arXiv subjects

B. Büchner

Publications and source records attributed to B. Büchner.

At least 73 records · Page 4Linked to original sources

arXiv2020

Coupling of lattice, spin and intra-configurational excitations of Eu3+ in Eu2ZnIrO6

In 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 4f 6 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 intra-configurational 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 ~ 12 K) till 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.

cond-mat.str-el↗

arXiv2020

Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnet CrCl$_3$

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

cond-mat.mtrl-sci↗

arXiv2020

Charge transfer energy in iridates: a hard x-ray photoelectron spectroscopy study

We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that regardless of the A and B cations, the A$_2$BIrO$_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge transfer energies. Hence, double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.

cond-mat.str-el↗

arXiv2020

Quasi-2D magnetic correlations in Ni$_2$P$_2$S$_6$ probed by ${}^{31}$P NMR

Detailed ${}^{31}$P nuclear magnetic resonance (NMR) measurements are presented on well-characterized single crystals of antiferromagnetic van der Waals Ni$_2$P$_2$S$_6$. An anomalous breakdown is observed in the proportionality of the NMR shift $K$ with the bulk susceptibility $χ$. This so-called $K$$-$$χ$ anomaly occurs in close proximity to the broad peak in $χ(T)$, thereby implying a connection to quasi-2D magnetic correlations known to be responsible for this maximum. Quantum chemistry calculations show that crystal field energy level depopulation effects cannot be responsible for the $K$$-$$χ$ anomaly. Appreciable in-plane transferred hyperfine coupling is observed, which is consistent with the proposed Ni$-$S$-$Ni super- and Ni$-$S$-$S$-$Ni super-super-exchange coupling mechanisms. Magnetization and spin$-$lattice relaxation rate ($T_1^{-1}$) measurements indicate little to no magnetic field dependence of the Néel temperature. Finally, $T_1^{-1}(T)$ evidences relaxation driven by three-magnon scattering in the antiferromagnetic state.

cond-mat.str-el↗

arXiv2020

The incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pmδ$), where $δ$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

cond-mat.mtrl-sci↗

arXiv2020

Momentum dependent $d_{xz/yz}$ band splitting in LaFeAsO

We performed angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure 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.

cond-mat.supr-con↗

arXiv2020

Kramer doublets, phonons, crystal-field excitations and their coupling in Nd2ZnIrO6

We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in temperature range of 4-330 K, and spanning a broad spectral range from 20 cm-1 to 5500 cm-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 quasi-particle excitations have 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 line-width. The phonon frequency shows abrupt hardening and line-width 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 intra-configurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution.

cond-mat.str-el↗

arXiv2019

Coherent spin dynamics of solitons in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Cl, Br)

We studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [P. Foury-Leylekian et al. Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. Such defects polarize the antiferromagnetically coupled spins in their vicinity, thereby leading to a finite local alternating magnetization around the defect site which can be described in terms of a soliton, i.e. a spin 1/2 quasiparticle built of many correlated spins, pinned to the defect. In addition, contributions of triplon excitations of the spin-dimerized state to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence.

cond-mat.str-el↗

arXiv2019

Electronic Structure Studies of FeSi: A Chiral Topological System

Most recent observation of topological Fermi arcs on the surface of manyfold degenerate B20 systems, CoSi and RhSi, have attracted enormous research interests. Although an another isostructural system, FeSi, has been predicted to show bulk chiral fermions, it is yet to be clear theoretically and as well experimentally that whether FeSi possesses the topological surface Fermi arcs associated with the exotic chiral fermions in vicinity of the Fermi level. In this contribution, using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), we present the low-energy electronic structure of FeSi. We further report the surface state calculations to provide insights into the surface band structure of FeSi near the Fermi level. Unlike in CoSi or RhSi, FeSi has no topological Fermi arcs near the Fermi level as confirmed both from ARPES and surface state calculations. Further, the ARPES data show spin-orbit coupling (SOC) band splitting of 40 meV, which is in good agreement with bulk band structure calculations. We noticed an anomalous temperature dependent resistivity in FeSi which can be understood through the electron-phonon interactions as we find a Debye energy of 80 meV from the ARPES data.

