arXiv subjects

D. Mandrus

Publications and source records attributed to D. Mandrus.

At least 145 records · Page 8Linked to original sources

arXiv2004

Luttinger liquid ARPES spectra from samples of Li$_{0.9}$Mo$_6$O$_{17}$ grown by the temperature gradient flux technique

Angle resolved photoemission spectroscopy line shapes measured for quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$ samples grown by a temperature gradient flux technique are found to show Luttinger liquid behavior, consistent with all previous data by us and other workers obtained from samples grown by the electrolyte reduction technique. This result eliminates the sample growth method as a possible origin of considerable differences in photoemission data reported in previous studies of Li$_{0.9}$Mo$_6$O$_{17}$.

cond-mat.str-el↗

arXiv2004

Magnetism in purple bronze Li$_{0.9}$Mo$_6$O$_{17}$

Muon spin relaxation measurements around the 25 K metal-insulator transition in Li$_{0.9}$Mo$_6$O$_{17}$ elucidate a profound role of disorder as a possible mechanism for this transition. The relaxation rate $1/T_1$ and the muon Knight shift are incompatible with the transition to a SDW state and thus exclude it.

cond-mat.str-el↗

arXiv2004

Non-Fermi liquid behavior near the ferromagnetic quantum critical point in the series Ca(x)Sr(1-x)RuO(3)

A series of epitaxial films were grown across the solid solution Ca(x)Sr(1-x)RuO(3) in order to pinpoint the ferromagnetic to paramagnetic quantum phase transition in this system and to study the evolution of transport and magnetic properties in its vicinity. The ferromagnetic Tc of SrRuO(3) was found to decrease linearly with Ca doping levels up to 70%. Further doping resulted in the abrupt elimination of ferromagnetic order, and the onset of low temperature (< 10K) non-Fermi liquid (NFL) resistivity of the form rho ~ rho(0 + AT^(1.5) for samples with x <= 0.75 <= 1.0. The resistivity exponent of 1.5 matches that previously observed for MnSi, indicating possible universality of the NFL behavior of itinerant ferromagnets. Field-dependent specific heat measurements on bulk samples at compositions near the quantum phase transition provide additional evidence for NFL behavior (C/T ~ log T) and show the conditions nder which spin fluctuations contribute to the specific heat.

cond-mat.str-el↗

arXiv2004

Fermi surface topology of Ca_{1.5}Sr_{0.5}RuO_{4} determined by ARPES

We report ARPES results of the Fermi surface of Ca_{1.5}Sr_{0.5}RuO$_{4}, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t_{2g} energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr_{2}RuO$_{4}. We find that while the Fermi surface topology of the alpha, beta (d_{yz, zx}) sheets remains almost the same in these two materials, the gamma (d_{xy}) sheet exhibits a hole-like Fermi surface in Ca_{1.5}Sr_{0.5}RuO_{4} in contrast to being electron-like in Sr_{2}RuO_{4}. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca_{1.5}Sr_{0.5}RuO_{4}.

cond-mat.str-el↗

arXiv2004

Infrared probe of the electronic structure and charge dynamics in Na$_{0.7}$CoO$_2$

We present measurements of the optical spectra on Na$_{0.7}$CoO$_2$ single crystals. The optical conductivity shows two broad interband transition peaks at 1.6 eV and 3.2 eV, and a weak midinfrared peak at 0.4 eV. The intraband response of conducting carriers is different from that of a simple Drude metal. A peak at low but finite frequency is observed, which shifts to higher frequencies with increasing temperature, even though the dc resistivity is metallic. We discussed the origin of the interband transitions and the low-frequency charge dynamics based on the results of band structure calculations and quasiparticle behavior observed in ARPES experiment.

cond-mat.supr-con↗

arXiv2004

Four-well tunneling states and elastic response of clathrates

We present resonant ultrasound elastic constant measurements of the clathrate compounds Eu8Ga16Ge30 and Sr8Ga16Ge30. The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLS's are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLS's is of the same order as the Einstein oscillator frequency. This explains why the observed harmonic oscillator type specific heat is not modified by tunneling states. In addition the quadrupolar interaction of FLS's with elastic strains explains the pronounced depression observed in elastic constant measurements. In the case of the Sr clathrate, we show that the shallow dip in the elastic constant c44 is explained using the same type of quadrupolar interaction with a soft Einstein mode instead of a FLS.

cond-mat.mtrl-sci↗

arXiv2004

Evidence and Characterization of a SDW Transition in Na0.75CoO2 Single Crystals

The magnetic, thermal and transport properties of Na0.75CoO2 single crystals grown by the floating zone (FZ) method are reported. Magnetic susceptibility, resistivity, magnetoresistance, and heat capacity data from these crystals indicate a bulk phase transition at T1 = 22 K. These data are most consistent with the formation of an antiferromagnet spin-density-wave (SDW) at 22 K with the easy axis for magnetization nearly along the c axis. Weak and soft ferromagnetism is observed for applied magnetic fields less than 0.5 T, which suggests a slight canting of the SDW magnetization with respect to the c axis. The jump in the heat capacity at the SDW transition is 0.45 J/K-mole-Co or about 50% of the value expected from mean-field weak-coupling theory. The reduced jump and the decrease in the resistivity below T1 are consistent with a gap for only part of the Fermi surface. The magnetoresistance is small at the SDW transition but increases in both directions reaching a value of 100% at 2 K for applied fields of 8 Tesla. The magnetoresistance data imply that the mobility of the remaining carriers is large and increases below T1. The observation of a SDW transition in this material is found to be sensitive to the preparation conditions and the degree of order in the Na layers. No SDW transition is observed in our polycrystalline powder with the same nominal composition (Na0.75CoO2) and lattice constants. Differential scanning calorimetry data, however, show distinct differences between the powder and crystal, suggesting a higher degree of order in the Na layers within the crystal. The crystal exhibits a sharp phase transition at T2 = 340 K while for the powder this transition is smeared over the temperature range from 250 to 310 K.

cond-mat.str-el↗

arXiv2004

Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7

A class of materials known as ``ferroelectric metals'' was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)], but to date no examples of this class have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ``ferroelectric metal'' in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a two-fold degenerate mode with Eu symmetry, and that motions of the O ions forming the O octahedra dominate the energetics of the transition.

cond-mat.mtrl-sci↗

arXiv2004

Thermal Spin Valves

The ability of an insulating solid to conduct heat is rarely effected by the application of a magnetic field. We have found, however, that the low temperature heat conduction of some solids increases by more than a factor of two with the application of a modest magnetic field. The effect occurs in low-dimensional magnetically ordered materials when a small gap, δ, in the acoustic magnon (spin wave) spectra is closed using a magnetic field H > δ/gμ_B. Since all magnetically ordered materials must have a gap in the magnon spectra for magnons with k = 0, this may be a very general effect. Extra heat is carried through the solid only when the magentic field exceeds the critical value (H > δ/gμ_B). At this critical field the tiny atomic magnets in the solid abruptly change the direction they point which results in more heat flowing through the material. The magnetic field thus acts as a heat switch. We have observed this effect in three quite different magnetically ordered materials: K_2V_3O_8, Nd_2CuO_4 and Pr_2CuO_4. Several possible explanations for these effects will be discussed.

cond-mat.str-el↗

arXiv2003

Optical properties of pyrochlore oxide $Pb_{2}Ru_{2}O_{7-δ}$

We present optical conductivity spectra for $Pb_{2}Ru_{2}O_{7-δ}$ single crystal at different temperatures. Among reported pyrochlore ruthenates, this compound exhibits metallic behavior in a wide temperature range and has the least resistivity. At low frequencies, the optical spectra show typical Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a broad absorption feature is observed. Our analysis suggests that the low frequency responses can be understood from two Drude components arising from the partially filled Ru $t_{2g}$ bands with different plasma frequencies and scattering rates. The high frequency broad absorption may be contributed by two interband transitions: from occupied Ru $t_{2g}$ states to empty $e_{g}$ bands and from the fully filled O 2p bands to unoccupied Ru $t_{2g}$ states.

cond-mat.mtrl-sci↗

arXiv2003

Distribution of exchange energy in a bond-alternating S=1 quantum spin chain

The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP is studied by single crystal inelastic neutron scattering. Parameters of the measured dispersion relation for magnetic excitations are compared to existing numerical results and used to determine the magnitude of bond-strength alternation. The measured neutron scattering intensities are also analyzed using the 1st-moment sum rules for the magnetic dynamic structure factor, to directly determine the modulation of ground state exchange energies. These independently determined modulation parameters characterize the level of spin dimerization in NTENP. First-principle DMRG calculations are used to study the relation between these two quantities.

cond-mat.str-el↗

arXiv2003

ARPES on Na$_{0.6}$CoO$_{2}$: Fermi surface, extended flat dispersion, and unusual band splitting

The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c \sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($Γ$-$K$, $Γ$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $Γ$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $Γ$-$M$ direction, which differs from the predicted bilayer splitting.

cond-mat.supr-con↗

arXiv2003

Observation of Bulk Superconductivity in NaxCoO2.yH2O and NaxCoO2.yD2O Powder and Single Crystals

