arXiv subjects

E. Pomjakushina

Publications and source records attributed to E. Pomjakushina.

At least 55 records · Page 3Linked to original sources

arXiv2018

Chemical- and hydrostatic-pressure effects on the Kitaev honeycomb material Na$_2$IrO$_3$

The low-temperature magnetic properties of \tcr{polycrystalline} Na$_2$IrO$_3$, a candidate material for the realization of a quantum spin-liquid state, were investigated by means of muon-spin relaxation and nuclear magnetic resonance methods under chemical and hydrostatic pressure. The Li-for-Na chemical substitution promotes an inhomogeneous magnetic order, whereas hydrostatic pressure (up to 3.9\,GPa) results in an enhancement of the ordering temperature $T_\mathrm{N}$. In the first case, the inhomogeneous magnetic order suggests either short- or long-range correlations of broadly distributed $j=\,$\textonehalf\ Ir$^{4+}$ magnetic moments, reflecting local disorder. The increase of $T_\mathrm{N}$ under applied pressure points at an increased strength of three dimensional interactions arising from interlayer compression.

cond-mat.str-el↗

arXiv2018

Evidence of Coulomb interaction induced Lifshitz transition and robust hybrid Weyl semimetal in Td MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td MoTe2. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe2, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.

cond-mat.str-el↗

arXiv2018

Cooling a polaronic liquid: Phase mixture and pseudogap-like spectra in superconducting $Ba_{1-x}K_{x}BiO_{3}$

Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor $Ba_{1-x}K_{x}BiO_{3}$ near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature $T_{c}$, and near an insulating phase with long-range lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature $T_{p} > T_{c}$, which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near $E_{F}$ into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.

cond-mat.supr-con↗

arXiv2018

High-pressure polymorphism of BaFe2Se3

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is intrinsic to the BaFe2Se3 phase and not to FeSe with Tc > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder X-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFe2Se3 transforms into δ-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transitions is characterized by a significant shortening of the b lattice parameter of γ-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

cond-mat.supr-con↗

arXiv2018

Formation of single-phase disordered CsxFe2-ySe2 at high pressure

A single-phase high pressure (HP) modification of CsxFe2-ySe2 was synthesized at 11.8 GPa at ambient temperature. Structurally this polymorph is similar to the minor low pressure (LP) superconducting phase, namely they both crystallize in a ThCr2Si2-type structure without ordering of the Fe vacancies within the Fe-deficient FeSe4 layers. The HP CsxFe2-ySe2 polymorph is found to be less crystalline and nearly twice as soft compared to the parent major and minor phases of CsxFe2-ySe2. It can be quenched to low pressures and is stable at least on the scale of weeks. At ambient pressure the HP polymorph of CsxFe2-ySe2 is expected to exhibit different superconducting properties compared to its LP minor phase (Tc = 27 K).

cond-mat.supr-con↗

arXiv2018

Revisiting the Magnetic Structure and Charge Ordering in La$_{1/3}$Sr$_{2/3}$FeO$_3$ by Neutron Powder Diffraction and Mössbauer Spectroscopy

The magnetic ordering of La$_{1/3}$Sr$_{2/3}$FeO$_3$ perovskite has been studied by neutron powder diffraction and $^{57}$Fe Mössbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector k = (0,0,1) in R-3c space group. In the former model, the magnetic moments of Fe adopt the magnetic space group P3$_{2}$21 and have helical and antiferromagnetic ordering propagating along the c axis. The model allows only one Fe site, with a magnetic moment of 3.46(2) $μ_{\rm{B}}$ at 2 K. In the latter model, the magnetic moments of iron ions adopt the magnetic space group C2/c or C2'/c' and are aligned collinearly. The model allows the presence of two inequivalent Fe sites with magnetic moments of amplitude 3.26(3) $μ_{\rm{B}}$ and 3.67(2) $μ_{\rm{B}}$, respectively. The neutron diffraction pattern is equally well fitted by either model. The Mössbauer spectroscopy study suggests a single charge state Fe$^{3.66+}$ above the magnetic transition and a charge disproportionation into Fe$^{(3.66-ζ)+}$ and Fe$^{(3.66+2ζ)+}$ below the magnetic transition. The compatibility of the magnetic structure models with the Mössbauer spectroscopy results is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Trivial topological phase of CaAgP and the topological nodal-line transition in CaAg(P1-xAsx)