cond-mat.mtrl-sci↗

arXiv2019

Kitaev Magnetism and Fractionalized Excitations in Double Perovskite Sm2ZnIrO6

The quest for Kitaev spin liquids in particular three dimensional solids is long sought goal in condensed matter physics, as these states may give rise to exotic new types of quasi-particle 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 provide 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 evidence the existence of Majorana fermions from spin fractionalization in double perovskites structure as theoretically conjectured in a Kitaev-Heisenberg geometrically frustrated double perovskite systems.

cond-mat.str-el↗

arXiv2019

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $ΔT_c$ assisted by the triplet superconducting spin-valve effect $ΔT_c^{trip}$. In the second case, we observed a giant magnitude of $ΔT_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in Appendix. We find a good agreement between our experimental data and theoretical results.

cond-mat.supr-con↗

arXiv2019

Complex magnetic properties in the mixed 4f -5d double perovskite iridates Ln2ZnIrO6 (Ln = Nd, Sm, Eu & Gd)

In this work, we report on the synthesis and magnetic properties of a series of double perovskites Ln$_2$ZnIrO$_6$ with Ln = Nd, Sm, Eu & Gd. These compounds present new examples of the rare case of double perovskites (general formula A$_2$BB'O$_6$) with a magnetic 4f -ion on the A-site in combination with the strongly spin-orbit coupled 5d-transition metal ion Ir$^{4+}$ on the B-sublattice. We discuss the impact of different rare earths on the macroscopic magnetic properties. Gd$_2$ZnIrO$_6$ and Eu$_2$ZnIrO$_6$ show weak canted antiferromagnetic order below T$_N$ = 23 K and T$_N$ = 12 K, respectively. Sm$_2$ZnIrO$_6$ orders antiferromagnetically at T$_N$ = 13 K. Nd$_2$ZnIrO$_6$ exhibits complex magnetic properties with strong field dependence ranging from a two-step behavior at H = 0.01 T to an antiferromagnetic ground state at intermediate external fields and a spin-flop phase at H$\geq$4 T, which suggests complex interplay between Nd$^{3+}$ and Ir$^{4+}$ . To further shed light on this magnetic interaction, the magnetic structure of Nd$_2$ZnIrO$_6$'s ground state is examined via neutron powder diffraction.

cond-mat.str-el↗

arXiv2019

Investigation of the surface properties of different highly aligned N-MWCNT carpets

We investigated the physicochemical surface properties of different highly aligned nitrogen-doped multi-walled carbon nanotube (N-MWCNT) carpets, synthesized using toluene/pyrazine, toluene/benzylamine and acetonitrile via a sublimation-based chemical vapor deposition (SCVD) method at 760°C. The surfaces of the N-MWCNT carpets synthesized using toluene/pyrazine and toluene/benzylamine were very hydrophobic. In contrast, we observed a complete wetting of the N-MWCNT carpets synthesized using acetonitrile. The difference in the wetting behavior of these N-MWCNT carpets is the main focus in this study and was not investigated before. Here, we show that not only the presence or concentration of nitrogen inside the carbon lattice, but especially it's kind of incorporation have an important influence on the surface polarity.

cond-mat.mes-hall↗

arXiv2019

Disordered magnetism in the double perovskite LaCaScIrO6 with a distorted fcc lattice of Ir4+

The synthesis and characterization of the previously unknown material LaCaScIrO$_6$ is reported. LaCaScIrO$_6$ presents a new example of the rare case of a double perovskite with the strongly spin-orbit coupled 5\textit{d}-ion Ir$^{4+}$ as its only magnetic species, forming a monoclinically distorted version of the frustrated fcc lattice. Magnetization measurements show a weak anomaly at 8~K. The Curie-Weiss temperature Θ$_{CW}$ and effective magnetic moment μ$_{eff}$ of LaCaScIrO$_6$ are in close proximity to the related compound La$_2$MgIrO$_6$ but differ from La$_2$ZnIrO$_6$. This suggests that the nature of the non-magnetic \textit{B}-ion, namely its \textit{d}-orbital filling has a strong influence on the magnetic properties. The \textit{d}$^{0}$-ions Sc$^{3+}$ and Mg$^{2+}$ allow a different kind of exchange interactions within the Ir-sublattice than the \textit{d}$^{10}$-ion Zn$^{2+}$. In addition, ac-susceptibility data does not show signs of a spin-glass ground state. The nature of the magnetism in LaCaScIrO$_6$ has been further elucidated using muon spin relaxation measurements. The zero-field measurements reveal the absence of well defined oscillations down to 1.6\,K, while temperature dependent $μ$SR studies show an anomaly at 8\,K. Overall, our results suggest the presence of two different magnetic environments or domains in LaCaScIrO$_6$, which is likely related to its structural features.

