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- ItemStrong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6(College Park, MD : APS, 2022) Spachmann, S.; Elghandour, A.; Selter, S.; Büchner, B.; Aswartham, S.; Klingeler, R.Cr2Ge2Te6 is a quasi-two-dimensional semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room-temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr2Ge2Te6 single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (∂TC/∂pc=24.7 K/GPa and ∂TC/∂pab=−15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr2Ge2Te6 up to room temperature. Moreover, unambiguous signs of short-range magnetic order up to 200 K are found.
- ItemMomentum dependent dxz/yz band splitting in LaFeAsO(Berlin : Springer Nature, 2020) Huh, S.S.; Kim, Y.S.; Kyung, W.S.; Jung, J.K.; Kappenberger, R.; Aswartham, S.; Büchner, B.; Ok, J.M.; Kim, J.S.; Dong, C.; Hu, J.P.; Cho, S.H.; Shen, D.W.; Denlinger, J.D.; Kim, Y.K.; Kim, C.The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the dxz and dyz hole bands near the Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.
- ItemSeparate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe(London : Nature Publishing Group, 2020) Baek, S.-H.; Ok, J.M.; Kim, J.S.; Aswartham, S.; Morozov, I.; Chareev, D.; Urata, T.; Tanigaki, K.; Tanabe, Y.; Büchner, B.; Efremov, D.V.The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.
- ItemCoupling of lattice, spin, and intraconfigurational excitations of Eu3+ in Eu2ZnIrO6(Washington, DC : American Association for the Advancement of Science, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, PradeepIn Eu2ZnIrO6, effectively two atoms are active; i.e., Ir is magnetically active, which results in complex magnetic ordering within the Ir sublattice at low temperature. On the other hand, although Eu is a Van Vleck paramagnet, it is active in the electronic channels involving 4f6 crystal-field split levels. Phonons, quanta of lattice vibration involving vibration of atoms in the unit cell, are intimately coupled with both magnetic and electronic degrees of freedom (DOF). Here, we report a comprehensive study focusing on the phonons as well as intraconfigurational excitations in double-perovskite Eu2ZnIrO6. Our studies reveal strong coupling of phonons with the underlying magnetic DOF reflected in the renormalization of the phonon self-energy parameters well above the spin-solid phase (TN∼12K) until temperature as high as ∼3TN evidences broken spin rotational symmetry deep into the paramagnetic phase. In particular, all the observed first-order phonon modes show softening of varying degree below ∼3TN, and low-frequency phonons become sharper, while the high-frequency phonons show broadening attributed to the additional available magnetic damping channels. We also observed a large number of high-energy modes, 39 in total, attributed to the electronic transitions between 4f levels of the rare-earth Eu3+ ion and these modes shows anomalous temperature evolution as well as mixing of the crystal-field split levels attributed to the strong coupling of electronic and lattice DOF.
- ItemStrain derivative of thermoelectric properties as a sensitive probe for nematicity([London] : Nature Publishing Group, 2021) Caglieris, F.; Wuttke, C.; Hong, C.; Sykora, S.; Kappenberger, R.; Aswartham, S.; Wurmehl, S.; Büchner, B.; Hess, C.The nematic instability is an undebatable ingredient of the physics of iron-based superconductors. Yet, its origin remains enigmatic as it involves a fermiology with an intricate interplay of lattice-, orbital-, and spin degrees of freedom. It is well known that thermoelectric transport is an excellent probe for revealing even subtle signatures of instabilities and pertinent fluctuations. In this paper, we report a strong response of the thermoelectric transport properties of two underdoped 1111 iron-based superconductors to a vanishingly small strain. By introducing the strain derivative of the Seebeck and the Nernst coefficients, we provide a description of the nematic order parameter, proving the existence of an anisotropic Peltier-tensor beside an anisotropic conductivity tensor. Our measurements reveal that the transport nematic phenomenology is the result of the combined effect of both an anisotropic scattering time and Fermi surface distortions, pointing out that in a realistic description, abreast of the spin fluctuations also the orbital character is a fundamental ingredient. In addition, we show that nematic fluctuations universally relax in a Curie–Weiss fashion above TS in all the elasto-transport measurements and we provide evidences that nematicity must be band selective.
