A. Günther

University of California, San Diego, San Diego, California, United States

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Publications (30)64.15 Total impact

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    ABSTRACT: We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.
    Scientific Reports 08/2014; 4:6079. · 5.08 Impact Factor
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    ABSTRACT: Magnetic properties as well as the specific heat of the spin-dimer system La2Ru1-yMnyO5 were investigated for manganese concentrations 0 \leq y \leq 0.25. The magnetic (DC-) susceptibility of the unsubstituted (y = 0) system shows a step-like decrease close to 160 K reflecting a magneto-structural transition into a dimerized ground state. With increasing manganese concentration this behavior (typical for singlet formation) becomes continuously suppressed and the susceptibility bears the signatures of the emergence of new magnetic ground states. The high-temperature Curie-Weiss susceptibility can be described by Ru^4+ (S = 1) and Mn^4+ (S = 3/2) spin moments, with a dramatic decrease of the Curie-Weiss temperatures by almost 30\% close to y = 0.1, indicating significant changes in the average mean magnetic exchange. Field cooled and zero-field cooled experiments as well as AC-susceptibility measurements provide clear evidence for the formation of a spin-glass state, well below the characteristic dimerization temperature. The relaxation dynamics can be described by a Vogel-Fulcher-Tammann behavior and indicates high fragility when characterized in terms of glassy dynamics of canonical super-cooled liquids. Additional electron-spin resonance experiments indicate different spin-glass regimes and a rather dynamic nature of the dimerized phase. In the Mn-substituted compounds a linear contribution to the heat capacity at low temperatures can be ascribed to the spin-glass formation. With increasing manganese concentration the anomaly in the specific heat caused by the spin-singlet formation is shifted to lower temperatures and becomes continuously suppressed and smeared out. On the basis of these results we propose a (y,T)-phase diagram indicating the competition of the spin-glass and the dimerized state. We stress the similarities with doped CuGeO3, the canonical inorganic spin-Peierls system.
    Physical Review B 07/2014; 90:024407. · 3.66 Impact Factor
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    ABSTRACT: We report results of a muon spin rotation and relaxation ($\mu$SR) study of dilute Pd$_{1-x}$Ni$_x$ alloys, with emphasis on Ni concentrations $x =$ 0.0243 and 0.025. These are close to the critical value $x_\mathrm{cr}$ for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar $\mu$SR properties. Both samples are fully magnetic, with average muon local fields $\langle B^\mathrm{loc}\rangle =$ 2.0 and 3.8 mT and Curie temperatures $T_C =$ 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at $T = 0$. The temperature dependence of $\langle B^\mathrm{loc}\rangle$ suggests ordering of Ni spin clusters rather than isolated spins. Just above $T_C$ a two-phase region is found with separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above $T_C$. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of $T_C$ extrapolate to $x_\mathrm{cr}$ = 0.0236 $\pm$ 0.0027. The dynamic muon spin relaxation in the vicinity of $T_C$ differs for the two samples: a relaxation-rate maximum at $T_C$ is observed for $x$ = 0.0243, reminiscent of critical slowing down, whereas for $x =$ 0.025 no dynamic relaxation is observed within the $\mu$SR time window. The data suggest a mean-field-like transition in this alloy.
    02/2014; 89(6).
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    ABSTRACT: We report on neutron diffraction, thermal expansion, dielectric and specific heat measurements on polycrystalline FeCr2S4 samples in external magnetic fields. The ferrimagnetic and orbital ordering temperatures, T_C = 170K and T_OO = 10 K are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the orbitally ordered state below T_OO. This contraction does not significantly change in external fields up to 7.5 T. The magnetic moments of the Cr and Fe ions are reduced from the spin-only values throughout the magnetically ordered regime. The moments start to increase in the orbitally ordered regime when the magnetic fields become larger than 4.5 T and approach the expected spin-only values above about 5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample due to magnetic domains below about 60 K. In the orbitally ordered state this contraction is followed by a moderate expansion of the sample for fields larger than about 4.5 T, coinciding with the onset of the increase of the magnetic moments. The orbital ordering transition is accompanied by an anomaly in the dielectric constant. Below T_OO the dielectric constant depends on both the strength of the external magnetic field as well as the orientation of the external magnetic field with respect to the applied electric field.
