arXiv subjects

V. B. Filipov

Publications and source records attributed to V. B. Filipov.

At least 19 recordsLinked to original sources

arXiv2025

Nodeless Hybridization as Proof of Trivial Topology in Samarium Hexaboride

Calculations unanimously predict samarium hexaboride to be a topological Kondo insulator, the first topological insulator driven by strong electron correlation. As of today it appears also experimentally established as the only representative of this material class. Here, we investigate the three-dimensional band structure of SmB$_6$ and show that it is incompatible with a topological Kondo insulator which must have hybridization nodes at high-symmetry points. In addition we clarify the remaining questions concerning the nature of the surface states with new data. We address consequences for the search for correlated topological insulators.

cond-mat.str-el↗

arXiv2022

Hall effect anisotropy in the paramagnetic phase of Ho0.8Lu0.2B12 induced by dynamic charge stripes

A detailed study of charge transport in the paramagnetic phase of Ho0.8Lu0.2B12 strongly correlated antiferromagnet was carried out at temperatures 1.9-300 K in magnetic fields up to 80 kOe. Four mono-domain single crystals with different orientation of normal vectors to the lateral surface of Ho0.8Lu0.2B12 samples were investigated in order to establish the changes in Hall effect due to the anisotropy, induced by (i) the electronic phase separation (dynamic charge stripes) and (ii) formation of the disordered cage-glass state below 60 K. It was demonstrated that in magnetic fields above 40 kOe directed along the 001 and 110 axes in fcc crystals a considerable intrinsic anisotropic positive component Ranxy appears in addition to the ordinary negative Hall resistivity contribution. The relation Ranxy prop. Ranxx 1.7 was found between anomalous components of the resistivity tensor for H along 001 below 60 K, and the power law Ranxy prop. Ranxx 0.83 was detected for the orientation H along 110 at temperatures T below TS about 15 K. It is argued that below TS about 15 K the anomalous odd Ranxy(T) and even Ranxx(T) parts of the resistivity tensor may be interpreted in terms of formation of a large size clusters in the filamentary structure of fluctuating charges (stripes). We assume that these Ranxy(001) and Ranxy(110) components represent the intrinsic (Berry phase contribution) and extrinsic (skew scattering) mechanism, respectively. An additional ferromagnetic contribution to anomalous Hall effect (AHE) for both ordinary and anisotropic components in Hall signal was registered and attributed to the effect of magnetic polarization of 5d states (ferromagnetic nano-domains) in the conduction band of Ho0.8Lu0.2B12.

cond-mat.str-el↗

arXiv2021

Checkerboard-type patterns of charge stripes in two-gap superconductor ZrB12

Inhomogeneous superconductivity in the high quality single crystals of ZrB12 (Tc = 6 K) has been studied using the heat capacity and x-ray diffraction (XRD) data. Evidence of two-band superconductivity with two branches of upper critical field Hc2(Tc) is obtained in a magnetic field applied along the [110] axis of the crystal. On the contrary, at H //[100], the only dependence Hc2(Tc) is observed. This finding is supplemented with the checkerboard-type patterns of the charge stripes in ZrB12 deduced from the detailed analysis of XRD data. These patterns are compared to the structure of the charge stripes in the weakly bound superconductor LuB12, whose Tc is 15 times lower than that of ZrB12. Probable nature of the two-gap superconductivity in ZrB12 with strongly enhanced characteristics is discussed.

cond-mat.supr-con↗

arXiv2021

Crystal field potential and short-range order effects in inelastic neutron scattering, magnetization and heat capacity of the cage-glass compound HoB12

The strongly correlated system Ho11B12 with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry and heat capacity measurements at temperatures in the range 3-300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field at holmium sites, B4=- 0.333 meV and B6= -2.003 meV (in Stevens notations), with an unconventional large ratio B6/B4 pointing on the dominant role of conduction electrons in the formation of a crystal field potential. The molecular field in the antiferromagnetic state, Bloc = (1.75+- 0.1) T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted Lu0.99Ho0.01B12 and concentrated HoB12 single crystals showed a strong suppression of Ho magnetic moments by antiferromagnetic exchange interactions in holmium dodecaboride. To account explicitly for the short-range antiferromagnetic correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of HoB12 in external magnetic fields.

