arXiv subjects

K. Yu. Povarov

Publications and source records attributed to K. Yu. Povarov.

At least 37 records · Page 2Linked to original sources

arXiv2020

Finite-temperature dynamics and the role of disorder in nearly critical Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$

Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $ x = 0.14(1)$, tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows $\hbarω/T$ scaling behavior characteristic of a quantum critical point. The deviation of the measured critical exponent from spin wave theoretical expectations are attributed to disorder. Another effect of disorder is local excitations above the magnon band. Their energy, structure factor and temperature dependence are well explained by simple strong-bond dimers associated with Br-impurity sites.

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arXiv2020

Anisotropy-induced soliton excitation in magnetized strong-rung spin ladders

We report low temperature electron spin resonance experimental and theoretical studies of an archetype $S=1/2$ strong-rung spin ladder material (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$. Unexpected dynamics is detected deep in the Tomonaga-Luttinger spin liquid regime. Close to the point where the system is half-magnetized (and believed to be equivalent to a gapless easy plane chain in zero field) we observed orientation-dependent spin gap and anomalous $g$-factor values. Field theoretical analysis demonstrates that the observed low-energy excitation modes in magnetized (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ are solitonic excitations caused by Dzyaloshinskii-Moriya interaction presence.

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arXiv2020

Magnetic phase diagram of the linear quantum ferro-antiferromagnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$

A single-crystal sample of the frustrated quasi one-dimensional quantum magnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ is investigated by magnetic and thermodynamic measurements.A combination of specific heat and magnetic torque measurements maps out the entire $H$-$T$ phase diagram for three orientations.Remarkably, a new phase emerges below the saturation field, irrespective of the crystal orientation. It is suggested that the presaturation phase represents spin-nematic order or other multi-magnon condensate. The phase diagrams within the long-range ordered dome are qualitatively different for each geometry. In particular, multiple transitions are identified in the field along the chain direction.

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arXiv2019

Microwave dynamics of pure and doped anisotropic S=1 chain antiferromagnet NiCl2-4SC(NH2)2

We studied electron spin resonance in a quantum magnet NiCl2-4SC(NH2)2, demonstrating a field-induced quantum phase transition from a quantum-disordered phase to an antiferromagnet. We observe two branches of the antiferromagnetic resonance of the ordered phase, one of them has a gap and the other is a Goldstone mode with zero frequency at a magnetic field along the four-fold axis. This zero frequency mode acquires a gap at a small tilting of the magnetic field with respect to this direction. The upper gap was found to be reduced in the doped compound Ni(Cl(1-x)Br(x))2-4SC(NH2)2 with $x=0.21$. This reduction is unexpected because of the previously reported rise of the main exchange constant in a doped compound. Further, a nonresonant diamagnetic susceptibility $χ^{\prime}$ was found for the ordered phase in a wide frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as large as the dynamic susceptibility of the paramagnetic resonance. We speculate that it originates from a two-magnon absorption band of low-frequency dispersive magnon branch.

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arXiv2019

Presaturation phase with no dipolar order in a quantum ferro-antiferromagnet

Magnetization, magnetocaloric, calorimetric, neutron and X-ray diffraction and inelastic neutron scattering measurements are performed on single crystals of BaCdVO(PO$_4$)$_2$. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a novel spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at $H_{c1}\simeq 4.0$ T. It is a distinct thermodynamic phase with a well-defined phase boundary at $H_{c2}\simeq 6.5$ T and is clearly separate from the fully saturated phase. Yet, it shows no conventional (dipolar) magnetic long range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state.

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arXiv2019

Electron spin resonance study of spin relaxation in the strong-leg spin ladder with nonmagnetic dilution

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet ({C}$_{7}$H$_{10}$N)$_{2}$Cu$_{(1-x)}$Zn$_{x}$Br$_{4}$ (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with non-zero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with non-magnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

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arXiv2019

One- and three-dimensional quantum phase transitions and anisotropy in Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$

Single crystal samples of the frustrated quasi one-dimensional quantum magnet Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ are investigated by magnetic, thermodynamic, and electron spin resonance (ESR) measurements. Quantum phase transitions between the gapped, magnetically ordered and fully saturated phases are observed. Surprisingly, the former has a distinctive three-dimensional character, while the latter is dominated by one-dimensional quantum spin fluctuations. The entire $H$-$T$ phase diagram is mapped out and found to be substantially anisotropic. In particular, the lower critical fields differ by over 50\% depending on the direction of applied field, while the upper ones are almost isotropic, as is the magnetization above saturation. The ESR spectra are strongly dependent on field orientation and point to a helical structure with a rigidly defined spin rotation plane.