Poly- and single-crystalline Na$_x$CoO$_2$ has been successfully intercalated with H$_2$O and D$_2$O as confirmed by X-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with T$_c$ close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on T$_c$ of less than 0.2 K. Investigation of the resistivity anisotropy of Na$_x$CoO$_2\cdot$yH$_2$O single crystals shows: (a) almost zero resistivity below T$_c$, and (b) an abrupt upturn at $T^* \sim$ 52 K in both the $\it {ab}$ plane and the $\it {c}$ direction. The implications of our results on the possible superconducting mechanism will be discussed.

cond-mat.supr-con↗

arXiv2003

The electrical properties of Cd2Re2O7 under pressure

We examine the resistivity and thermopower of single crystal specimens of the pyrochlore oxide Cd2Re2O7 at pressures up to 2GPa. Thermopower proves to be a sensitive tool in the study of the phase diagram of Cd2Re2O7. The 200K metal-to-metal phase transition is accompanied by a strong increase of the absolute value of the thermopower. A weaker anomaly allows to identify a second phase transition at 125K. Following the temperature dependence of this anomaly, we obtain the corresponding phase boundary up to 1.2GPa, and argue that it must drop to T=0 before p reaches 1.8GPa. There is a wide temperature range where the electrical properties are fairly sensitive to pressure, indicating the strong coupling of the electronic degrees of freedom to the lattice.

cond-mat.str-el↗

arXiv2002

Doping evolution of the phonon density of states and electron-lattice interaction in Nd$_{2-x}$Ce$_x$CuO$_{4+δ}$

We use inelastic neutron scattering to study the evolution of the generalized phonon density of states (GDOS) of the $n$-type high-$T_c$ superconductor Nd$_{2-x}$Ce$_x$CuO$_{4+δ}$ (NCCO), from the half-filled Mott-insulator ($x=0$) to the $T_c=24$ K superconductor ($x=0.15$). Upon doping the CuO$_2$ planes in Nd$_2$CuO$_{4+δ}$ (NCO) with electrons by Ce substitution, the most significant change in the GDOS is the softening of the highest phonon branches associated with the Cu-O bond stretching and out-of-plane oxygen vibration modes. However, the softening occurs within the first few percent of Ce-doping and is not related to the electron doping induced nonsuperconducting-superconducting transition (NST) at $x\approx 0.12$. These results suggest that the electron-lattice coupling in the $n$-type high-$T_c$ superconductors is different from that in the $p$-type materials.

cond-mat.supr-con↗

arXiv2002

Orbital-resolved Soft X-Ray Spectroscopy in NaV2O5

We demonstrate that angle-resolved soft x-ray spectroscopy can resolve absorption by inequivalent oxygen sites and by different orbitals belonging to the same site in NaV2O5. By rotating the polarization direction, we see a dramatic change in the absorption spectra at the oxygen K edge. Our theory identifies the detailed composition of the spectra and predicts a correct energy-ordering of the orbitals of three inequivalent oxygen atoms. Because different orbitals dominate absorption spectra at different energies and angles, one can excite at a specific site and ``orbital''. In contrast, absorption at the vanadium L edge does not show large changes when varying the polarization direction. The reason for this is that different excitation channels (involving different initial states for the excited electron) overlap in energy and vary in compensating ways, obscuring each channel's sensitive polarization dependence.

cond-mat.str-el↗

arXiv2002

Searching for the Slater Transition in the Pyrochlore Cd$_{2}$Os$_{2}$O$_{7}$ with Infrared Spectroscopy

Infrared reflectance measurements were made on the single crystal pyrochlore Cd$_{2}$Os$_{2}$O$_{7}$ in order to examine the transformations of the electronic structure and crystal lattice across the boundary of the metal insulator transition at $T_{MIT}=226K$. All predicted IR active phonons are observed in the conductivity over all temperatures and the oscillator strength is found to be temperature independent. These results indicate that charge ordering plays only a minor role in the MIT and that the transition is strictly electronic in nature. The conductivity shows the clear opening of a gap with $2Δ=5.2k_{B}T_{MIT}$. The gap opens continuously, with a temperature dependence similar to that of BCS superconductors, and the gap edge having a distinct $σ(ω)\thicksimω^{1/2}$ dependence. All of these observables support the suggestion of a Slater transition in Cd$_{2}% $Os$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2002

Structural Ordering and Symmetry Breaking in Cd_2Re_2O_7

Single crystal X-ray diffraction measurements have been carried out on Cd_2Re_2O_7 near and below the phase transition it exhibits at Tc' ~195 K. Cd_2Re_2O_7 was recently discovered as the first, and to date only, superconductor with the cubic pyrochlore structure. Superlattice Bragg peaks show an apparently continuous structural transition at Tc', however the order parameter displays anomalously slow growth to ~Tc'/10, and resolution limited critical-like scattering is seen above Tc'. High resolution measurements show the high temperature cubic Bragg peaks to split on entering the low temperature phase, indicating a (likely tetragonal) lowering of symmetry below Tc'.

cond-mat.str-el↗