arXiv subjects

P. Khalifah

Publications and source records attributed to P. Khalifah.

14 recordsLinked to original sources

arXiv2009

Infrared anomalous Hall effect in SrRuO$_3$: Evidence for crossover to intrinsic behavior

The origin of the Hall effect in many itinerant ferromagnets is still not resolved, with an anomalous contribution from the sample magnetization that can exhibit extrinsic or intrinsic behavior. We report the first mid-infared (MIR) measurements of the complex Hall ($θ_H$), Faraday ($θ_F$), and Kerr ($θ_K$) angles, as well as the Hall conductivity ($σ_{xy}$) in a SrRuO$_3$ film in the 115-1400 meV energy range. The magnetic field, temperature, and frequency dependence of the Hall effect is explored. The MIR magneto-optical response shows very strong frequency dependence, including sign changes. Below 200 meV, the MIR $θ_H (T)$ changes sign between 120 and 150 K, as is observed in dc Hall measurements. Above 200 meV, the temperature dependence of $θ_H$ is similar to that of the dc magnetization and the measurements are in good agreement with predictions from a band calculation for the intrinsic anomalous Hall effect (AHE). The temperature and frequency dependence of the measured Hall effect suggests that whereas the behavior above 200 meV is consistent with an intrinsic AHE, the extrinsic AHE plays an important role in the lower energy response.

cond-mat.str-el↗

arXiv2008

A New Unconventional Antiferromagnet, Yb$_3$Pt$_4$

We report the synthesis and basic properties of single crystals of a new binary compound, Yb$_{3}$Pt$_{4}$. The Yb ions in this compound are fully trivalent, and heat capacity measurements show that the crystal field scheme involves a doublet ground state, well separated from the excited states, which are fully occupied above $\sim$ 150 K. The heat capacity displays a large, weakly first order anomaly at 2.4 K, where a cusp is observed in the magnetic susceptibility signalling the onset of antiferromagnetic order. The entropy associated with this order is the full Rln2 of the doublet ground state, however the magnetic susceptibility in the ordered phase is dominated by a large and temperature independent component below the Neel temperature. The heat capacity in the ordered state originates with ferromagnetic spin waves, giving evidence for the inherently local moment character of the ordered state. The electrical resistivity is unusually large, and becomes quadratic in temperature exactly at the Neel temperature. The absence of analogous Fermi liquid behavior in the heat capacity and the magnetic susceptibility implies that Yb$_{3}$Pt$_{4}$ is a low electron density system, where the Fermi surface is further gapped by the onset of magnetic order.

cond-mat.str-el↗

arXiv2007

Incommensurate Charge Order Phase in Fe2OBO3 due to Geometrical Frustration

The temperature dependence of charge order in Fe2OBO3 was investigated by resistivity and differential scanning calorimetry measurements, Mossbauer spectroscopy, and synchrotron x-ray scattering, revealing an intermediate phase between room temperature and 340 K, characterized by coexisting mobile and immobile carriers, and by incommensurate superstructure modulations with temperature-dependent propagation vector (1/2,0,tau). The incommensurate modulations arise from specific anti-phase boundaries with low energy cost due to geometrical charge frustration.

cond-mat.str-el↗

arXiv2007

Charge Order Superstructure with Integer Iron Valence in Fe2OBO3

Solution-grown single crystals of Fe2OBO3 were characterized by specific heat, Mossbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mossbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe2OBO3 is the clearest example of ionic charge order so far.

cond-mat.str-el↗

arXiv2007

Determination of the infrared complex magneto-conductivity tensor in itinerant ferromagnets from Faraday and Kerr measurements

We present measurement and analysis techniques that allow the complete complex magneto-conductivity tensor to be determined from mid-infrared (11-1.6 micron; 100-800 meV) measurements of the complex Faraday (theta_F) and Kerr (theta_K) angles. Since this approach involves measurement of the geometry (orientation axis and ellipticity of the polarization) of transmitted and reflected light, no absolute transmittance or reflectance measurements are required. Thick film transmission and reflection equations are used to convert the complex theta_F and theta_K into the complex longitudinal conductivity sigma_xx and the complex transverse (Hall) conductivity sigma_xy. theta_F and theta_K are measured in a Ga_(1-x)Mn_xAs and SrRuO_3 films. The resulting sigma_xx is compared to the values obtained from conventional transmittance and reflectance measurements, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films.

cond-mat.str-el↗

arXiv2005

Kondo Lattice Behavior in the Ordered Dilute Magnetic Semiconductor Yb_{14-x}La_xMnSb_{11}

We report Hall, magnetic, heat capacity and doping studies from single crystals of Yb_14MnSb_11 and Yb_13.3La_0.7MnSb_11. These heavily doped semiconducting compounds are ferromagnetic below 53 K and 39 K respectively. The renormalization of the carrier mass from 2 me near room temperature to 15 me at 5 K, plus the magnetic evidence for partial screening of the Mn magnetic moments suggest that these compounds represent rare examples of an underscreened Kondo lattice with T_K = 350 K.

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

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↗

arXiv2003

MgB2 Energy Gap Determination by Scanning Tunneling Spectroscopy

We report scanning tunneling spectroscopy (STS) measurements of the gap properties of both ceramic MgB2 and c-axis oriented epitaxial MgB2 thin films. Both show a temperature dependent zero bias conductance peak and evidence for two superconducting gaps. We report tunneling spectroscopy of superconductor-insulator-superconductor (S-I-S) junctions formed in two ways in addition to normal metal-insulator-superconductor (N-I-S) junctions. We find a gap delta=2.3-2.8 meV, with spectral features and temperature dependence that are consistent between S-I-S junction types. In addition, we observe evidence of a second, larger gap, delta=7.2 meV, consistent with a proposed two-band model.

cond-mat.supr-con↗

arXiv2003

Magnetic Behavior of La7Ru3O18

Rhombohedral La7Ru3O18 can be considered to be nearly geometrically frustrated due to its close structural similarity to the strongly geometrically frustrated compound La4.87Ru2O12. The magnetic ordering of La7Ru3O18 was explored using a combination of dc and ac magnetic susceptibility measurements. The magnetic phase diagram shows two different ordering regimes. A low field magnetic phase occurs at applied fields of 0-3T and temperatures below 10K, while a high field phase is found at fields of 3-5T and temperatures below 9K.

cond-mat.str-el↗

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↗

arXiv2001

Giant anharmonicity and non-linear electron-phonon coupling in MgB$_{2}$; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $σ$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$

cond-mat.mtrl-sci↗

arXiv2001

Loss of superconductivity and structural transition in Mg1-xAlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. Therefore the recent announcement of superconductivity at 39 K in the simple binary ceramic compound MgB2 is surprising. This compound, available from common chemical suppliers, and used as a starting material for chemical metathesis reactions, has been known and structurally characterized since the mid 1950's. Here we show that the addition of electrons to MgB2 through partial substitution of Al for Mg results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near 10% Al content. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

cond-mat.supr-con↗

arXiv2001

Strongly linked current flow in polycrystalline forms of the new superconductor MgB2

The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.

cond-mat.supr-con↗