    09/2013;
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    ABSTRACT: La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru1-yTiyO5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.
    Journal of Physics Condensed Matter 02/2013; 25(12):126002. · 2.22 Impact Factor
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    ABSTRACT: Ultrasound and magnetization studies of the frustrated spinel ZnCr2Se4 are performed as a function of temperature and magnetic field up to 14 T. In zero field, the sound velocity and attenuation reveal significant anomalies at the antiferromagnetic transition at TN≈ 21 K indicating strong spin-lattice coupling. External magnetic fields shift these anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. At 2 K, the sound velocity as a function of magnetic field manifests three pronounced anomalies: a deep minimum at 5.4 T related to an inflection point of the magnetization followed by two plateaus with distinct stiffness at fields above 7 and 10 T. The first plateau is ascribed to a transformation from a tetragonal to a cubic phase, while the second one corresponds to a state with fully polarized magnetization. The evolution of magnetic and structural states is discussed within a H-T phase diagram and compared with related frustrated magnetic spinels with strong spin-lattice coupling.
    Physical review. B, Condensed matter 09/2012; 86(10). · 3.77 Impact Factor
  • Physical review. B, Condensed matter 07/2012; 86(3). · 3.77 Impact Factor
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    ABSTRACT: We report on the phonon spectrum of the spin-gapped quantum magnet Ba3Cr2O8 determined by infrared spectroscopy, and on specific heat measurements across the Jahn-Teller transition in magnetic fields up to 9 T. Phonon modes split below the Jahn-Teller transition, which occurs at TJT=70 K as detected by specific-heat measurements. The field-dependent specific heat data is analyzed in terms of the contributions from lattice, magnetic and orbital degrees of freedom. In contrast to the isostructural compound Sr3Cr2O8, our analysis does not indicate the existence of orbital fluctuations below the Jahn-Teller transition in Ba3Cr2O8.
    Physical review. B, Condensed matter 06/2012; 85(22). · 3.77 Impact Factor
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    ABSTRACT: We report on the phonon spectrum of Ba3Cr2O8 determined by infrared spectroscopy, and on specific heat measurements across the Jahn-Teller transition in magnetic fields up to 9 T. Phonon modes split below the Jahn-Teller transition, which occurs at T_{JT} = 70 K as detected by specific heat measurements. The field-dependent specific heat data is analyzed in terms of the contributions from lattice, magnetic and orbital degrees of freedom. In contrast to the isostructural compound Sr3Cr2O8 our analysis does not indicate the existence of orbital fluctuations below the Jahn-Teller transition in Ba3Cr2O8.
    05/2012;
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    ABSTRACT: Anisotropic magnetic properties of a layered kagome-like system Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At TN=27.4 K the system develops an alternating antiferromagnetic order of (ab) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro- and antiferro-magnetic intralayer exchange interactions. A magnetic field BC∼0.8 T applied along the c axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the b axis (∼7 T) or towards the a axis (∼15 T). The estimates of the exchange coupling constants and features indicative of an XY character of this quasi-2D system are presented.
    Physical review. B, Condensed matter 03/2012; 86(14). · 3.77 Impact Factor
