Sergey L. Bud’ko

Pressure Induced Superconductivity inCaFe2As2

CaFe2As2 has been found to be exceptionally sensitive to the application of hydrostatic pressure and can be tuned to reveal all the salient features associated with FeAs superconductivity without introducing any disorder. The ambient pressure, 170 K, structural/magnetic, first-order phase transition is suppressed to 128 K by 3.5 kbar. At 5.5 kbar a new transition is detected at 104 K, increasing to above 300 K by 19 kbar. A low temperature, superconducting dome (T(c) approximately 12 K) is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity too dramatically.

Temperature-Induced Lifshitz Transition in WTe2.

We use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the X-Γ-X direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at T≃160  K, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potential μ(T) due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at T≈175  K and a breakdown of Kohlers rule in the 70-140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.

Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor

Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in most Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors, (π,0) and (0,π). Consequently, three different types of magnetic order are possible. Of these, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-doped and Co-doped CaKFe4As4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe4As4 structure. Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.Iron-based superconductors: making a hedgehog spin-vortex crystalThe magnetic texture of a new superconductor adopts a in-out spin, spin-vortex crystal motif, fulfilling theoretical predictions. Many iron-based superconductors have magnetic phases arising from combining two basic magnetic structures, but only two of three possible combinations had previously been observed. A team led by Paul Canfield of Iowa State University and Ames Laboratory have synthesised a material with the third type of magnetic structure called a hedgehog spin-vortex crystal. The authors began with a compound with spatial symmetry that could help stabilise the structure, but without magnetic order. By tuning the chemical composition they induced magnetism and successfully obtained the desired phase. The sensitivity of the magnetic state to the symmetry and composition indicates that different phases are energetically close, suggesting magnetic fluctuations may play a significant role in the physics of iron-based superconductors.

Mössbauer studies of Ba(Fe1-xNix)2As2

We present detailed (57)Fe Mössbauer effect spectroscopy (MS) measurements at various temperatures for Ba(Fe(1-x)Ni(x))(2)As(2). The isomer shift values for all samples are in the range of 0.42 ± 0.02 mm s(-1), indicating a typical metallic state of divalent Fe ions. The MS spectra of the magnetic samples (up to x = 0.024) are well reproduced by spin density waves subspectra. Both the magnetic ordering temperatures T(M) and the average magnetic hyperfine fields H(eff), decrease with x. The H(eff) values scale linearly with T(M). For higher x values the samples become superconducting and the MS spectra below and above T(C) are almost identical, indicating that the MS technique are not sensitive enough to the superconducting transition.

Contactless measurements of Shubnikov-de Haas oscillations in the magnetically ordered state of CeAgSb2 and SmAgSb2 single crystals

Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic

Tricritical wings and modulated magnetic phases in LaCrGe3 under pressure

\mathrm{Sm}\mathrm{Ag}{\mathrm{Sb}}_{2}

57Fe Mössbauer study of stoichiometric iron-based superconductor CaKFe4As4: a comparison to KFe2As2 and CaFe2As2

and ferromagnetic

Ce3-xMgxCo9: Transformation of a Pauli Paramagnet into a Strong Permanent Magnet

\mathrm{Ce}\mathrm{Ag}{\mathrm{Sb}}_{2}

Collapsed tetragonal phase transition in LaRu2P2

using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of

Extremely large magnetoresistance and Kohler's rule in PdSn4 : A complete study of thermodynamic, transport, and band-structure properties

\ensuremath{\Delta}\ensuremath{\rho}\ensuremath{\simeq}20\phantom{\rule{0.3em}{0ex}}\mathrm{p}\ensuremath{\Omega}