]2+ and insulating [Bi2I8]2- layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well screened towards the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both, on the surface and in the bulk, i.e. it is not surface-doped due to iodine desorption. The well screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically non-trivial surface layer which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.

cond-mat.mtrl-sci↗

arXiv2015

Direct observation of spin-orbit coupling in iron-based superconductors

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces.

cond-mat.supr-con↗

arXiv2015

Crystal growth and the electronic phase diagram of the 4$d$ doped Na$_{1-δ}$Fe$_{1-x}$Rh$_x$As in comparison with 3$d$ doped Na$_{1-δ}$Fe$_{1-x}$Co$_x$As

Single crystals of Na$_{1-δ}$Fe$_{1-x}$T$_x$As with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder X-ray diffraction, magnetic susceptibility and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration ($0\,\leq\,x\,\leq\,0.013$) is fully suppressed in the optimally doped sample. The superconducting transition temperature ($T_c$) is enhanced from $10$~K in Na$_{1-δ}$FeAs to $21$~K in the optimally doped sample ($x$ = 0.019) of the Na$_{1-δ}$Fe$_{1-x}$Rh$_x$As series and decreases for the overdoped compounds, revealing a typical shape for the superconducting part of the electronic phase diagram. Remarkably, the phase diagram is almost identical to that of Co-doped Na$_{1-δ}$FeAs, suggesting a generic phase diagram for both dopants.

cond-mat.supr-con↗

arXiv2013

Experimental Realization of a Three-Dimensional Dirac Semimetal

The three dimensional (3D) Dirac semimetal, which has been predicted theoretically, is a new electronic state of matter. It can be viewed as 3D generalization of graphene, with a unique electronic structure in which conduction and valence band energies touch each other only at isolated points in momentum space (i.e. the 3D Dirac points), and thus it cannot be classified either as a metal or a semiconductor. In contrast to graphene, the Dirac points of such a semimetal are not gapped by the spin-orbit interaction and the crossing of the linear dispersions is protected by crystal symmetry. In combination with broken time-reversal or inversion symmetries, 3D Dirac points may result in a variety of topologically non-trivial phases with unique physical properties. They have, however, escaped detection in real solids so far. Here we report the direct observation of such an exotic electronic structure in cadmium arsenide (Cd3As2) by means of angle-resolved photoemission spectroscopy (ARPES). We identify two momentum regions where electronic states that strongly disperse in all directions form narrow cone-like structures, and thus prove the existence of the long sought 3D Dirac points. This electronic structure naturally explains why Cd3As2 has one of the highest known bulk electron mobilities. This realization of a 3D Dirac semimetal in Cd3As2 not only opens a direct path to a wide spectrum of applications, but also offers a robust platform for engineering topologically-nontrivial phases including Weyl semimetals and Quantum Spin Hall systems.

cond-mat.mes-hall↗

arXiv2013

Stacked topological insulator built from bismuth-based graphene sheet analogues

Commonly materials are classified as either electrical conductors or insulators. The theoretical discovery of topological insulators (TIs) in 2005 has fundamentally challenged this dichotomy. In a TI, spin-orbit interaction generates a non-trivial topology of the electronic band-structure dictating that its bulk is perfectly insulating, while its surface is fully conducting. The first TI candidate material put forward -graphene- is of limited practical use since its weak spin-orbit interactions produce a band-gap of ~0.01K. Recent reinvestigation of Bi2Se3 and Bi2Te3, however, have firmly categorized these materials as strong three-dimensional TI's. We have synthesized the first bulk material belonging to an entirely different, weak, topological class, built from stacks of two-dimensional TI's: Bi14Rh3I9. Its Bi-Rh sheets are graphene analogs, but with a honeycomb net composed of RhBi8-cubes rather than carbon atoms. The strong bismuth-related spin-orbit interaction renders each graphene-like layer a TI with a 2400K band-gap.

cond-mat.mtrl-sci↗

arXiv2010

The absence of surface states for LiFeAs

We investigate the cleaving behavior of LiFeAs and determine its surface electronic structure by detailed density functional calculations. We show that due to the neutral surface of LiFeAs after cleaving, barely any influence of the surface on the electronic states is present. Therefore the data of surface sensitive probes such as angle resolved photoemission spectroscopy (ARPES) represent to a high degree the bulk electronic structure. This we highlighted by a direct comparison of the calculations to ARPES spectra.

cond-mat.supr-con↗

arXiv2006

Dressing of the charge carriers in high-Tc superconductors

In this contribution we review recent ARPES results on the dressing of the charge carriers in high-temperature superconductors. After an introduction into the cuprates and their electronic structure, ARPES, the spectral function and the self-energy function in the normal state and in the superconducting state is discussed.. Finally results on the dressing of the charge carriers in the system Bi2Sr2caCu2O7 at the nodal and the antinodal point are presented.

cond-mat.supr-con↗

arXiv2000

The electronic structure of cuprates from high energy spectroscopy

We report studies of the electronic structure and elementary excitations of doped and undoped cuprate chains, ladders and planes. Using high energy spectroscopies such as x-ray absorption, core level photoemission and angle resolved photoemission spectroscopy, important information regarding the charge distribution and hole dynamics can be obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations, and offers insight into the important physical quantities governing the electronic structure of these materials. Recurring themes include the importance of the dimensionality of the Cu-O network (1D->2D) and the crucial role played by the spin-background in determining the dynamics of low-lying excitations in these strongly correlated systems.

cond-mat.supr-con↗