By performing angle-resolved photoemission spectroscopy and first-principles calculations, we address the topological phase of CaAgP and investigate the topological phase transition in CaAg(P1-xAsx). We reveal that in CaAgP, the bulk band gap and surface states with a large bandwidth are topologically trivial, in agreement with hybrid density functional theory calculations. The calculations also indicate that application of "negative" hydrostatic pressure can transform trivial semiconducting CaAgP into an ideal topological nodal-line semimetal phase. The topological transition can be realized by partial isovalent P/As substitution at x = 0.38.

cond-mat.mtrl-sci↗

arXiv2018

Pauling entropy, metastability and equilibrium in Dy$_2$Ti$_2$O$_7$ spin ice

Determining the fate of the Pauling entropy in the classical spin ice material Dy$_2$Ti$_2$O$_7$ with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice - the dipolar spin ice model - predicts an ordering transition at $T\approx 0.15$ K, but recent experiments by Pomaranski $et\ al.$ suggest an entropy recovery over long time scales at temperatures as high as $0.5$ K, much too high to be compatible with theory. Using neutron scattering and specific heat measurements at low temperatures and with long time scales ($0.35$ K$/10^6$ s and $0.5$ K$/10^5$ s respectively) on several isotopically enriched samples we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Further, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez $et\ al.$ are, after all, essentially correct: the short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

cond-mat.str-el↗

arXiv2018

Nodeless superconductivity and time-reversal symmetry breaking in the noncentrosymmetric superconductor Re24Ti5

The noncentrosymmetric superconductor Re$_{24}$Ti$_{5}$, a time-reversal symmetry (TRS) breaking candidate with $T_c = 6$\,K, was studied by means of muon-spin rotation/relaxation ($μ$SR) and tunnel-diode oscillator (TDO) techniques. At a macroscopic level, its bulk superconductivity was investigated via electrical resistivity, magnetic susceptibility, and heat capacity measurements. The low-temperature penetration depth, superfluid density and electronic heat capacity all evidence an $s$-wave coupling with an enhanced superconducting gap. The spontaneous magnetic fields revealed by zero-field $μ$SR below $T_c$ indicate a time-reversal symmetry breaking and thus the unconventional nature of superconductivity in Re$_{24}$Ti$_{5}$. The concomitant occurrence of TRS breaking also in the isostructural Re$_6$(Zr,Hf) compounds, hints at its common origin in this superconducting family and that an enhanced spin-orbital coupling does not affect pairing symmetry.

cond-mat.str-el↗

arXiv2017

Scavenging of oxygen vacancies at modulation-doped oxide interfaces: Evidence from oxygen isotope tracing

The introduction of manganite buffer layers, La7/8Sr1/8MnO3 (LSMO) in particular, at the metallic interface between SrTiO3 (STO) and another band insulator suppresses the carrier density of the interfacial two-dimensional electron gas (2DEG) and improves significantly the electron mobility. However, the mechanisms underlying the extreme mobility enhancement remain elusive. Herein, we used 18O isotope exchanged SrTi18O3 as substrates to create 2DEG at room temperature with and without the LSMO buffer layer. By mapping the oxygen profile across the interface between STO18 and disordered LaAlO3 or yttria-stabilized zirconia (YSZ), we provide unambiguous evidence that redox reactions occur at oxide interfaces even grown at room temperature. Moreover, the manganite buffer layer not only suppresses the carrier density but also strongly suppresses the oxygen exchange dynamics of the STO substrate, which likely prevents the reduction of STO during the formation of the 2DEG. The underlying mechanism on the enhanced electron mobility at buffered oxide interfaces is also discussed.