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arXiv2019

Thermodynamics of a frustrated quantum magnet on a square lattice

We report the magnetic and calorimetric measurements in single crystal samples of the square lattice $J_{1}-J_{2}$ quantum antiferromagnet BaCdVO(PO$_4$)$_2$. An investigation of the scaling of magnetization reveals a "dimensionality reduction" indicative of a strong degree of geometric frustration. Below a characteristic temperature of $T^{\ast}\simeq150$~mK we observe the emergence of an additional strongly fluctuating quantum phase close to full magnetic saturation. It is separated from the magnetically ordered state by first- and second-order phase transitions, depending on the orientation of the applied magnetic field. We suggest that this phase may indeed be related to the theoretically predicted spin-nematic state.

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arXiv2018

Spin waves near the edge of halogen substitution induced magnetic order in Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$

We report an inelastic neutron scattering study of magnetic excitations in a quantum paramagnet driven into a magnetically ordered state by chemical substitution, namely Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$ with $x=0.21(2)$. The measured spectrum is well accounted for by the generalized spin wave theory (GSWT) approach [M. Matsumoto and M. Koga, [J. Phys. Soc. Jap. 76, 073709 (2007)]. This analysis allows us to determine the effective Hamiltonian parameters for a direct comparison with those in the previously studied parent compound and `underdoped' system. The issue of magnon lifetimes due to structural disorder is also addressed.

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arXiv2018

Spin gap in a quasi-1D S=1/2 antiferromagnet K2CuSO4Cl2

Electron spin resonance experiments in the quasi-1D S=1/2 antiferromagnet K$_2$CuSO$_4$Cl$_2$ reveal opening of a gap in absence of magnetic ordering, as well as an anisotropic shift of the resonance magnetic field. These features of magnetic excitation spectrum are explained by a crossover between a gapped spinon-type doublet ESR formed in a 1D antiferromagnet with uniform Dzyaloshinskii-Moriya interaction and a Larmor-type resonance of a quasi-1D Heisenberg system

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arXiv2018

Magnetic phase diagram of the strongly frustrated quantum spin chain system PbCuSO$_4$(OH)$_2$ in tilted magnetic fields

We report the $\mathbf{H}-T$ phase diagram of $S=1/2$ strongly frustrated anisotropic spin chain material linarite PbCuSO$_4$(OH)$_2$ in tilted magnetic fields up to 10 T and temperatures down to 0.2 K. By means of torque magnetometry we investigate the phase diagram evolution as the magnetic field undergoes rotation in $\mathbf{ba}^{\ast}$ and $\mathbf{bc}$ planes. The key finding is the robustness of the high field spin density wave-like phase, which may persist even as the external field goes orthogonal to the chain direction $\mathbf{b}$. In contrast, the intermediate collinear antiferromagnetic phase collapses at moderate deflection angles with respect to $\mathbf{b}$ axis.

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arXiv2017

Quantum criticality in a three dimensional spin system at zero field and pressure

We report spontaneous appearance of antiferromagnetic order in a model gapped quantum paramagnet Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$ induced by a change in bromine concentration x. This transition is qualitatively similar to a z = 1 magnetic quantum critical point. However, the observed critical scaling of thermodynamic and magnetic properties has rather unusual critical exponents.

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arXiv2017

Anisotropic magnetic interactions and spin dynamics in the spin-chain compound Cu(py)$_2$Br$_2$: An experimental and theoretical study