cond-mat.str-el↗

arXiv2019

Strong Spin Resonance Mode associated with suppression of soft magnetic ordering in Hole-doped Ba1-xNaxFe2As2

Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state.

cond-mat.supr-con↗

arXiv2019

Superconducting switching due to triplet component in the Pb/Cu/Ni/Cu/Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ spin-valve structure

We report the superconducting properties of the Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$/Cu/Ni/Cu/Pb spin-valve structure which magnetic part comprises the Heusler alloy layer HA = Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ with a high degree of spin polarization (DSP) of the conduction band and the Ni layer of variable thickness. We obtained that the separation between the superconducting transition curves measured for the parallel ($α= 0^\circ$) and perpendicular ($α= 90^\circ$) orientation of the magnetizations of the HA and Ni layers reaches up to 0.5 K ($α$ is an angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature $T_c$ on $α$ demonstrates a deep minimum in the vicinity of the perpendicular configuration of magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of the superconducting spin-valve multilayer comprising a half-metallic ferromagnet recently proposed by A. Singh et al., Phys. Rev. X 5, 021019 (2015) in achieving large values of the switching effect.

cond-mat.supr-con↗

arXiv2019

Correlated Paramagnetism and Interplay of Magnetic and Phononic Degrees of Freedom in 3d-5d Coupled La2CuIrO6

Conventional Paramagnetism - a state with finite magnetic moment per ion sans long range magnetic ordering, but with lowering temperature the moment on each ion picks up a particular direction, breaking rotational symmetry, and results into long range magnetic ordering. However, in systems with competing multiple degrees of freedom this conventional notion may easily breaks and results into short range correlation much above the global magnetic transition temperature. La2CuIrO6 with complex interplay of spins (s =1/2) on Cu site and pseudo-spin (j =1/2) on Ir site owing to strong spin-orbit coupling provides fertile ground to observe such correlated phenomena. By a comprehensive temperature dependent Raman study, we have shown the presence of such a correlated paramagnetic state in La2CuIrO6 much above the long range magnetic ordering temperature (TN ). Our observation of strong interactions of phonons, associated with Cu/Ir octahedra, with underlying magnetic degrees of freedom mirrored in the observed Fano asymmetry, which remarkably persists as high as ~ 3.5TN clearly signals the existence of correlated paramagnetism hence broken rotational symmetry. Our detailed analysis also reveals anomalous changes in the self-energy parameters of the phonon modes, i.e. mode frequencies and linewidth, below TN, providing a useful gauge for monitoring the strong coupling between phonons and magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2019

Metal-chalcogen bond-length induced electronic phase transition from semiconductor to topological semimetal in ZrX$_2$ (X = Se and Te)

Using angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations we studied the low-energy electronic structure of bulk ZrTe$_2$. ARPES studies on ZrTe$_2$ demonstrate free charge carriers at the Fermi level, which is further confirmed by the DFT calculations. An equal number of hole and electron carrier density estimated from the ARPES data, points ZrTe$_2$ to a semimetal. The DFT calculations further suggest a band inversion between Te $p$ and Zr $d$ states at the $Γ$ point, hinting at the non-trivial band topology in ZrTe$_2$. Thus, our studies for the first time unambiguously demonstrate that ZrTe$_2$ is a topological semimetal. Also, a comparative band structure study is done on ZrSe$_2$ which shows a semiconducting nature of the electronic structure with an indirect band gap of 0.9 eV between $Γ(A) $ and $M (L)$ high symmetry points. In the below we show that the metal-chalcogen bond-length plays a critical role in the electronic phase transition from semiconductor to a topological semimetal ingoing from ZrSe$_2$ to ZrTe$_2$.

cond-mat.mtrl-sci↗