arXiv subjects

P. Babkevich

Publications and source records attributed to P. Babkevich.

At least 19 recordsLinked to original sources

arXiv2020

Exchange Interactions Mediated by Non-Magnetic Cations in Double Perovskites

Establishing the physical mechanism governing exchange interactions is fundamental for exploring exotic phases such as the quantum spin liquids (QSLs) in real materials. In this work, we address exchange interactions in Sr2CuTe$_{1-x}$W$_{x}$O, a series of double perovskites that realize the spin-1/2 square lattice and were suggested to harbor a QSL ground state arising from random distribution of non-magnetic ions. Our {\it ab initio} multi-reference configuration interaction calculations show that replacing Te atoms with W atoms changes the dominant couplings from nearest to next-nearest neighbor explained by the crucial role of unoccupied states of non-magnetic ions in the super-superexchange mechanism. Combined with spin-wave theory simulations, our calculated exchange couplings provide an excellent description of the inelastic neutron scattering spectra of the end compounds, as well as explain the magnetic excitations in Sr2CuTe$_{0.5}$W$_{0.5}$O as emerging from the bond-disordered exchange couplings. Our results provide crucial understanding of the role of non-magnetic cations in exchange interactions paving the way to further exploration of QSL phases in bond-disordered materials.

cond-mat.str-el↗

arXiv2018

$A$-cation control of magnetoelectric quadrupole order in $A$(TiO)Cu$_4$(PO$_4$)$_4$ ($A$ = Ba, Sr, and Pb)

Ferroic magnetic quadrupole order exhibiting macroscopic magnetoelectric activity is discovered in the novel compound $A$(TiO)Cu$_4$(PO$_4$)$_4$ with $A$ = Pb, which is in contrast with antiferroic quadrupole order observed in the isostructural compounds with $A$ = Ba and Sr. Unlike the famous lone-pair stereochemical activity which often triggers ferroelectricity as in PbTiO$_3$, the Pb$^{2+}$ cation in Pb(TiO)Cu$_4$(PO$_4$)$_4$ is stereochemically inactive but dramatically alters specific magnetic interactions and consequently switches the quadrupole order from antiferroic to ferroic. Our first-principles calculations uncover a positive correlation between the degree of $A$-O bond covalency and a stability of the ferroic quadrupole order.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic structure of Ba(TiO)Cu$_4$(PO$_4$)$_4$ probed using spherical neutron polarimetry

The antiferromagnetic compound Ba(TiO)Cu$_4$(PO$_4$)$_4$ contains square cupola of corner-sharing CuO$_4$ plaquettes, which were proposed to form effective quadrupolar order. To identify the magnetic structure, we have performed spherical neutron polarimetry measurements. Based on symmetry analysis and careful measurements we conclude that the orientation of the Cu$^{2+}$ spins form a non-collinear in-out structure with spins approximately perpendicular to the CuO$_4$ motif. Strong Dzyaloshinskii-Moriya interaction naturally lends itself to explain this phenomenon. The identification of the ground state magnetic structure should serve well for future theoretical and experimental studies into this and closely related compounds.

cond-mat.str-el↗

arXiv2017

Electronic and magnetic excitations in the "half-stuffed" Cu--O planes of Ba$_2$Cu$_3$O$_4$Cl$_2$ measured by resonant inelastic x-ray scattering

We use resonant inelastic x-ray scattering (RIXS) at the Cu L$_3$ edge to measure the charge and spin excitations in the "half-stuffed" Cu--O planes of the cuprate antiferromagnet Ba$_2$Cu$_3$O$_4$Cl$_2$. The RIXS line shape reveals distinct contributions to the $dd$ excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO$_2$ sublattice, similar to the case of the "fully-stuffed" planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr$_2$CuO$_2$Cl$_2$, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO.

cond-mat.str-el↗

arXiv2017

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba$_2$Cu$_3$O$_4$Cl$_2$

We report detailed neutron scattering studies on Ba$_2$Cu$_3$O$_4$Cl$_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as Cu$_{\rm A}$ and Cu$_{\rm B}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the Cu$_{\rm A}$ sublattice extending up to 300 meV and a much weaker dispersion of Cu$_{\rm B}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the Cu$_{\rm B}$ dispersion that cannot be explained by conventional spin-wave theory.

cond-mat.str-el↗

arXiv2017

$J_1$-$J_2$ square lattice antiferromagnetism in the orbitally quenched insulator MoOPO$_4$