We compare theoretical results for electron spin resonance (ESR) properties of the Heisenberg-Ising Hamiltonian with ESR experiments on the quasi-one-dimensional magnet Cu(py)$_2$Br$_2$ (CPB). Our measurements were performed over a wide frequency and temperature range giving insight into spin dynamics, spin structure, and magnetic anisotropy of this compound. By analyzing the angular dependence of ESR parameters (resonance shift and linewidth) at room temperature we show that the two weakly coupled inequivalent spin chain types inside the compound are well described by Heisenberg-Ising chains with their magnetic anisotropy axes perpendicular to the chain direction and almost perpendicular to each other. We further determine the full $g$-tensor from these data. In addition, the angular dependence of the linewidth at high temperatures gives us access to the exponent of the algebraic decay of a dynamical correlation function of the isotropic Heisenberg chain. From the temperature dependence of static susceptibilities we extract the strength of the exchange coupling ($J/k_B = 52.0\,\text{K}$) and the anisotropy parameter ($δ\approx -0.02$) of the model Hamiltonian. An independent compatible value of $δ$ is obtained by comparing the exact prediction for the resonance shift at low temperatures with high-frequency ESR data recorded at $4\,\text{K}$. The spin structure in the ordered state implied by the two (almost) perpendicular anisotropy axes is in accordance with the propagation vector determined from neutron scattering experiments. In addition to undoped samples we study the impact of partial substitution of Br by Cl ions on spin dynamics. From the dependence of the ESR linewidth on doping level we infer an effective decoupling of the anisotropic component $Jδ$ from the isotropic exchange $J$ in these systems.

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arXiv2016

Multiferroic phases of the frustrated quantum spin-chain compound linarite

The dielectric properties of the prototypical frustrated ferromagnetic spin-chain compound PbCuSO$_4$(OH)$_2$ known as linarite, are studied across its strongly anisotropic magnetic phase diagram in single crystal samples. The ferroelectric character of the principal low-field spin spiral phase is confirmed. The measured polarization is fully consistent with the previously proposed magnetic structure. Spontaneous polarization is also detected in two other field-induced phases but in some cases is incompatible with previously suggested models for the spin arrangement.

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arXiv2016

Electron Spin Resonance in a Spin-1/2 Heisenberg Strong-rung Ladder

Cu(C$_8$H$_6$N$_2$)Cl$_2$, a strong-rung spin-1/2 Heisenberg ladder compound, is probed by means of electron spin resonance (ESR) spectroscopy in the field-induced gapless phase above $H_{c1}$. The temperature dependence of the ESR linewidth is analyzed in the quantum field theory framework, suggesting that the anisotropy of magnetic interactions plays a crucial role, determining the peculiar low-temperature ESR linewidth behavior. In particular, it is argued that the uniform Dzyaloshinskii-Moriya interaction (which is allowed on the bonds along the ladder legs) can be the source of this behavior in Cu(C$_8$H$_6$N$_2$)Cl$_2$.

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arXiv2016

Emergent interacting spin islands in a depleted strong-leg Heisenberg ladder

Properties of the depleted Heisenberg spin ladder material series (C$_7$H$_{10}$N)$_2$Cu$_{1-z}$Zn$_{z}$Br$_4$ have been studied by the combination of magnetic measurements and neutron spectroscopy. Disorder-induced degrees of freedom lead to a specific magnetic response, described in terms of emergent strongly interacting "spin island" objects. The structure and dynamics of the spin islands is studied by high-resolution inelastic neutron scattering. This allows to determine their spatial shape and to observe their mutual interactions, manifested by strong spectral in-gap contributions.

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arXiv2015

Giant dielectric nonlinearities at a magnetic Bose-Einstein condensation

We experimentally investigate the dielectric response of the low-dimensional gapped quantum magnet Cu$_2$Cl$_{4}\cdot$H$_8$C$_4$SO$_2$ near a magnetic field-induced quantum critical point, which separates the quantum-disordered and helimagnetic ground states. The observed magnetocapacitive effect originates from an improper ferroelectric nature of the transition, which itself is perhaps one of the best known realizations of Bose--Einstein condensation of magnons. Despite that, we find that the magnetocapacitive effect associated with the transition exhibits huge and very unusual anharmonicities.

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arXiv2015

Dynamics of a bond-disordered $S=1$ quantum magnet near $z=1$ criticality

Neutron scattering is used to study NiCl$_{2-2x}$Br$_{2x}\cdot$4SC(NH$_2$)$_2$, $x=0.06$, a bond-disordered modification of the well-known gapped $S=1$ antiferromagnetic quantum spin system NiCl$_{2}\cdot$4SC(NH$_2$)$_2$. The magnetic excitation spectrum throughout Brillouin zone is mapped out at $T=60$ mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy $Δ\simeq0.2$ meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.

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