cond-mat.str-el↗

arXiv2020

Effect of pressure on electric transport properties of carbon-doped EuB6

We report about influence of external pressure on electrical resistivity of EuB5.99C0.01, the compound believed to be intrinsically inhomogeneous due to fluctuation of carbon content. Our results show that the low-temperature resistivity maximum shifts to lower temperature with applied pressure, opposite to the behavior reported for stoichiometric EuB6. The origin of such qualitative difference we associate with the increasing volume fraction of the phase that is not compatible with ferromagnetic ordering (originating in regions with relatively higher carbon concentratio) with enhancing pressure. Our results support a recent proposition that carbon-rich regions strongly influence magnetotransport properties of carbon-doped EuB6, such as they play a role of spacers, which prevent percolation of ferromagnetic phase.

cond-mat.str-el↗

arXiv2020

Field-angle resolved magnetic excitations as a probe of hidden-order symmetry in CeB6

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order". Previously the hidden order in phase II was identified as primary antiferroquadrupolar (AFQ) and field-induced octupolar (AFO) order. Here we present a combined experimental and theoretical investigation of collective excitations in the phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in rotating field is calculated within a localized approach using the pseudo-spin presentation for the Gamma8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of inter-multipolar interactions that stabilize hidden-order phases.

cond-mat.str-el↗

arXiv2019

Maltese Cross anisotropy in Ho0.8Lu0.2B12 antiferromagnetic metal with dynamic charge stripes

The model strongly correlated electron system Ho0.8Lu0.2B12 which demonstrates a cooperative Jahn-Teller instability of the boron sub-lattice in combination with rattling modes of Ho(Lu) ions, dynamic charge stripes and unusual antiferromagnetic (AF) ground state has been studied in detail at low temperatures by magnetoresistance, magnetization and heat capacity measurements. Based on received results it turns out that the angular H-fi-T magnetic phase diagrams of this non-equilibrium AF metal can be reconstructed in the form of a Maltese cross. The dramatic AF ground state symmetry lowering of this dodecaboride with fcc crystal structure can be attributed to the redistribution of conduction electrons which leave the RKKY oscillations of the electron spin density to participate in the dynamic charge stripes providing with extraordinary changes in the indirect exchange interaction between magnetic moments of Ho3+ ions and resulting in the emergence of a number of various magnetic phases. It is also shown that the two main contributions to magnetoresistance in the complex AF phase, the (i) positive linear on magnetic field and the (ii) negative quadratic component can be separated and analyzed quantitatively, correspondingly, in terms of charge carrier scattering on spin density wave (5d) component of the magnetic structure and on local 4f-5d spin fluctuations of holmium sites.

cond-mat.str-el↗

arXiv2018

A consistent view of the samarium hexaboride terminations to resolve the nature of its surface states

The research effort prompted by the prediction that SmB$_6$ could be the first topological Kondo insulator has produced a wealth of new results, though not all of these seem compatible. A major discrepancy exists between scanning tunneling microscopy / spectroscopy (STM/S) and angle-resolved photoemission spectroscopy (ARPES), because the two experimental methods suggest a very different number of terminations of the (100) surface with different properties. Here we tackle this issue in a combined STM/S and ARPES study. We find that two of the well-ordered topographies reported in earlier STM studies can be associated with the crystal terminations identified using photoemission. We further observe a reversal of the STM contrast with bias voltage for one of the topographies. We ascribe this result to a different energy dependence of Sm and B-derived states, and show that it can be used to obtain element specific images of SmB$_6$ and identify which topography belongs to which termination. We finally find STS results to support a modification of the low-energy electronic structure at the surface that has been proposed as the trivial origin of surface metallicity in this material.

