arXiv subjects

S. S. Saxena

Publications and source records attributed to S. S. Saxena.

15 recordsLinked to original sources

arXiv2019

Pressure dependence of ferroelectric quantum critical fluctuations

The effect of ferroelectric fluctuations on the temperature dependent dielectric constant of SrTiO$_3\,$(STO) has been long studied. Those fluctuations have been shown in recent years to be quantum critical and STO demonstrated to form the archetypal quantum critical paraelectric. The effect of those same fluctuations on the pressure and temperature dependence of the ferroelectric soft phonon mode as the system is tuned away from criticality are reported for the first time in this paper. We show that the mean field approximation is confirmed experimentally. Furthermore, using a self-consistent model of the quantum critical excitations including coupling to the volume strain and without adjustable parameters, we determine logarithmic corrections that would be observable only very close to the quantum critical point. Thus, the mean-field character of the pressure dependence is much more robust to the fluctuations than is the temperature dependence. We predict stronger corrections for lower dimensionalities however. The same calculation confirms that the Lydanne-Sachs-Teller relation is valid over the whole pressure and temperature range considered. Therefore, the measured dielectric constant can be used to extract the frequency of the soft mode down to 1.5 K and up to 20 kbar of applied pressure. The soft mode is observed to stiffen further, raising the low-temperature energy gap and returning towards the expected shallow temperature dependence of an optical mode. This behavior is consistent with the existence of a quantum critical point on the pressure-temperature phase diagram of STO, which applied pressure tunes the system away from. This work represents the first experimental measurement of the stiffening of a soft phonon mode as a system is tuned away from criticality, a potentially universal phenomenon across a variety of phase transitions and systems in condensed matter physics.

cond-mat.str-el↗

arXiv2019

Isostructural Mott Transition in 2D honeycomb antiferromagnet V$_{0.9}$PS$_3$

We present the observation of an isostructural Mott insulator-metal transition in van-der-Waals honeycomb antiferromagnet V$_{0.9}$PS$_3$ through high-pressure x-ray diffraction and transport measurements. The MPX$_3$ family of magnetic van-der-Waals materials (M denotes a first row transition metal and X either S or Se) are currently the subject of broad and intense attention, but the vanadium compounds have until this point not been studied beyond their basic properties. We observe insulating variable-range-hopping type resistivity in V$_{0.9}$PS$_3$, with a gradual increase in effective dimensionality with increasing pressure, followed by a transition to a metallic resistivity temperature dependence between 112 and 124 kbar. The metallic state additionally shows a low-temperature upturn we tentatively attribute to the Kondo Effect. A gradual structural distortion is seen between 26-80 kbar, but no structural change at higher pressures corresponding to the insulator-metal transition. We conclude that the insulator-metal transition occurs in the absence of any distortions to the lattice - an isostructural Mott transition in a new class of two-dimensional material, and in strong contrast to the behavior of the other MPX$_3$ compounds.

cond-mat.str-el↗

arXiv2018

Structural and electronic phase transitions in FePS$_3$ under the application of pressure

Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. In this paper we present work on a new metal to insulator transition system FePS$_3$ . This compound is a two-dimensional van-der-Waals antiferromagnetic Mott insulator. Here we report the discovery of an insulator-metal transition in FePS$_3$, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure and resistivity measurements show evidence of onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.

cond-mat.str-el↗

arXiv2018

Superconductivity in the vicinity of a ferroelectric quantum phase transition

Superconductivity has been observed in doped SrTiO$_3$ at charge-carrier densities below 10$^{18}$ cm$^{-3}$, where the density of states at the Fermi level of the itinerant electrons is several orders of magnitude lower than that of conventional metals. In terms of the Bardeen-Cooper-Schrieffer description, this implies the existence of an extraordinarily strong interaction driving the formation of Cooper pairs, potentially comparable in order of magnitude to that in some high Tc superconductors. Under suitable conditions the interaction might remain effective at densities approaching metallic densities, leading to the possibility of pair formation at elevated temperatures. Here we investigate the pressure dependence of the resistivity and superconducting transition temperature, Tc, of SrTiO$_3$ at a carrier density near to optimal doping. Our experiments show that Tc collapses rapidly with pressure and hence with increasing frequency of the soft transverse-optical phonon mode connected to the ferroelectric quantum critical point. We show that the superconductivity phase diagram can be understood in terms of the coupling of electrons via two hybrid longitudinal polar modes, based on a model of dipolar fluctuations of the charge carrier-ion system. In particular, we predict that for carrier densities above the order of 10$^{18}$ cm$^{-3}$, Tc can be strongly enhanced on approaching the ferroelectric quantum critical point, as seen in our measurements of SrTiO$_3$ and as found in many electrically conducting magnetic analogues. However below this density we predict the reverse behaviour, namely that Tc is suppressed on approaching the ferroelectric quantum critical point. Our model is also relevant to superconductivity found in gated ferroelectric quantum critical systems such as KTaO$_3$ and can guide searches for new superconductors in a diversity of materials.

