H. Hodovanets

Iowa State University, Ames, Iowa, United States

Are you H. Hodovanets?

Claim your profile

Publications (20)29.09 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigated the occurrence and nature of superconductivity in single crystals of YFe$_2$Ge$_2$ grown out of Sn flux by employing x-ray diffraction, electrical resistivity, and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as $\sim$60, by decanting the crystals from the molten Sn at $\sim$350$^\circ$C and/or by annealing at temperatures between 550$^\circ$C and 600$^\circ$C. We found that samples with RRR $\gtrsim$ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition $T_c\approx1.4$ K. RRR values vary between 35 and 65 with, on average, no systematic change in $T_c$ value, indicating that systematic changes in RRR do not lead to comparable changes in $T_c$. Specific heat measurements on samples that showed clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain stabilized superconductivity associated with small amounts of stressed YFe$_2$Ge$_2$ (perhaps at twin boundaries or dislocations) or is a second crystallographic phase present at levels below detection capability of conventional powder x-ray techniques.
    08/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report in-plane thermoelectric power measurements on single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (0.44 $\leq x \leq$ 1). We observe a minimum in the S$|_{T=const}$ versus x at x ~ 0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ~ 75 K. Thermoelectric power also shows a change in the x ~ 0.8 - 0.9 range, where maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the M point into four blades as observed by ARPES measurements.
    05/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present data on the anisotropic magnetic properties, heat capacity and transport properties of CeGe2-x (x = 0.24) single crystals. The electronic coefficient of the heat capacity, γ ∼ 110 mJ mol(-1) K(-2), is enhanced; three magnetic transitions, with critical temperatures of ≈7, ≈5 and ≈4 K are observed in thermodynamic and transport measurements. The ground state has a small ferromagnetic component along the c-axis. Small applied field, below 10 kOe, is enough to bring the material to an apparent saturated paramagnetic state (with no further metamagnetic transitions up to 55 kOe) with a reduced, below 1.2μB, saturated moment.
    Journal of Physics Condensed Matter 04/2014; 26(14):146005. · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Thorough resistivity measurements on single crystals of CeZn_{11} under pressure p and magnetic field H are presented. At ambient pressure, CeZn_{11} orders antiferromagnetically at T_{N}=2 K. The pressure dependence of the resistivity reveals an increase of the Kondo effect. We determine the pressure evolution of the magnetic exchange interaction between conduction and localized 4f electrons. It qualitatively reproduces the pressure evolution of the magnetic ordering temperature T_{O_{1}} (with T_{O_{1}}=T_{N} at ambient pressure). In addition to T_{O_{1}}, a new anomaly T_{O_{2}} appears under pressure. Both anomalies are found to increase with applied pressure up to 4.9 GPa, indicating that CeZn_{11} is far from a pressure induced quantum critical point. Complex T-H phase diagrams are obtained under pressure which reveal the instability of the ground state in this compound.
    Physical Review B 11/2013; 88(19). · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe1−xCox)2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02<x<0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and nonreconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical xc≈0.05, close to the highest superconducting Tc. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.
    Physical review. B, Condensed matter 06/2013; 87(22). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present detailed temperature and field dependent data obtained from magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric power measurements performed on single crystals of CeZn11. The compounds orders antiferromagnetically at $\sim$ 2 K. The zero-field resistivity and TEP data show features characteristic of a Ce-based intermetallic with crystal electric field splitting and possible correlated, Kondo lattice effects. We constructed the T-H phase diagram for the magnetic field applied along the easy, [110], direction which shows that the magnetic field required to suppress T_N below 0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data, H||[110], was observed only for H=45 kOe for 0.46 K<T<1.96 K followed by the Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe< H<60 kOe. From the analysis of our data, it appears that CeZn11 is a weakly to moderately correlated local moment compound with rather small Kondo temperature. The thermoelectric and transport properties of CeZn11 are mostly governed by the CEF effects. Given the very high quality of our single crystals, quantum oscillations are found for both CeZn11 and its non-magnetic analogue, LaZn11.
    Physical review. B, Condensed matter 04/2013; 88(5). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In most Ce-based intermetallic compounds, the magnetic exchange is assumed to be due to the RKKY interaction. This interaction competes with the Kondo interaction, leading to the suppression of the magnetic order and the possibility of field and/or pressure induced quantum criticality. In order to study this competition in CeZn11, a compound that orders antiferromagnetically below TN= 2 K, we performed electrical resistivity measurements on a single crystal of CeZn11 under pressure up to 5 GPa in a Bridgman pressure cell modified to use a liquid pressure transmitting medium (1:1 mixture of n-pentane: iso-pentane). TN(p) slightly increases and approaches a broad maximum in the studied pressure range. At ambient pressure, the antiferromagnetic order is suppressed by a magnetic field along the [1,1,0] direction of the tetragonal crystal structure. The temperature versus magnetic field phase diagram at 5 GPa will be compared to the one at ambient pressure.