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    ABSTRACT: We report on optical excitations observed recently in Sr3Cr2O8 by THz and infrared spectroscopy. Low-energy excitations below 3 THz are detected by THz time domain spectroscopy. These excitations can be divided into two different classes according to the temperature-dependent properties. One is emergent right below the Jahn-Teller transition temperature, which is determined by specific heat measurement to occur at 285 K [1, 2]. The other appears only below 100 K, where the fluctuations are sufficiently suppressed, consistent with the temperature dependence of low-energy Raman modes [3]. Infrared transmission measurements reveal a broad crystal-field excitation, which can be associated with an electronic transition from E to T2 orbital states. [4pt] [1] Zhe Wang et al., Phys. Rev. B 83, 201102 (2011) [0pt] [2] D. L. Quintero-Castro et al., Phys. Rev. B 81, 014415 (2010) [0pt] [3] D. Wulferding et al., Phys. Rev. B 84, 064419 (2011)
    02/2012;
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    ABSTRACT: We report on the structural, magnetic, and superconducting properties of Rb1-xFe2-ySe2 single crystals. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400 K. For 1.53 <2-y< 1.6 superconductivity is found, whereas for Fe concentrations 2-y< 1.5 and 2-y> 1.6 insulating and semiconducting behavior is observed, respectively. The sharpest transition to the superconducting state and the highest transition temperature Tc of 32.4 K is found for compositions close to Rb2Fe4Se5. Comparison of the magnetic behavior of non-superconducting and superconducting samples provides evidence for the coexistence of superconductivity and static antiferromagnetic order [1]. THz time-domain transmission spectroscopy in superconducting samples evidences a metallic response and the superconducting transition of the system [2].[4pt] [1] V. Tsurkan et al., Phys. Rev. B 84, 144520 (2011).[0pt] [2] A. Charnukha et al., arXiv:1108.5698.
    02/2012;
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    ABSTRACT: We report on magnetization and specific heat measurements on single-crystalline CuTe2O5. The experimental data are directly compared to theoretical results for two different spin structures, namely an alternating spin-chain and a two-dimensional (2D) coupled dimer model, obtained by Das et al. [Phys. Rev. B 77, 224437 (2008)]. While the analysis of the specific heat does not allow to distinguish between the two models, the magnetization data is in good agreement with the 2D coupled dimer model.
    Physics of Condensed Matter 11/2011; 84(3). · 1.28 Impact Factor
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    ABSTRACT: We report on the magnetic and phononic excitation spectrum of Sr$_3$Cr$_2$O$_8$ determined by THz and infrared (IR) spectroscopy, specific heat, and electron spin resonance measurements. We identify the singlet-triplet excitations in the dimerized ground state and observe an extended temperature range $T^{*}(=$ 125 K$)
    03/2011;
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    ABSTRACT: We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe0.5Te0.5 single crystals grown by self-flux and Bridgman methods. The lowest values of the susceptibility in the normal state, the highest transition temperature T c of 14.4 K, and the largest heat-capacity anomaly at T c were obtained for pure (oxygen-free) samples. The critical current density j c of 8.6 × 104 A/cm2 (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the anion sites. The samples containing an impurity phase of Fe3O4 show increased j c up to 2.3 × 105 A/cm2 due to additional pinning centers. The upper critical field H_{c2} of 500 kOe is estimated from the resistivity study in magnetic fields parallel to the c-axis using a criterion of a 50% drop of the normal state resistivity R n . The anisotropy of the upper critical field γ H c2 = H ab c2/ H c2 c reaches a value 6 at Tlongrightarrow T_c. Extremely low values of the residual Sommerfeld coefficient γ_r of about 1 mJ/mol K2, compared to the normal state Sommerfeld coefficient γ n = 25 mJ/mol K2 for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap Δ(0 K) = 27(1) K. A broad cusp-like anomaly in the electronic specific heat observed at low temperatures in samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the Fe2+ ions at the 2c sites. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.
    Physics of Condensed Matter 01/2011; 79:289-299. · 1.28 Impact Factor
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    ABSTRACT: We report on the optical excitation spectra in Sr2VO4. The phonon modes are assigned and their evolution with temperature is discussed in the frame of the different phase transitions crossed upon cooling. Besides the expected infrared-active phonons, we observe two additional excitations at about 290 and 840 cm-1, which could correspond to electronic transitions of the V4+ ions. Our experimental results are discussed in the context of recent experimental and theoretical studies of this material with a unique spin-orbital ground state.