- ItemIntertwined electronic and magnetic structure of the van-der-Waals antiferromagnet Fe2P2S6([London] : Nature Publishing Group, 2023) Koitzsch, A.; Klaproth, T.; Selter, S.; Shemerliuk, Y.; Aswartham, S.; Janson, O.; Büchner, B.; Knupfer, M.Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type MPS3 (M = V, Mn, Fe, Ni..), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and, especially for Fe2P2S6, giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of Fe2P2S6 using angle-resolved photoemission spectroscopy, q-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory. Fe2P2S6 is a Mott insulator with a gap of E gap ≈ 1.4 eV and zigzag antiferromagnetism below T N = 119 K. The low energy excitations are dominated by Fe 3d states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in Fe2P2S6 and related materials.
- ItemKitaev magnetism and fractionalized excitations in double perovskite Sm2ZnIrO6(College Park, ML : American Physical Society, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, PradeepThe quest for Kitaev spin liquids in particular three-dimensional solids is a long sought goal in condensed matter physics, as these states may give rise to exotic new types of quasiparticle excitations carrying fractional quantum numbers, namely Majorana fermionic excitations. Here we report the experimental signature of this characteristic feature of the Kitaev spin liquid via Raman measurements. Sm2ZnIrO6 is a strongly spin-orbit-coupled Mott insulator where Jeff=1/2 controls the physics, which provides striking evidence for this characteristic feature of the Kitaev spin liquid. As the temperature is lowered, we find that the spin excitations form a continuum in contrast to the conventional sharp modes expected in ordered antiferromagnets. Our observation of a broad magnetic continuum and anomalous renormalization of the phonon self-energy parameters shows the existence of fractionalization excitations in the double-perovskite structure, as theoretically conjectured in a Kitaev-Heisenberg geometrically frustrated double-perovskite system.
- ItemLinkage between scattering rates and superconductivity in doped ferropnictides(Woodbury, NY : Inst., 2021) Fink, J.; Rienks, E.D.L.; Yao, M.; Kurleto, R.; Bannies, J.; Aswartham, S.; Morozov, I.; Wurmehl, S.; Wolf, T.; Hardy, F.; Meingast, C.; Jeevan, H.S.; Maiwald, J.; Gegenwart, P.; Felser, C.; Buechner, B.We report an angle-resolved photoemission study of a series of hole- and electron-doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the inner hole pocket, which is the hot spot in these compounds. More specifically, we determine the energy (E)-dependent scattering rate Γ(E) as a function of the 3d count. Moreover, for the compounds K0.4Ba0.6Fe2As2 and BaCr2As2, we derive the energy dependence of the renormalization function Z(E) and the imaginary part of the self-energy function ImΣ(E). We obtain a non-Fermi liquidlike linear in energy scattering rate Γ(E≫kBT), independent of the dopant concentration. The main result is that the slope β=Γ(E≫kBT)/E reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Γ(E)/E≈1. This result, together with the energy dependence of the renormalization function Z(E), signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.
- ItemKramers doublets, phonons, crystal-field excitations, and their coupling in Nd2ZnIrO6(College Park, ML : American Physical Society, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, PradeepWe report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in a temperature range of 4–330 K, spanning a broad spectral range from 20 to 5500cm−1. The paper focuses on lattice vibrations and electronic transitions involving Kramers doublets of the rare-earth Nd3+ ion with local C1 site symmetry. Temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling between lattice and electronic degrees of freedom in Nd2ZnIrO6. Strong coupling between phonons and crystal-field excitation is observed via renormalization of the self-energy parameter of the phonons, i.e., peak frequency and linewidth. The phonon frequency shows abrupt hardening and linewidth narrowing below ∼100 K for the majority of the observed first-order phonons. We observed splitting of the lowest Kramers doublets of ground state (4I9/2) multiplets, i.e., lifting of the Kramers degeneracy, prominently at low temperature (below ∼100 K), attributed to the Nd-Nd/Ir exchange interactions and the intricate coupling with the lattice degrees of freedom. The observed splitting is of the order of ∼2–3 meV and is consistent with the estimated value. We also observed a large number of high-energy modes, 46 in total, attributed to the intraconfigurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution.