Y. A. Ovchenkov

Highly mobile carriers in iron-based superconductors

Y.A. Ovchenkov, ∗ D.A. Chareev, 3 V.A. Kulbachinskii, V.G. Kytin, D.E. Presnov, O.S. Volkova, 3, 5 and A.N. Vasiliev 3, 5 Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia Institute of Experimental Mineralogy, RAS, Chernogolovka, 123456, Russia Ural Federal University, 620002 Ekaterinburg, Russia Skobeltsyn Institute of Nuclear Physics, Moscow 119991, Russia National University of Science and Technology ’MISiS’ , Moscow 119049, Russia (Dated: August 19, 2016)

Magnetic anisotropy and the phase diagram of chiral MnSb2O6

The magnetic phase diagram and low-energy magnon excitations of structurally and magnetically chiral MnSb2O6 are reported. The specific heat and the static magnetization are investigated in magnetic fields up to 9 and 30 T, respectively, while the dynamicmagnetic properties are probed by X-band as well as tunable high-frequency electron spin-resonance spectroscopy. Below TN = 11.5 K, we observe antiferromagnetic resonance modes which imply small but finite planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are well described by means of an easy-plane two-sublattice model with the anisotropy field BA = 0.02 T. The exchange field BE = 13 T is obtained from the saturation field derived from the pulsed-field magnetization. A crucial role of the small anisotropy for the spin structure is reflected by competing antiferromagnetic phases appearing, at T = 2 K, in small magnetic fields at BC1

Vehement Competition of Multiple Superexchange Interactions and Peculiar Magnetically Disordered State in Cu(OH)F

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Scanning tunneling microscopy study of morphology and electronic properties in (K0.7Na0.3)Fe2−ySe2 single crystal

0.5 T and BC2 = 0.9 T. We discuss the results in terms of spin reorientation and of small magnetic fields favoring helical spin structure over the cycloidal ground state which, at B = 0, is stabilized by the planar anisotropy. Above TN, short-range magnetic correlations up to >=60 K and magnetic entropy changes well above TN reflect the frustrated triangular arrangement of Mn2+ ions in MnSb2O6.

Single crystal growth, transport and scanning tunneling microscopy and spectroscopy of FeSe1−xSx

The mixed anion copper compound Cu(OH)F was studied in measurements of dc- and ac-magnetic susceptibility, static and pulsed field magnetization, specific heat, X-band electron magnetic resonance and muon-spin spectroscopy. In variance with its layered structure, the magnetic behavior shows no evidence of low-dimensionality. Cu(OH)F reaches short range static antiferromagnetic order at TN = 9.5–11.5 K and experiences the spin-flop transition at B ∼ 3.5 T. This behavior is in a sharp contrast with physical properties of earlier reported isostructural compound Cu(OH)Cl. The first principle calculations reveal highly competitive nature of ferromagnetic and antiferromagnetic superexchange interactions, the details being rather sensitive to choice of magnetic structure employed in the extraction of magnetic interaction. Rather broad anomaly in Cp(T) dependence at phase transition and smeared magnetization curve M(B) at low temperatures suggest static disorder at low temperatures however no frequency dependence...

The influence of titanium substitution on the magnetic, magnetocaloric, and magnetoelastic properties of Gd5Si2Ge2

We investigated the microstructure of the iron selenide superconductor (K0.7Na0.3)Fe2−ySe2 with a Tc = 32 K and a near 100% Meissner screening volume fraction. Topography and electron transport properties were studied using electron microscopy and ultra-high vacuum scanning tunneling microscopy (STM) techniques. Room temperature STM measurements reliably identify spatial variations of the local electronic properties of this material. The studied crystals consist of continuous regions with significantly different shapes of current-voltage curves reflecting different electronic transport properties of these regions. Fitting of the local current-voltage curves with the Simmons model for metal-dielectric-metal structure confirmed a phase separation in the sample to a metal and semiconducting phases. The observed regions have dimensions in the range of several tenths of a micrometer and indicate a phase separation in the sample.

Magnetotransport properties of FeSe in fields up to 50 T

Single crystals of sulfur-substituted iron selenide, FeSe1−xSx, were grown within eutectics of molten halides, AlCl3/KCl, AlCl3/KCl/NaCl or AlCl3/KBr, under permanent temperature gradient. The innovative “ampoule in ampoule” design of a crystallization vessel allows obtaining mm-sized plate-like single crystals with a sulfur content up to x ∼ 0.19. The sharp anomalies in the physical properties indicate the superconducting and nematic phase transitions in FeSe0.96 at TC = 8.4 K and TN = 90 K, respectively. Scanning tunneling microscopy reveals the presence of dumbbell defects associated with Fe vacancies and dark defects at the chalcogen site associated with S within the FeSe1−xSx series of compounds. Scanning tunneling spectroscopy shows the presence of two different superconducting gaps at both hole and electron pockets of the Fermi surface for low S content levels. As a function of sulfur content, TC follows the conventional dome-shaped curve while TN decreases with x. The overall appearance of the T–x phase diagram of FeSe1−xSx suggests the importance of nematic fluctuations for the formation of the superconducting state in these compounds.

Zigzag antiferromagnetic quantum ground state in monoclinic honeycomb lattice antimonates A3Ni2SbO6 (A=Li, Na)

The magnetic, magnetocaloric, thermal expansion, magnetostriction, and heat capacity measurements of Gd5−xTixSi2Ge2 (x = 0, 0.05, 0.1) compounds have been carried out in a wide range of fields and temperatures. The Ti addition was found to increase the temperature of magnetic ordering and change the order of phase transition with preservation of high magnetocaloric effect values. At the same time, the large temperature and field hysteresis, observed for ΔT-effect in Gd5Si2Ge2, significantly decrease when Ti is added. The obtained results can be explained by the decoupling of magnetic and structural phase transition with Ti substitution. It is also shown that the magnetoelastic energy has a the strong influence on the type of phase transition for investigated compounds.The magnetic, magnetocaloric, thermal expansion, magnetostriction, and heat capacity measurements of Gd5−xTixSi2Ge2 (x = 0, 0.05, 0.1) compounds have been carried out in a wide range of fields and temperatures. The Ti addition was found to increase the temperature of magnetic ordering and change the order of phase transition with preservation of high magnetocaloric effect values. At the same time, the large temperature and field hysteresis, observed for ΔT-effect in Gd5Si2Ge2, significantly decrease when Ti is added. The obtained results can be explained by the decoupling of magnetic and structural phase transition with Ti substitution. It is also shown that the magnetoelastic energy has a the strong influence on the type of phase transition for investigated compounds.

Coexistence of superconductivity and magnetism in Fe1+δ Te1−xSex (x = 0.1, 0.2, 0.28, 0.4 and 0.45)

Abstract A study of the magnetotransport properties of a high-quality FeSe crystal in a wide temperature range and in magnetic fields up to 50 T shows that the main electron-like and hole-like bands have very similar values of carrier density and mobility, indicating good electron-hole symmetry in this compound. In addition to the main two bands, there is also a tiny, highly mobile, electron-like band which is responsible for the non-linear behavior of ρ xy ( B ) at low temperatures and some other peculiarities of FeSe. We observe the inversion of the ρ xx temperature coefficient at a magnetic field higher than about 20 T which is an implicit confirmation of the electron-hole symmetry in the main bands.