cond-mat.mtrl-sci↗

arXiv2017

Complementary response of static spin-stripe order and superconductivity to non-magnetic impurities in cuprates

We report muon-spin rotation and neutron-scattering experiments on non-magnetic Zn impurity effects on the static spin-stripe order and superconductivity of the La214 cuprates. Remarkably, it was found that, for samples with hole doping x = 1/8, the spin-stripe ordering temperature T_so decreases linearly with Zn doping y and disappears at around y = 4\%, demonstrating a high sensitivity of static spin-stripe order to impurities within a CuO_2 plane. Moreover, T_so is suppressed by Zn in the same manner as is the superconducting transition temperature T_c for samples near optimal hole doping. This surprisingly similar sensitivity suggests that the spin-stripe order is dependent on intertwining with superconducting correlations.

cond-mat.supr-con↗

arXiv2017

The Spin-Reorientation Transition in TmFeO3

X-ray magnetic circular and linear dichroism (XMCD and XMLD) have been used to investigate the Fe magnetic response during the spin reorientation transition (SRT) in TmFeO3. These experiments are complemented with resonant magnetic diffraction experiments at the Tm M5 edge to study simultaneously the induced magnetic order in the Tm 4f shell and the behavior of the Tm orbitals through the SRT. Comparing the Fe XMLD results with neutron diffraction and magnetization measurements on the same sample indicate that the SRT has an enhanced temperature range in the near surface region. This view is supported by the resonant soft x-ray diffraction results at the Tm M5 edge. These find an induced magnetic moment on the Tm sites, which is well-described by a dipolar mean field model originating from the Fe moments. Even though such a model can describe the 4f response in the experiments, it is insufficient to describe the SRT even when considering a change in the 4f anisotropy. Moreover, the results of the Fe XMCD are indicative of a decoupling of spin canting and antiferromagnetic spin rotation in the near surface regime close to the SRT, which remains to be understood.

cond-mat.str-el↗

arXiv2017

Global formation of topological defects in the multiferroic hexagonal manganites

The spontaneous transformations associated with symmetry-breaking phase transitions generate domain structures and defects that may be topological in nature. The formation of these defects can be described according to the Kibble-Zurek mechanism, which provides a generic relation that applies from cosmological to interatomic lengthscales. Its verification is challenging, however, in particular at the cosmological scale where experiments are impractical. While it has been demonstrated for selected condensed-matter systems, major questions remain regarding e.g. its degree of universality. Here we develop a global Kibble-Zurek picture from the condensed-matter level. We show theoretically that a transition between two fluctuation regimes (Ginzburg and mean-field) can lead to an intermediate region with reversed scaling, and we verify experimentally this behavior for the structural transition in the series of multiferroic hexagonal manganites. Trends across the series allow us to identify additional intrinsic features of the defect formation beyond the original Kibble-Zurek paradigm.

