arXiv subjects

A. A. Gippius

Publications and source records attributed to A. A. Gippius.

12 recordsLinked to original sources

arXiv2025

Two-step phase transitions in Fe(Se,Te)

In the studied crystals of FeSe0.7 Te0.3 , a structural phase transition occurs in two stages. At higher temperatures, the electronic subsystem undergoes a reconstruction, leading to a significant increase in elastoresistance. 77 Se NMR data show an abrupt change in the relaxation rate during this transition. The final transition occurs at a temperature several degrees below and is also accompanied by anomalies in the electronic properties. Thus, in the Fe(Se,Te) series, similarly to the behavior of pure FeSe under pressure, the type of transition changes and intermediate state appear before the structural transition is suppressed. This similarity between the corresponding phase diagrams is explained by the same deformation of the iron coordination environment in Fe(Se,Te) compounds and in FeSe under pressure. Our findings provide new and significant information on the phase diagram of Fe(Se,Te) compounds and in particular suggest the possible existence of a triple point near the quantum critical point.

cond-mat.supr-con↗

arXiv2020

NMR study of magnetic structure and hyperfine interactions in binary helimagnet FeP

We report a detailed study of the ground state helical magnetic structure in monophosphide FeP by means of ${}^{31}$P NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by Mössbauer spectroscopy. From field-sweep ${}^{31}$P NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4 - 7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ${}^{31}$P NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ${}^{31}$P spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the ($ab$)-plane with a phase shift of 36$^{\circ}$ and 176$^{\circ}$ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.

cond-mat.str-el↗

arXiv2019

Structural, thermodynamic, and local probe investigations of a honeycomb material Ag$_{3}$LiMn$_{2}$O$_{6}$

The system Ag[Li$_{1/3}$Mn$_{2/3}$]O$_{2}$ belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group $C\,2/m$ and the magnetic Mn$^{4+}$($S=3/2$) ions form a honeycomb network in the $ab$-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature $θ_{CW}\sim-51$ K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48\,K and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. The (ESR) line broadening on approaching the ordering temperature $T_{\rm N}$ could be described in terms of a Berezinski-Kosterlitz-Thouless (BKT) scenario with $T_{\rm KT}=40(1)$ K. $^{7}$Li NMR line-shift probed as a function of temperature tracks the static susceptibility (K$_{iso}$) of magnetically coupled Mn$^{4+}$ ions. The $^{7}$Li spin-lattice relaxation rate (1/$T$$_{1}$) exhibits a sharp decrease below about 50 K. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag$_{3}$LiMn$_{2}$O$_{6}$.Our ab-initio electronic structure calculations suggest that in the $ab$-plane, the nearest neighbor (NN) exchange interaction is strong and AFM, while the next NN and the third NN exchange interactions are FM and AFM respectively. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.

cond-mat.str-el↗

arXiv2019

Unconventional magnetism in the 4d$^{4}$ based ($S=1$) honeycomb system Ag$_{3}$LiRu$_{2}$O$_{6}$

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($θ_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S = 1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $μ$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.

cond-mat.str-el↗

arXiv2018

Antiferromagnetic ground state in the MnGa$_4$ intermetallic compound

Magnetism of the binary intermetallic compound MnGa$_4$ is re-investigated. Band-structure calculations predict antiferromagnetic behavior in contrast to Pauli paramagnetism reported previously. Magnetic susceptibility measurements on single crystals indeed reveal an antiferromagnetic transition at $T_N=393$ K. Neutron powder diffraction and $^{69,71}$Ga nuclear quadrupole resonance spectroscopy show collinear antiferromagnetic order with magnetic moments alligned along the [111] direction of the cubic unit cell. The magnetic moment of 0.80(3)$μ_B$ at 1.5 K extracted from the neutron data is in good agreement with the band-structure results.

