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D. V. Evtushinsky

Publications and source records attributed to D. V. Evtushinsky.

At least 37 records · Page 2Linked to original sources
arXiv2011

Electronic band structure of ferro-pnictide superconductors from ARPES experiment

ARPES experiments on iron based superconductors show that the differences between the measured and calculated electronic band structures look insignificant but can be crucial for understanding of the mechanism of high temperature superconductivity. Here we focus on those differences for 111 and 122 compounds and discuss the observed correlation of the experimental band structure with the superconductivity.

cond-mat.supr-con↗
arXiv2011

Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering

LiFeAs is unique among the broad family of FeAs-based superconductors, because it is superconducting with a rather large $T_c\simeq 18$ K under ambient conditions although it is a stoichiometric compound. We studied the electrical transport on a high-quality single crystal. The resistivity shows quadratic temperature dependence at low temperature giving evidence for strong electron-electron scattering and a tendency towards saturation around room temperature. The Hall constant is negative and changes with temperature, what most probably arises from a van Hove singularity close to the Fermi energy in one of the hole-like bands. Using band structure calculations based on angular resolved photoemission spectra we are able to reproduce all the basic features of both the resistivity as well as the Hall effect data.

cond-mat.supr-con↗
arXiv2011

Fusion of bogoliubons in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and similarity of energy scales in high temperature superconductors

Usually the superconducting pairing is considered to modify electronic states only in a narrow momentum range close to the Fermi surface. Here we present a direct experimental observation of fusion of Bogoliubov dispersion branches originating from the antipodal Fermi crossings by means of angle-resolved photoemission spectroscopy (ARPES). Uncommon discernibility and brightness of bogoliubons' fusion stems from comparability of the superconducting gap magnitude and the distance from the Fermi level to the band's top, and strong electron scattering on a mode with similar energy. Such similarity of the electronic and pairing energy scales seems to be a persistent associate of high-temperature superconductivity (HTSC) rather than just a mere coincidence.

cond-mat.supr-con↗
arXiv2011

Evidence for conventional superconductivity in SrPd2Ge2

Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band-structure calculations within the local-density approximation (LDA). The STS measurements show single s-wave superconducting energy gap Δ(0) = 0.5 meV. Photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the "122" family.

cond-mat.supr-con↗
arXiv2011

Surface and bulk electronic structure of unconventional superconductor Sr_2RuO_4: unusual splitting of the beta-band

We present an angle resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr$_2$RuO$_4$. As the early studies of its electronic structure by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface aging was previously proposed as a possible remedy to access the bulk states. Here we suggest an alternative way by demonstrating that, in the case of Sr$_2$RuO$_4$, circularly polarised light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface $β$ band. We propose that spin--orbit splitting might be responsible for this, and discuss possible relations of the newly observed surface feature to topological matter.

cond-mat.supr-con↗
arXiv2011

Photoemission induced gating of topological insulator

The recently discovered topological insulators exhibit topologically protected metallic surface states which are interesting from the fundamental point of view and could be useful for various applications if an appropriate electronic gating can be realized. Our photoemission study of Cu intercalated Bi2Se3 shows that the surface states occupancy in this material can be tuned by changing the photon energy and understood as a photoemission induced gating effect. Our finding provides an effective tool to investigate the new physics coming from the topological surface states and suggests the intercalation as a recipe for synthesis of the material suitable for electronic applications.

cond-mat.mes-hall↗
arXiv2010

The superconducting gaps in LiFeAs: Joint study of specific heat and ARPES

We present specific heat, c_P, and ARPES data on single crystals of the stoichiometric superconductor LiFeAs. A pronounced anomaly is found in c_P at the superconducting transition. The electronic contribution can be described by two s-type energy gaps with magnitudes of approximately Delta1 = 1.2 meV and Delta2 = 2.6 meV and a normal-state gamma coefficient of 10 mJ/mol K^2. All these values are in remarkable agreement with ARPES results.

cond-mat.supr-con↗
arXiv2010

An ARPES view on the high-Tc problem: phonons vs spin-fluctuations

We review the search for a mediator of high-Tc superconductivity focusing on ARPES experiment. In case of HTSC cuprates, we summarize and discuss a consistent view of electronic interactions that provides natural explanation of both the origin of the pseudogap state and the mechanism for high temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. On the other hand, a similar analysis being applied to the iron pnictides reveals especially strong electron-phonon coupling that suggests important role of phonons for high-Tc superconductivity in pnictides.

cond-mat.supr-con↗
arXiv2010

Time of life as it is in LiFeAs

The time of life of fermionic quasiparticles, the distribution of which in the momentum-energy space can be measured by angle resolved photoemission (ARPES), is the first quantity to look for fingerprints of interaction responsible for the superconducting pairing. Such an approach has been recently used for superconducting cuprates, but its direct application to pnictides was not possible due to essential three-dimensionality of the electronic band structure and magnetic ordering. Here, we report the investigation of the quasiparticle lifetime in LiFeAs, a non-magnetic stoichiometric superconductor with a well separated two-dimensional band. We have found two energy scales: the lower one contains clear fingerprints of optical phonon modes while the higher scale indicates a presence of strong electron-electron interaction. The result suggests that LiFeAs is a phonon mediated superconductor with strongly enhanced electronic density of states at the Fermi level.

cond-mat.supr-con↗
arXiv2010

Weak superconducting pairing and a single isotropic energy gap in stoichiometric LiFeAs

We report superconducting (SC) properties of stoichiometric LiFeAs (Tc = 17 K) studied by small-angle neutron scattering (SANS) and angle-resolved photoemission (ARPES). Although the vortex lattice exhibits no long-range order, well-defined SANS rocking curves indicate better ordering than in chemically doped 122-compounds. The London penetration depth of 210 nm, determined from the magnetic field dependence of the form factor, is compared to that calculated from the ARPES band structure with no adjustable parameters. Its temperature dependence is best described by a single isotropic SC gap of 3.0 meV, which agrees with the ARPES value of 3.1 meV and corresponds to the ratio 2Delta/kTc = 4.1, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor, similar to conventional metals.

cond-mat.supr-con↗
arXiv2010

Superconductivity without magnetism in LiFeAs

The particular shape of the Fermi surface can give rise to a number of collective quantum phenomena in solids, such as density wave orderings or even superconductivity. In many new iron superconductors this shape, the 'nested' Fermi surface, is indeed observed, but its role in the formation of spin-density waves or superconductivity is not clear. We have studied the electronic structure of the non-magnetic LiFeAs (Tc~18K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of three, high density of states at the Fermi level caused by a Van Hove singularity, and no evidence for either a static or fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations set a new hierarchy of the electronic properties necessary for the superconductivity in iron pnictides and, possibly, in other materials.

cond-mat.supr-con↗
arXiv2009

Fermi surface of Ba{1-x}KxFe2As2 as probed by angle-resolved photoemission

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba1-xKxFe2As2 (Tc = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in G-X and G-Y directions.

cond-mat.supr-con↗
arXiv2009

Two-gap superconductivity in Ba_1-xK_xFe_2As_2: A complementary study of the magnetic penetration depth by \muSR and ARPES

We investigate the magnetic penetration depth λin superconducting Ba_1-xK_xFe_2As_2 (T_c\simeq32K) with muon-spin rotation (\muSR) and angle-resolved photoemission (ARPES). Using \muSR, we find the penetration-depth anisotropy γ_λ=λ_c/λ_{ab} and the second-critical-field anisotropy γ_{H_c2} to show an opposite T-evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB_2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth λ_ab around 7K. The complementarity of \muSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T-evolution of λ_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.

cond-mat.supr-con↗
arXiv2009

Momentum-resolved superconducting gap in the bulk of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ from combined ARPES and $μ$SR measurements

Here we present a calculation of the temperature-dependent London penetration depth, $λ(T)$, in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (BKFA) on the basis of the electronic band structure [1,2] and momentum-dependent superconducting gap [3] extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of $λ(T)$ by muon spin rotation ($μ$SR) [4]. The value of $λ(T=0)$, calculated with \emph{no} adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence $λ(T)$ is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new iron-based superconductors in general allows us to state that all hole-doped of them bear two nearly isotropic gaps with coupling constants $2Δ/k_{\rm B}T_{\rm c}=2.5\pm1.5$ and $7\pm2$.

cond-mat.supr-con↗
arXiv2009

Momentum dependence of the superconducting gap in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor with Tc = 32K (BKFA) was determined from angle-resolved photoemission spectroscopy (ARPES) via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner Gamma-barrel, X-pocket, and "blade"-pocket, and equals 9 meV, while the gap on the outer Gamma-barrel is estimated to be less than 4 meV, resulting in 2Delta/kT_c=6.8 for the large gap, and 2Delta/kT_c<3 for the small gap. A large (77 \pm 3%) non-superconducting component in the photoemission signal is observed below T_c. Details of gap extraction from ARPES data are discussed in Appendix.

cond-mat.supr-con↗
arXiv2009

Excitation energy map of the high-energy dispersion anomalies in cuprates

The anomalous high-energy dispersion of the conductance band in the high-Tc superconductor Pb-Bi2212 has been extensively mapped by angle-resolved photoemission (ARPES) as a function of excitation energy in the range from 34 to 116 eV. Two distinctive types of dispersion behavior are observed around 0.6 eV binding energy, which alternate as a function of photon energy. The continuous transitions observed between the two kinds of behavior near 50, 70, and 90 eV photon energies allow to exclude the possibility that they originate from the interplay between the bonding and antibonding bands. The effects of three-dimensionality can also be excluded as a possible origin of the excitation energy dependence, as the large period of the alterations is inconsistent with the lattice constant in this material. We therefore confirm that the strong photon energy dependence of the high-energy dispersion in cuprates originates mainly from the photoemission matrix element that suppresses the photocurrent in the center of the Brillouin zone.

cond-mat.supr-con↗