We report magnetic and thermodynamic properties of a $4d^1$ (Mo$^{5+}$) magnetic insulator MoOPO$_4$ single crystal, which realizes a $J_1$-$J_2$ Heisenberg spin-$1/2$ model on a stacked square lattice. The specific-heat measurements show a magnetic transition at 16 K which is also confirmed by magnetic susceptibility, ESR, and neutron diffraction measurements. Magnetic entropy deduced from the specific heat corresponds to a two-level degree of freedom per Mo$^{5+}$ ion, and the effective moment from the susceptibility corresponds to the spin-only value. Using {\it ab initio} quantum chemistry calculations we demonstrate that the Mo$^{5+}$ ion hosts a purely spin-$1/2$ magnetic moment, indicating negligible effects of spin-orbit interaction. The quenched orbital moments originate from the large displacement of Mo ions inside the MoO$_6$ octahedra along the apical direction. The ground state is shown by neutron diffraction to support a collinear Néel-type magnetic order, and a spin-flop transition is observed around an applied magnetic field of 3.5 T. The magnetic phase diagram is reproduced by a mean-field calculation assuming a small easy-axis anisotropy in the exchange interactions. Our results suggest $4d$ molybdates as an alternative playground to search for model quantum magnets.

cond-mat.str-el↗

arXiv2016

Probing strongly hybrid nuclear-electronic states in a model quantum ferromagnet

We present direct local-probe evidence for strongly hybridized nuclear-electronic spin states of an Ising ferromagnet LiHoF$_4$ in a transverse magnetic field. The nuclear-electronic states are addressed via a magnetic resonance in the GHz frequency range using coplanar resonators and a vector network analyzer. The magnetic resonance spectrum is successfully traced over the entire field-temperature phase diagram, which is remarkably well reproduced by mean-field calculations. Our method can be directly applied to a broad class of materials containing rare-earth ions for probing the substantially mixed nature of the nuclear and electronic moments.

cond-mat.str-el↗

arXiv2016

Magnetic excitations and electronic interactions in Sr$_2$CuTeO$_6$: a spin-1/2 square lattice Heisenberg antiferromagnet

Sr$_2$CuTeO$_6$ presents an opportunity for exploring low-dimensional magnetism on a square lattice of $S=1/2$ Cu$^{2+}$ ions. We employ ab initio multi-reference configuration interaction calculations to unravel the Cu$^{2+}$ electronic structure and to evaluate exchange interactions in Sr$_2$CuTeO$_6$. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr$_2$CuTeO$_6$ is an almost realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.

cond-mat.str-el↗

arXiv2016

Phase diagram of diluted Ising ferromagnet LiHo$_x$Y$_{1-x}$F$_4$

We present a systematic study of the phase diagram of LiHo$_x$Y$_{1-x}$F$_4$ ($0.25<x<1$) Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.

cond-mat.str-el↗

arXiv2016

Magnetodielectric detection of magnetic quadrupole order in Ba(TiO)Cu$_4$(PO$_4$)$_4$ with Cu$_4$O$_{12}$ square cupolas

In vortex-like spin arrangements, multiple spins can combine into emergent multipole moments. Such multipole moments have broken space-inversion and time-reversal symmetries, and can therefore exhibit linear magnetoelectric (ME) activity. Three types of such multipole moments are known: toroidal, monopole, and quadrupole moments. So far, however, the ME-activity of these multipole moments has only been established experimentally for the toroidal moment. Here, we propose a magnetic square cupola cluster, in which four corner-sharing square-coordinated metal-ligand fragments form a noncoplanar buckled structure, as a promising structural unit that carries an ME-active multipole moment. We substantiate this idea by observing clear magnetodielectric signals associated with an antiferroic ME-active magnetic quadrupole order in the real material Ba(TiO)Cu$_4$(PO$_4$)$_4$. The present result serves as a useful guide for exploring and designing new ME-active materials based on vortex-like spin arrangements.

cond-mat.str-el↗

arXiv2016

Direct evidence for charge stripes in a layered cobalt oxide

Recent experiments indicate that static stripe-like charge order is generic to the hole-doped copper oxide superconductors and competes with superconductivity. Here we show that a similar type of charge order is present in La$_{5/3}$Sr$_{1/3}$CoO$_4$, an insulating analogue of the copper oxide superconductors containing cobalt in place of copper. The stripe phase we have detected is accompanied by short-range, quasi-one-dimensional, antiferromagnetic order, and provides a natural explanation for the distinctive hourglass shape of the magnetic spectrum previously observed in neutron scattering measurements of La$_{5/3}$Sr$_{1/3}$CoO$_4$ and many hole-doped copper oxide superconductors. The results establish a solid empirical basis for theories of the hourglass spectrum built on short-range, quasi-static, stripe correlations.

cond-mat.str-el↗

arXiv2016

Dimensional Reduction in Quantum Dipolar Antiferromagnets

We report ac susceptibility, specific heat, and neutron scattering measurements on a dipolar-coupled antiferromagnet LiYbF$_4$. For the thermal transition, the order-parameter critical exponent is found to be 0.20(1) and the specific-heat critical exponent -0.25(1). The exponents agree with the 2D XY/h$_4$ universality class despite the lack of apparent two-dimensionality in the structure. The order-parameter exponent for the quantum phase transitions is found to be 0.35(1) corresponding to (2+1)D. These results are in line with those found for LiErF$_4$ which has the same crystal structure, but largely different T$_N$, crystal field environment and hyperfine interactions. Our results therefore experimentally establish that the dimensional reduction is universal to quantum dipolar antiferromagnets on a distorted diamond lattice.