cond-mat.str-el↗

arXiv2018

Collective infrared excitation in rare-earth Gd$_x$La$_{1-x}$B$_6$ hexaborides

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides Gd$_x$La$_{1-x}$B$_6$, $x$(Gd)$=0$, 0.01, 0.1, 0.78, 1 are determined in the frequency range 30$-$35000$~cm^{-1}$. In all compounds, in addition to the Drude free-carrier spectral component, a broad excitation is discovered with the unusually large dielectric contribution $Δ$$\varepsilon$=5000 -- 15000 and non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with the dynamic cooperative Jahn-Teller effect of B$_6$ clusters. Analysis of the spectra together with the results of DC and Hall resistivity measurements shows that only 30$-$50$\%$ of the conduction band electrons are contributing to the free carrier conductivity with the rest being involved in the formation of an overdamped excitation, thus providing possible explanation of remarkably low work function of thermoemission of Gd$_x$La$_{1-x}$B$_6$ and non-Fermi-liquid behavior in GdB$_6$ crystals.

cond-mat.mtrl-sci↗

arXiv2018

Observation of dynamic charge stripes in Tm0.19Yb0.81B12 at the metal-insulator transition

Higher accuracy low temperature charge transport measurements in combination with precise X-ray diffraction experiment have allowed detecting the symmetry lowering in the single domain Tm0.19Yb0.81B12 crystals of the family of dodecaborides with metal-insulator transition. Basing on the fine structure analysis we discover formation of dynamic charge stripes within the semiconducting matrix of Tm0.19Yb0.81B12. The charge dynamics in these metallic nano-size conducting channels is characterized by broad-band optical spectroscopy that allowed estimating the frequency (~2.4 10^11 Hz) of quantum motion of the charge carriers. It is suggested that caused by cooperative Jahn-Teller effect in the boron sub-lattice, the large amplitude rattling modes of the Tm and Yb ions are responsible for modulation of the conduction band along [110] direction through the variation of 5d-2p hybridization of electron states.

cond-mat.str-el↗

arXiv2018

Chemical and valence reconstruction at the surface of SmB6 revealed with resonant soft x-ray reflectometry

Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant $M_{4,5}$ absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB$_6$. We show that upon cleaving, the highly polar (001) surface of SmB$_6$ undergoes substantial chemical and valence reconstruction, resulting in boron termination and a Sm$^{3+}$ dominated sub-surface region. Whereas at room temperature, the reconstruction occurs on a time scale of less than two hours, it takes about 24 hours below 50 K. The boron termination is eventually established, irrespective of the initial termination. Our findings reconcile earlier depth resolved photoemission and scanning tunneling spectroscopy studies performed at different temperatures and are important for better control of polarity and, as a consequence, surface states in this system.

cond-mat.str-el↗

arXiv2017

Doping-induced redistribution of magnetic spectral weight in substituted hexaborides Ce$_{1-x}$La$_x$B$_6$ and Ce$_{1-x}$Nd$_x$B$_6$

We investigate the doping-induced changes in the electronic structure of CeB$_6$ on a series of substituted Ce$_{1-x}R_x$B$_6$ samples ($R$ = La, Nd) using diffuse neutron scattering. We observe a redistribution of magnetic spectral weight across the Brillouin zone, which we associate with the changes in the Fermi-surface nesting properties related to the modified charge carrier concentration. In particular, a strong diffuse peak at the corner of the Brillouin zone ($R$ point), which coincides with the propagation vector of the elusive antiferroquadrupolar (AFQ) order in CeB$_6$, is rapidly suppressed by both La and Nd doping, like the AFQ order itself. The corresponding spectral weight is transferred to the $X(00\frac{1}{2})$ point, ultimately stabilizing a long-range AFM order at this wave vector at the Nd-rich side of the phase diagram. At an intermediate Nd concentration, a broad diffuse peak with multiple local maxima of intensity is observed around the $X$ point, evidencing itinerant frustration that gives rise to multiple ordered phases for which Ce$_{1-x}$Nd$_x$B$_6$ is known. On the La-rich side of the phase diagram, however, dilution of the magnetic moments prevents the formation of a similar $(00\frac{1}{2})$-type order despite the presence of nesting. Our results demonstrate how diffuse neutron scattering can be used to probe the nesting vectors in complex f-electron systems directly, without reference to the single-particle band structure, and emphasize the role of Fermi surface geometry in stabilizing magnetic order in rare-earth hexaborides.

cond-mat.str-el↗

arXiv2017

Large positive correlation between the effective electron mass and the multipolar fluctuation in the heavy-fermion metal Ce$_{1-x}$La$_x$B$_6$