cond-mat.mtrl-sci↗

arXiv2017

Effect of Sr doping on the magnetic exchange interactions in manganites

Strontium doping transforms manganites of type La(1-x)Sr(x)MnO(3) from an insulating antiferromagnet (x=0) to a metallic ferromagnet (x>0.16) due to the induced charge carriers (holes). Neutron scattering experiments were employed to investigate the effect of Sr doping on a tailor-made compound of composition La(0.7)Sr(0.3)Mn(0.1)Ti(0.3)Ga(0.6)O(3). By the simultaneous doping with Sr2+ and Ti4+ ions the compound remains in the insulating state, so that the magnetic interactions for large Sr doping can be studied in the absence of charge carriers. At TC=215 K there is a first-order reconstructive phase transition from the trigonal R-3c structure to the orthorhombic Pnma structure via an intermediate virtual configuration described by the common monoclinic subgroup P21/c. The magnetic excitations associated with Mn3+ dimers give evidence for two different nearest-neighbor ferromagnetic exchange interactions, in contrast to the undoped compound LaMnyA(1-y)O(3) where both ferromagnetic and antiferromagnetic interactions are present. The doping induced changes of the exchange coupling originates from different Mn-O-Mn bond angles determined by neutron diffraction. The large fourth-nearest-neighbor interaction found for metallic manganites is absent in the insulating state. We argue that the Ruderman-Kittel-Kasuya-Yosida interaction reasonably accounts for all the exchange couplings derived from the spin-wave dispersion in metallic manganites.

cond-mat.mtrl-sci↗

arXiv2017

Distinct evolutions of Weyl fermion quasiparticles and Fermi arcs with bulk band topology in Weyl semimetals

The Weyl semimetal phase is a recently discovered topological quantum state of matter characterized by the presence of topologically protected degeneracies near the Fermi level. These degeneracies are the source of exotic phenomena, including the realization of chiral Weyl fermions as quasiparticles in the bulk and the formation of Fermi arc states on the surfaces. Here, we demonstrate that these two key signatures show distinct evolutions with the bulk band topology by performing angle-resolved photoemission spectroscopy, supported by first-principle calculations, on transition-metal monophosphides. While Weyl fermion quasiparticles exist only when the chemical potential is located between two saddle points of the Weyl cone features, the Fermi arc states extend in a larger energy scale and are robust across the bulk Lifshitz transitions associated with the recombination of two non-trivial Fermi surfaces enclosing one Weyl point into a single trivial Fermi surface enclosing two Weyl points of opposite chirality. Therefore, in some systems (e.g. NbP), topological Fermi arc states are preserved even if Weyl fermion quasiparticles are absent in the bulk. Our findings not only provide insight into the relationship between the exotic physical phenomena and the intrinsic bulk band topology in Weyl semimetals, but also resolve the apparent puzzle of the different magneto-transport properties observed in TaAs, TaP and NbP, where the Fermi arc states are similar.

cond-mat.mes-hall↗

arXiv2017

Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.

cond-mat.str-el↗

arXiv2017

Cooperative coupling of static magnetism and bulk superconductivity in the stripe phase of La_2-xBa_xCuO_4: Pressure- and doping-dependent studies

Static spin-stripe order and superconductivity were systematically studied in La_2-xBa_xCuO_4 (0.11 - 0.17) at ambient pressure by means of magnetization and muSR experiments. We find that all the investigated La_2-xBa_xCuO_4 samples exhibit static spin-stripe order and that the quasi two-dimensional superconducting (SC) transition temperature T_c1 and the static spin-stripe order temperature T_so have very similar values throughout the phase diagram. Moreover, the magnetic and the SC properties of the x = 0.155 (LBCO-0.155) and x = 0.17 (LBCO-0.17) samples were studied under hydrostatic pressure. As a remarkable result, in these bulk cuprate superconductors the three-dimensional SC transition temperature T_c and T_so nearly coincide at all pressure investigated (0 - 2.3 GPa). We also observed a pressure induced transition from long-range spin stripe order to a disordered magnetic state at 1.6 GPa in LBCO-0.155, coexisting with a SC state with substantial superfluid density. In LBCO-0.17 a disordered magnetic state is present at all p. The present results indicate that static magnetic order and SC pairing correlations develop in a cooperative fashion in La_2-xBa_xCuO_4, and provide a new route of understanding the complex interplay between static magnetism and superconductivity in the stripe phase of cuprates.

cond-mat.supr-con↗

arXiv2016

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La$_{2-x}$Sr$_{x}$CuO$_{4}$

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $α_{T^{\ast}}$ systematically decreases from $α_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $α_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $α_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

cond-mat.supr-con↗