cond-mat.str-el↗

arXiv2015

Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in Li$R$F$_4$ ($R$ = Gd, Ho, Er, Tm, and Yb)

We present a systematic study of the crystal field interactions in the Li$R$F$_4$, $R$ = Gd, Ho, Er, Tm and Yb, family of rare-earth magnets. Using detailed inelastic neutron scattering measurements we have been able to quantify the transition energies and wavefunctions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to lineshape broadening in LiTmF$_4$ and level splitting in LiYbF$_4$. Furthermore, using high resolution neutron scattering from LiHoF$_4$, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.

cond-mat.str-el↗

arXiv2014

Superconductivity in Semimetallic Bi3O2S3

Here we report further investigation on the thermodynamic, and transport properties, and the first assessment of theoretical calculations for the BiS2-layered Bi3O2S3 superconductor. The polycrystalline sample is synthesized with the superconducting transition temperature of Tc_onset = 5.75 K, that drops to 3.3 K by applying hydrostatic pressure of 6 kbar. Density-of-states calculations give substantial hybridization between Bi, O and S, with the largest Bi component at DOS, which supports the idea that BiS2 layer is relevant for producing electron-phonon coupling. The electron charge carrier concentration of only 1.5 x 10^19 cm-3 for Bi3O2S3 is additionally suggestive of strong electron-phonon interaction in Bi-O-S system. The analysis of Seebeck coefficient results strongly suggests that Bi3O2S3 is a semimetal. In fact, the semimetallic or narrow band gap behavior may be general in the BiS2-layered class of materials, including that of Bi4O4S3.

cond-mat.supr-con↗

arXiv2014

Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_{4}$

We present a study of the model spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_4$ using simultaneous AC susceptibility, magnetization and magnetocaloric effect measurements along with small angle neutron scattering (SANS) at sub-Kelvin temperatures. All measured bulk quantities reveal hysteretic behavior when the field is applied along the crystallographic c axis. Furthermore avalanche-like relaxation is observed in a static field after ramping from the zero-field-cooled state up to $200 - 300$ Oe. SANS measurements are employed to track the microscopic spin reconfiguration throughout both the hysteresis loop and the related relaxation. Comparing the SANS data to inhomogeneous mean-field calculations performed on a box of one million unit cells provides a real-space picture of the spin configuration. We discover that the avalanche is being driven by released Zeeman energy, which heats the sample and creates positive feedback, continuing the avalanche. The combination of SANS and mean-field simulations reveal that the conventional distribution of cluster sizes is replaced by one with a depletion of intermediate cluster sizes for much of the hysteresis loop.

cond-mat.str-el↗

arXiv2014

Pressure dependence of the magnetic order in CrAs: a neutron diffraction investigation

The suppression of magnetic order with pressure concomitant with the appearance of pressure- induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground-state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa and at various temperatures. The helimagnetic long-range order is sustained under pressure and the magnetic propagation vector does not show any considerable change. The average ordered magnetic moment is reduced from 1.73(2) μB at ambient pressure to 0.4(1) μB close to the critical pressure Pc=0.7 GPa, at which magnetic order is completely suppressed. The width of the magnetic Bragg peaks strongly depends on temperature and pressure, showing a maximum in the region of the onset of superconductivity. We interpret this as associated with competing ground-states in the vicinity of the superconducting phase.

cond-mat.str-el↗

arXiv2014

Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator La5/3Sr1/3CoO4

We report on an investigation into the dynamics of the stripe phase of La5/3Sr1/3CoO4, a material recently shown to have an hour-glass magnetic excitation spectrum. A combination of magnetic susceptibility, muon-spin relaxation and nuclear magnetic resonance measurements strongly suggest that the physics is determined by a disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are strongly dynamic at high temperature and which freeze out in a glassy manner as the temperature is lowered. Our results broadly confirm a recent theoretical prediction, but show that the charge quenching remains incomplete well below the charge ordering temperature and reveal, in detail, the manner in which the magnetic degrees of freedom are frozen.

cond-mat.str-el↗

arXiv2012

Spin-wave excitations and superconducting resonant mode in Cs(x)Fe(2-y)Se2

We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with previous measurements on Rb(x)Fe(2-y)Se2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating A(x)Fe(2-y)Se2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave vector of (0.25, 0.5). The resonance peak increases below Tc = 27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of Rb(x)Fe(2-y)Se2 and now of Cs(x)Fe(2-y)Se2 suggests that this is a common feature of superconductivity in this family. The low energy spin-wave excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases.

cond-mat.supr-con↗