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: We report the crystal growth and structural, magnetic, conductivity, and specific heat investigations of Rb1-xFe2-ySe2 single crystals with varying stoichiometry prepared by self-flux and Bridgman methods. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400 K. Bulk superconductivity is found in samples with Fe concentrations 1.53
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: We report on the magnetic and phononic excitation spectrum of Sr3Cr2O8 determined by THz and infrared (IR) spectroscopy, and electron spin resonance (ESR) measurements across the Jahn-Teller transition, which is detected by specific-heat measurements to occur at TJT=285 K. We identify the singlet-triplet excitations in the dimerized ground state and estimate the exchange couplings in the system. ESR absorptions were observed up to T*=120 K with a linewidth ∝exp(-Δ/kBT) and Δ/kB=388 K, indicating a phonon-mediated spin relaxation via the excited orbital state of the Cr e doublet in the orbitally ordered state. Upon entering the low-symmetry Jahn-Teller distorted phase below TJT, we find an extended regime T*<T<TJT, where the IR active phonons change only gradually with decreasing temperature. This regime is associated with strong fluctuations in the orbital and lattice degrees of freedom, in agreement with the loss of the ESR signal above T*. Using the measured magnetic and phononic excitation spectrum we model the orbital contribution to the specific heat and find the persistence of strong fluctuations far below TJT.
    Physical review. B, Condensed matter 11/2010; 83(20). · 3.77 Impact Factor
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    ABSTRACT: We report on structural, susceptibility, conductivity, and heat-capacity studies of FeSe0.5Te0.5 single crystals with 2% substitution of Mn for Fe. Mn-doped samples show a higher onset temperature, a narrower width of the superconducting transition, and a higher magnitude of the jump in the specific heat at Tc in compared to undoped samples. From the resistivity data in magnetic fields parallel to the c axis we derived an upper critical field Hc2 of ~580 kOe for doped samples compared to 490 kOe for pure samples. Using a single-band BCS model we can describe the electronic specific heat in the superconducting state with a gap Delta(T=0) = 31 K for the Mn doped sample in comparison to 26 K for pure FeSe0.5Te0.5.
    Superconductor Science and Technology 10/2010; 24(4). · 2.76 Impact Factor
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    ABSTRACT: We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe$_{0.5}$Te$_{0.5}$ single crystals grown by self-flux and Bridgman methods. The samples were prepared from starting materials of different purity at various temperatures and cooling rates. The lowest values of the susceptibility in the normal state, the highest transition temperature $T_c$ of 14.5 K, and the largest heat-capacity anomaly at $T_c$ were obtained for pure (oxygen-free) samples. The critical current density $j_c$ of $8 \times 10^4$ A/cm$^2$ (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the cation and anion sites. The impure samples show increased $j_c$ up to $2.3 \times 10^5$ A/cm$^2$ due to additional pinning centers of Fe$_3$O$_4$. The upper critical field $H_{c2}$ of $\sim 500$ kOe is estimated from the resistivity study in magnetic fields parallel to the \emph{c}-axis. The anisotropy of the upper critical field $\gamma_{H_{c2}} = H_{_{c2}}^{ab}/H_{_{c2}}^{c}$ reaches a value $\sim 6$ at $T\longrightarrow T_c$. Extremely low values of the residual Sommerfeld coefficient for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap $\Delta_0 = 27(1)$ K. A broad cusp-like anomaly in the electronic specific heat of samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the interstitial Fe$^{2+}$ ions. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.
    Physics of Condensed Matter 06/2010; 79(3):289-299. · 1.28 Impact Factor

Publication Stats

108 Citations
64.15 Total Impact Points

Institutions

  • 2014
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States
  • 2008–2014
    • Universität Augsburg
      • Institute of Physics
      Augsberg, Bavaria, Germany
  • 2010
    • Academy of Sciences of Moldova
      • Institute of Applied Physics
      Chişinău, Municipiul Chisinau, Moldova