cond-mat.supr-con↗

arXiv2015

Fermi surfaces and orbital polarization in superconducting CeO$_{0.5}$F$_{0.5}$BiS$_{2}$ revealed by angle-resolved photoemission spectroscopy

We have investigated the electronic structure of BiS$_2$-based CeO$_{0.5}$F$_{0.5}$BiS$_2$ superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around X point, the straight portion parallel to the $k_y$ direction is dominated by Bi $6p_x$ character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the BiS$_2$ system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

cond-mat.supr-con↗

arXiv2015

Magnetoelastic relaxations in EuTiO3

The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300 K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below ~2.8 K. Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls.

cond-mat.mtrl-sci↗

arXiv2011

Magnetism in graphite oxide: The role of epoxy groups

We investigate the magnetism in graphite by controlled oxidation. Our approach renders graphite an insulator while maintaining its structure. Fourier transform infrared spectroscopy and X-ray absorption near edge structure spectra reveal that graphite oxide has epoxy groups on its surface and it is not thermally stable. Magnetic susceptibility data exhibit negative Curie temperature, field irreversibility, and slow relaxation. The magnetic properties diminish after the epoxy groups are destroyed. The overall results indicate the unexpected magnetism is associated with the presence of epoxy groups.

cond-mat.str-el↗

arXiv2010

Neutron scattering study of the high energy graphitic phonons in superconducting CaC$_6$

We present the results of a neutron scattering study of the high energy phonons in the superconducting graphite intercalation compound CaC$_6$. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC$_6$, and in particular any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density functional theory (DFT). A description is given of the analysis methods developed to account for the highly-textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC$_6$. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.

cond-mat.supr-con↗

arXiv2009

Quantum criticality in ferroelectrics

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

cond-mat.str-el↗

arXiv2008

Optical spectra of the heavy fermion uniaxial ferromagnet UGe$_2$

We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $ω>$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.

cond-mat.str-el↗

arXiv2006

Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C$_6$Yb

We report measurements of the in-plane electrical resistivity $ρ$ and the thermal conductivity $κ$ of the intercalated graphite superconductor C$_6$Yb to temperatures as low as $T_c$/100. When a field is applied along the c-axis, the residual electronic linear term $κ_0/T$ evolves in an exponential manner for $H_{c1} < H < H_{c2}$. This activated behaviour establishes the order parameter as unambiguously s-wave, and rules out the possibility of multi-gap or unconventional superconductivity in this system.

cond-mat.supr-con↗

arXiv2006

Superconductivity induced by spark erosion in ZrZn2

We show that the superconductivity observed recently in the weak itinerant ferromagnet ZrZn2 [C. Pfleiderer et al., Nature (London) 412, 58 (2001)] is due to remnants of a superconducting layer induced by spark erosion. Results of resistivity, susceptibility, specific heat and surface analysis measurements on high-quality ZrZn2 crystals show that cutting by spark erosion leaves a superconducting surface layer. The resistive superconducting transition is destroyed by chemically etching a layer of 5 microns from the sample. No signature of superconductivity is observed in rho(T) of etched samples at the lowest current density measured, J=675 Am-2, and at T < 45 mK. EDX analysis shows that spark-eroded surfaces are strongly Zn depleted. The simplest explanation of our results is that the superconductivity results from an alloy with higher Zr content than ZrZn2.

cond-mat.supr-con↗

arXiv2005

Pressure-induced insulating state in (La,Sr)CoO$_{3}$

We have investigated the effect of pressure on the electronic, magnetic, and structural properties on a single crystal of conducting, ferromagnet (T$_{C}$=157K) La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ located near the boundary of the metal-insulator transition. Contrary to the results reported on related systems, we find a transition from the conducting state to an insulating state and a decrease of T$_{C}$ with increasing pressure while the lattice structure remains unchanged. We show that this unusual behavior is driven by a gradual change of the spin state of Co$^{3+}$ ions from magnetic intermediate-spin (t$_{2g}^5$e$_{g}^{1}$; S=1) to a nonmagnetic low-spin (t$_{2g}^6$e$_{g}^{0}$; S=0) state.

cond-mat.str-el↗

arXiv2001

Observation of the Transverse Optical Plasmon in SmLa0.8Sr0.2CuO4-d

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-d, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)O block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

cond-mat.supr-con↗

arXiv1998

Anomalous Low Temperature States in CeNi2Ge2

Ambient pressure studies on high purity single crystals of the stoichiometric 4f-electron metal CeNi2Ge2 reveal anomalous low temperature forms of the resistivity which challenge our understanding of the metallic state. Comparisons are made with the isostructural and isoelectronic compound CePd2Si2 near the border of magnetism at high pressure, and possible reasons for this novel non-Fermi liquid form of the resistivity are discussed. Phase diagrams of further anomalies are presented, which involve a loss of resistance at low temperature in some samples of CeNi2Ge2 and unexpected high pressure phases.

cond-mat.str-el↗