For the last few decades, researchers have been intrigued by multipolar ordering phenomena while looking for the related quantum criticality in the heavy-fermion Kondo system Ce$_{1-x}$La$_{x}$B$_6$. However, critical phenomena induced by substitution level ($x$), temperature ($T$), and magnetic field ($B$) are poorly understood despite a large collection of experimental results is available. In this work, we present $T$-$B$, $x$-$T$, and $x$-$B$ phase diagrams of Ce$_{1-x}$La$_x$B$_6$ ($\mathbf{B}\parallel[110]$). These are completed by analyzing heat capacity, magnetocaloric effect (MCE), and elastic neutron scattering. A drastic increase of the Sommerfeld coefficient $γ_0$, which is estimated from the heat capacity down to 0.05 K, is observed with increasing $x$. The precise $T$-$B$ phase diagram which includes an unforeseen high-entropy region is drawn by analyzing the MCE for the first time in Ce$_{1-x}$La$_x$B$_6$. The $x$-$B$ phase diagram, which supports the existence of a QCP at $x>0.75$, is obtained by the same analysis. A detailed interpretation of phase diagrams strongly indicates positive correlation between the fluctuating multipoles and the effective electron mass.

cond-mat.str-el↗

arXiv2016

Raman study of coupled electronic and phononic excitations in LuB12

Electronic Raman scattering and optical phonon self-energies are studied on single crystals of $LuB_{12}$ with different isotopic composition in the temperature region 10-650K and at pressures up to 10 GPa. The shape and energy position of the spectral peaks depend on the magnitude of the probed wave vector, temperature, and symmetry of excitations. We simulated experimental spectra using electronic structure obtained in the density function theory and taking into account the electron-phonon scattering. The emergence of a broad continuum in the spectra is identified with the inelastic scattering of light from the electronic intraband excitations. Their coupling to non-fully symmetric phonon modes is the source of both the Fano interference and temperature-dependent phonon self-energies. In addition, long wavelength vibrations of the boron atoms are in nonadiabatic regime, so the electronic contribution to their self-energies provides a temperature dependence that is similar to the anharmonic contribution. Comparison of calculation and experiment allowed us to determine the coupling constant $λ_{ep}$=0.32, which gives correct critical temperature of the transition to the superconducting state.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic field dependence of the neutron spin resonance in CeB6

In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn5 is similar to the recently discovered exciton peak in the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB6. The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a non-monotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields.

cond-mat.str-el↗

arXiv2015

Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6)

CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with non-magnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to T_N upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.

cond-mat.str-el↗

arXiv2015

Surface properties of SmB6 from x-ray photoelectron spectroscopy

We have investigated the properties of cleaved SmB$_6$ single crystals by x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved samples a surface core level shift is observed which vanishes when the temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the relative intensities of the Sm$^{2+}$ and Sm$^{3+}$ multiplets. The B/Sm intensity ratio obtained from the core levels is always larger than the stoichiometric value. Possible reasons for this deviation are discussed. The B $1s$ signal shows an unexpected complexity: an anomalous low energy component appears with increasing temperature and is assigned to the formation of a suboxide at the surface. While several interesting intrinsic and extrinsic properties of the SmB$_6$ surface are elucidated in this manuscript no clear indication of a trivial mechanism for the prominent surface conductivity is found.

cond-mat.str-el↗

arXiv2015

Samarium hexaboride: A trivial surface conductor

Recent theoretical and experimental studies suggest that SmB$_6$ is the first topological Kondo insulator: A material in which the interaction between localized and itinerant electrons renders the bulk insulating at low temperature, while topological surface states leave the surface metallic. While this would elegantly explain the material's puzzling conductivity, we find the experimentally observed candidates for both predicted topological surface states to be of trivial character instead: The surface state at $\barΓ$ is very heavy and shallow with a mere $\sim 2$ meV binding energy. It exhibits large Rashba splitting which excludes a topological nature. We further demonstrate that the other metallic surface state, located at $\bar{X}$, is not an independent in-gap state as supposed previously, but part of a massive band with much higher binding energy (1.7 eV). We show that it remains metallic down to 1 K due to reduced hybridization with the energy-shifted surface 4$f$ level.

cond-mat.str-el↗