cond-mat.mtrl-sci↗

arXiv2018

Exotic phases of frustrated antiferromagnet LiCu2O2

7Li NMR spectra were measured in a magnetic field up to 17 T at temperatures 5-30 K on single crystalline LiCu2O2. Earlier reported anomalies on magnetization curves correspond to magnetic field values where we observe changes of the NMR spectral shape. For the interpretation of the field and temperature evolutions of our NMR spectra, the magnetic structures were analyzed in the frame of the phenomenological theoretical approach of the Dzyaloshinskii-Landau theory. A set of possible planar and collinear structures was obtained. Most of these structures have an unusual configuration; they are characterized by a two-component order parameter and their magnetic moments vary harmonically not only in direction, but also in size. From the modeling of the observed spectra, a possible scenario of magnetic structure transformations is obtained.

cond-mat.str-el↗

arXiv2017

121,123Sb NQR as a microscopic probe in Te doped correlated semimetal FeSb2 : emergence of electronic Griffith phase, magnetism and metallic behavior %

$^{121,123}Sb$ nuclear quadrupole resonance (NQR) was applied to $Fe(Sb_{1-x}Te_x)_2$ in the low doping regime (\emph{x = 0, 0.01} and \emph{0.05}) as a microscopic zero field probe to study the evolution of \emph{3d} magnetism and the emergence of metallic behavior. Whereas the NQR spectra itself reflects the degree of local disorder via the width of the individual NQR lines, the spin lattice relaxation rate (SLRR) $1/T_1(T)$ probes the fluctuations at the $Sb$ - site. The fluctuations originate either from conduction electrons or from magnetic moments. In contrast to the semi metal $FeSb_2$ with a clear signature of the charge and spin gap formation in $1/T_1(T)T ( \sim exp/ (Δk_BT) ) $, the 1\% $Te$ doped system exhibits almost metallic conductivity and a almost filled gap. A weak divergence of the SLRR coefficient $1/T_1(T)T \sim T^{-n} \sim T^{-0.2}$ points towards the presence of electronic correlations towards low temperatures wheras the \textit{5\%} $Te$ doped sample exhibits a much larger divergence in the SLRR coefficient showing $1/T_1(T)T \sim T^{-0.72} $. According to the specific heat divergence a power law with $n\ =\ 2\ m\ =\ 0.56$ is expected for the SLRR. Furthermore $Te$-doped $FeSb_2$ as a disordered paramagnetic metal might be a platform for the electronic Griffith phase scenario. NQR evidences a substantial asymmetric broadening of the $^{121,123}Sb$ NQR spectrum for the \emph{5\%} sample. This has purely electronic origin in agreement with the electronic Griffith phase and stems probably from an enhanced $Sb$-$Te$ bond polarization and electronic density shift towards the $Te$ atom inside $Sb$-$Te$ dumbbell.

cond-mat.str-el↗

arXiv2013

Interplay between localized and itinerant magnetism in Co substituted FeGa$_3$

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe$_{1-x}$Co$_x$Ga$_3$ was studied by means of electrical resistivity, magnetization, ab-initio band structure calculations, and nuclear spin-lattice relaxation $1/T_1$ of the $^{69,71}$Ga nuclei. Temperature dependencies of the electrical resistivity reveal that the evolution from the semiconducting to the metallic state in the Fe$_{1-x}$Co$_x$Ga$_3$ system occurs at $0.025<x<0.075$. The $^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of $2\!-\!300$ K for the concentrations $x = 0.0,$ $0.5,$ and $1.0$. In the parent semiconducting compound FeGa$_3$, the temperature dependence of the $^{69}(1/T_1)$ exhibits a huge maximum at about $T\!\sim\!6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa$_3$, demonstrates a metallic Korringa behavior with $1/T_1$ $\propto T$. In Fe$_{0.5}$Co$_{0.5}$Ga$_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with both magnetization measurements, showing localized magnetism with a relatively low effective moment of about 0.7 $μ_B$/f.u., and ab initio band structure calculations, where a ferromagnetic state with an ordered moment of 0.5 $μ_B$/f.u. is predicted. The results are discussed in terms of the interplay betwen the localized and itinerant magnetizm including in-gap states and spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic structure of the frustrated S=1/2 chain magnet LiCu2O2 doped with nonmagnetic Zn