    03/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe1−x Cox )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rhx )2As2 (0 ≤ x ≤ 0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe1−x Cox )2As2 delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0 ≤ x ≤ 0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0 ≤ x ≤ 0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe1−x Rhx )2As2 show very similar behavior to that of Co substituted BaFe2As2. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.
    Philosophical Magazine 01/2013; 93(6). · 1.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The influence of local moment magnetism on the boron isotope effect of T$_c$ was studied on single crystals of ErNi$_2$B$_2$C. Values of the partial isotope effect exponent of $\alpha_B$=0.10$\pm$0.02 and $\alpha_B$=0.10$\pm$0.04 were obtained based on two different criteria applied to extract $T_c$. No significant change in the partial isotope effect exponent compared to the ones obtained for LuNi$_2$B$_2$C was observed. Based on this result we conclude that pair-breaking due to the Er local magnetic moment appears to have no detectable influence on boron isotope effect of T$_c$.
    Philosophical Magazine A 03/2012; 93(14).
  • [Show abstract] [Hide abstract]
    ABSTRACT: Temperature-dependent, in-plane, thermoelectric power (TEP) data will be presented for single crystals of Ba(Fe1-xCox)2As2, (0<=x <=0.025). Previously reported TEP data for this system showed a big jump in the TEP data from x=0.02 to x=0.024 suggesting a Lifshitz transition, a result which was later confirmed by ARPES measurement. Given that TEP and ARPES delineated a rather large region for the Lifshitz transition to occur, and the underdoped side of the phase diagram is poorly explored, newly careful measurements of TEP on tightly spaced concentrations of Co, 0<=x <=0.025, were carried out. The data show clear evidence of a Lifshitz transition, but instead of a discontinuous jump in TEP between 0<=x <=0.025, there is a more gradual evolution in the S(T) plots as x is increased.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: At ambient pressure CeGe2-x has an antiferromagnetic (AFM) transition at TN˜7 K and, on further cooling, a second transition, to a weak ferromagnetic (WFM) state, at TC˜4 K. Under pressure the TN goes through a broad maximum, and then is completely suppressed with a critical point at ˜85 kbar, grossly following the Kondo-necklace picture. The WFM phase evolves into apparent AFM phase under moderate pressure of ˜10 kbar, with TN1 joining TN at ˜60 kbar. Resulting P - T phase diagram and comparison with CeRu2Ge2 under pressure will be presented.
    02/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this article we review our studies of the K0.80Fe1.76Se2 superconductor, with an attempt to elucidate the crystal growth details and basic physical properties over a wide range of temperatures and applied magnetic field, including anisotropic magnetic and electrical transport properties, thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer measurements. We find that: (i) Single crystals of similar stoichiometry can be grown both by furnace-cooled and decanted methods; (ii) Single crystalline K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and electrical resistivity and a small modulation of stoichiometry of the crystal, which gives rise to broadened transitions; (iii) The upper critical field, Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits magnetic order, probably of the first order, from well below Tc to its Neel temperature of Tn = 532 +/- 2 K. It is very important to note that, although, at first glance there is an apparent dilemma posed by these data: high Tc superconductivity in a near insulating, large ordered moment material, analysis indicates that the sample may well consist of two phases with the minority superconducting phase (that does not exhibit magnetic order) being finely distributed, but connected with in an antiferromagnetic, poorly conducting, matrix, essentially making a superconducting aerogel.
    01/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Single crystals of Ba(Fe1-xMnx)2As2, 0<x<0.148, have been grown and characterized by structural, magnetic, electrical transport, and thermopower measurements. Although growths of single crystals of Ba(Fe1-xMnx)2As2 for the full 0⩽x⩽1 range were made, we find evidence for phase separation (associated with some form of immiscibility) starting for x>0.1–0.2. Our measurements show that whereas the structural/magnetic phase transition found in pure BaFe2As2 at 134 K is initially suppressed by Mn substitution, superconductivity is not observed at any substitution level. Although the effect of hydrostatic pressure up to 20 kbar in the parent BaFe2As2 compound is to suppress the structural/magnetic transition at the approximate rate of 0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound are not cumulative. Phase diagrams of transition temperature versus substitution concentration x based on electrical transport, magnetization, and thermopower measurements have been constructed and compared to those of the Ba(Fe1-xTMx)2As2 (TM= Co and Cr) series.
    Phys. Rev. B. 10/2011; 84(14).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Single crystals of K0.80Fe1.76Se2.02 were suscessfully grown from a ternary solution. We show that although crystals form when cooling a near stoichiometric melt, crystals are actually growing out of a ternary solution that remains liquid to at least 850 C. We investigated their chemical composition, anisotropic magnetic susceptibility and resistivity, specific heat, thermoelectric power, London penetration depth and flux penetration via magneto-optical imaging. Whereas the samples appear to be homogeneously superconducting at low temperatures, there appears to be a broadened transtion range close to Tc ~ 30 K that may be associated with small variations in stociometry.
    Superconductor Science and Technology 02/2011; 24(6). · 2.76 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We used angle-resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe1−xCox)2As2 high-temperature superconductor. Our data demonstrate that several Lifshitz transitions—topological changes of the Fermi surface—occur for large x. The central hole barrel changes to ellipsoids that are centered at Z at x∼0.11 and subsequently disappear around x∼0.2; changes in thermoelectric power occur at similar x values. Tc decreases and goes to zero around x∼0.15—between the two Lifshitz transitions. Beyond x=0.2 the central pocket becomes electron-like and superconductivity does not exist. Our observations reveal the importance of the underlying Fermiology in electron-doped iron arsenides. We speculate that a likely necessary condition for superconductivity in these materials is the presence of the central hole pockets rather than nesting between central and corner pockets.
    Phys. Rev. B. 01/2011; 84(2).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We used angle-resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe1-xCox)2As2 high-temperature superconductor. Our data demonstrate that several Lifshitz transitions---topological changes of the Fermi surface---occur for large x. The central hole barrel changes to ellipsoids that are centered at Z at x˜0.11 and subsequently disappear around x˜0.2; changes in thermoelectric power occur at similar x values. Tc decreases and goes to zero around x˜0.15---between the two Lifshitz transitions. Beyond x=0.2 the central pocket becomes electron-like and superconductivity does not exist. Our observations reveal the importance of the underlying Fermiology in electron-doped iron arsenides. We speculate that a likely necessary condition for superconductivity in these materials is the presence of the central hole pockets rather than nesting between central and corner pockets.
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report effects of local magnetic moment, Gd^3+, doping (x
    Physical review. B, Condensed matter 11/2010; 82(17). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Temperature-dependent, in-plane, thermoelectric power (TEP) data are presented for Ba(Fe1-xRux)2As2 (0 < x < 0.36) single crystals. The previously outlined x - T phase diagram for this system is confirmed. The analysis of TEP evolution with Ru-doping suggests significant changes in the electronic structure, correlations and/or scattering occurring near ~7% and ~30% of Ru-doping levels. These results are compared with an extended set of TEP data for the electron-doped Ba(Fe1-xCox)2As2 series.
    Physical review. B, Condensed matter 10/2010; · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report effects of local magnetic moment, Gd3+, doping (x =< 0.3) on superconducting and magnetic properties of the closely related Lu(1-x)GdxNi2B2C and Y(1-x)GdxNi2B2C series. The superconducting transition temperature decreases and the heat capacity jump associated with it drops rapidly with Gd-doping; qualitative changes with doping are also observed in the temperature-dependent upper critical field behavior, and a region of coexistence of superconductivity and spin-glass state is delineated on the x - T phase diagram. The evolution of superconducting properties can be understood within Abrikosov-Gor'kov theory of magnetic impurities in superconductors taking into account the paramagnetic effect on upper critical field with additional contributions particular for the family under study.
    09/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Temperature-dependent, in-plane, thermoelectric power (TEP) data are presented for Ba(Fe1-xRux)2As2 (0⩽x⩽0.36) single crystals. The previously outlined x-T phase diagram for this system is confirmed. The analysis of TEP evolution with Ru doping suggests significant changes in the electronic structure, correlations, and/or scattering occurring near ~7%, ~30%, and possibly ~20% of Ru-doping levels. These results are compared with an extended set of TEP data for the electron-doped Ba(Fe1-xCox)2As2 series for which initial angle-resolved photoemission spectroscopy and transport studies have identified x~0.02 as the concentration at which the Lifshitz transition takes place. In addition to x~0.02 the Co levels of x≈0.11 and 0.22 are identified as concentrations at which similar changes occur.
    Phys. Rev. B. 83(9).