We present the results of magnetization, ESR and NMR measurements on single crystal samples of the frustrated S=1/2 chain cuprate LiCu2O2 doped with nonmagnetic Zn^2+. As shown by the x-ray techniques the crystals of Li(Cu{1-x}Zn{x})2O2 with x<0.12 are single-phase, whereas for higher Zn concentrations the samples were polyphase. ESR spectra for all monophase samples (0<= x<0.12) can be explained within the model of a planar spin structure with a uniaxial type anisotropy. The NMR spectra of the highly doped single crystal sample Li(Cu0.9Zn0.1)2O2 can be described in the frame of a planar spin glass like magnetic structure with short range spiral correlations in the crystal (ab)-planes with strongest exchange bonds. The value of magnetic moments of Cu^2+ ions in this structure is close to value obtained for undoped crystals: (0.8 +- 0.1) mu_B.

cond-mat.str-el↗

arXiv2010

Spin polarization of the magnetic spiral in NaCu_2O_2, as seen by NMR

The incommensurate (IC) spin ordering in quasi-1D edge-shared cuprate NaCu_2O_2 has been studied by ^{23}Na nuclear magnetic resonance spectroscopy in an external magnetic field near 6 Tesla applied along the main crystallographic axes. The NMR lineshape evolution above and below T_N\approx12 K yields a clear signature of an IC static modulation of the local magnetic field consistent with a Cu^{2+} spin spiral polarized in the bc-plane rather than in the ab-plane as reported from earlier neutron diffraction data.

cond-mat.str-el↗

arXiv2004

Comment on "Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2"

The microscopic origin of the recently observed helical structure in LiCu2O2 [1] is considered. It is shown that the frustrated antiferromagnetic double chain scenario adopted in Ref.1 is unrealistic. It should be replaced by a frustrated single-chain (CuO2) scenario proposed in Ref. 2 with ferromagnetic nearest neighbor and antiferromagnetic next nearest neighbor exchange integrals \. [1] T. Masuda et al. Phys. Rev. Lett. 92 (2004), 177201. [2] A. Gippius et al. Phys. Rev. B 70 (2004), R01426; cond-mat/0312576.

cond-mat.str-el↗

arXiv2000

CuSiO_3 : a quasi - one - dimensional S=1/2 antiferromagnetic chain system

CuSiO_3, isotypic to the spin - Peierls compound CuGeO_3, was discovered recently as a metastable decomposition product of the silicate mineral dioptase, Cu_6Si_6O_{18}\cdot6H_2O. We investigated the physical properties of CuSiO_3 using susceptibility, magnetization and specific heat measurements on powder samples. The magnetic susceptibility χ(T) is reproduced very well above T = 8 K by theoretical calculations for an S=1/2 antiferromagnetic Heisenberg linear chain without frustration (α= 0) and a nearest - neighbor exchange coupling constant of J/k_{B} = 21 K, much weaker than in CuGeO_3. Below 8 K the susceptibility exhibits a substantial drop. This feature is identified as a second - order phase transition at T_{0} = 7.9 K by specific heat measurements. The influence of magnetic fields on T_{0} is weak, and ac - magnetization measurements give strong evidence for a spin - flop - phase at μ_0H_{SF} ~ 3 T. The origin of the magnetic phase transition at T_{0} = 7.9 K is discussed in the context of long - range antiferromagnetic order (AF) versus spin - Peierls(SP)order. Susceptibility and specific heat results support the AF ordered ground state. Additional temperature dependent ^{63,65}Cu nuclear quadrupole resonance experiments have been carried out to probe the Cu^{2+} electronic state and the spin dynamics in CuSiO_3.

cond-mat.str-el↗