arXiv subjects

Sunil Nair

Publications and source records attributed to Sunil Nair.

48 records · Page 3Linked to original sources

arXiv2008

Analysis of the normal state magnetotransport in CeIrIn${_5}$

We present an analysis of the normal state magnetotransport in the heavy fermion superconductor CeIrIn${_5}$. The Hall effect and the transverse magnetoresistance in this material do not appear to be uniquely correlated, as inferred from the field dependence of the current ratio ($R_σ = σ_{xy} / σ_{xx}^2 H$). The Hall coefficient is seen to satisfy a scaling equation of the form $R_H = f [H / (a + b T^c)]$. These results are compared to those observed earlier in CeCoIn$_5$, and are discussed in terms of the contrasting phase diagram which the CeIrIn$_5$ system exhibits in relation to its Co counterpart.

cond-mat.str-el↗

arXiv2008

Magnetotransport in the CeIrIn${_5}$ system: The influence of antiferromagnetic fluctuations

We present an overview of magnetotransport measurements on the heavy-fermion superconductor CeIrIn$_5$. Sensitive measurements of the Hall effect and magnetoresistance (MR) are used to elucidate the low temperature phase diagram of this system. The normal-state magnetotransport is highly anomalous, and experimental signatures of a pseudogap-like precursor state to superconductivity as well as evidence for two distinct scattering times governing the Hall effect and the MR are observed. Our observations point out the influence of antiferromagnetic fluctuations on the magnetotransport in this class of materials. The implications of these findings, both in the context of unconventional superconductivity in heavy-fermion systems as well as in relation to the high temperature superconducting cuprates are discussed.

cond-mat.str-el↗

arXiv2008

The precursor state to superconductivity in CeIrIn${_5}$: Unusual scaling of magnetotransport

We present an analysis of the normal-state Hall effect and magnetoresistance in the heavy fermion superconductor CeIrIn${_5}$. It is demonstrated that the modified Kohler's scaling--which relates the magnetoresistance to the Hall angle--breaks down prior to the onset of superconductivity due to the presence of a precursor state to superconductivity in this system. A model-independent, single-parameter scaling of the Hall angle governed solely by this precursor state is observed. Neither the Hall coefficient nor the resistivity exhibit this scaling implying that this precursor state preferentially influences the Hall channel.

cond-mat.str-el↗

arXiv2007

A precursor state to unconventional superconductivity in CeIrIn${_5}$

We present sensitive measurements of the Hall effect and magnetoresistance in CeIrIn${_5}$ down to temperatures of 50 mK and magnetic fields up to 15 T. The presence of a low temperature coherent Kondo state is established. Deviations from Kohler's rule and a quadratic temperature dependence of the cotangent of the Hall angle are reminiscent of properties observed in the high temperature superconducting cuprates. The most striking observation pertains to the presence of a \textit{precursor} state--characterized by a change in the Hall mobility--that appears to precede the superconductivity in this material, in similarity to the pseudogap in the cuprate high $T_c$ superconductors.

cond-mat.str-el↗

arXiv2005

Finite size effects and the effect of disorder on electronic phase separation in Pr_(0.5)Ca_(0.5)Mn_(1-x)Al_(x)O_(3)

We report a bulk magnetisation study of the low temperature magnetic phases in the Mn site substituted half doped manganite series Pr_(0.5)Ca_(0.5)Mn_(1-x)Al_(x)O_(3). Our results indicate the formation of self organised structures in this series of compounds which is driven by electronic phase separation and stabilised by the presence of finite quenched disorder, thus providing a basis for understanding the anomalously large supression of the CE phase in the presence of disorder. Striking magnetic memory effects similar to that seen in spin glasses and interacting superparamagnets are also observed. Increasing disorder is seen to make electronic phase separation weaker owing to the destabilisation of the CE state which results in an enhanced mobility of the charge carriers.

cond-mat.str-el↗

arXiv2005

Critical exponents and the correlation length in the charge exchange manganite spin glass Eu_{0.5}Ba_{0.5}MnO_{3}

The critical regime of the charge exchange (CE) manganite spin glass Eu_{0.5}Ba_{0.5}MnO_{3} is investigated using linear and non linear magnetic susceptibility and the divergence of the third ordered susceptibility (chi{_3}) signifying the onset of a conventional freezing transition is experimentally demonstrated. The divergence in chi{_3}, dynamical scaling of the linear susceptibility and relevant scaling equations are used to determine the critical exponents associated with this freezing transition, the values of which match well with the 3D Ising universality class. Magnetic field dependence of the spin glass response function is used to estimate the spin correlation length which is seen to be larger than the charge/orbital correlation length reported in this system.

cond-mat.str-el↗

arXiv2004

Formation of finite antiferromagnetic clusters and the effect of electronic phase separation in Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}

We report the first experimental evidence of a magnetic phase arising due to the thermal blocking of antiferromagnetic clusters in the weakened charge and orbital ordered system Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}. The third order susceptibility (χ_3) is used to differentiate this transition from a spin or cluster glass like freezing mechanism. These clusters are found to be mesoscopic and robust to electronic phase separation which only enriches the antiphase domain walls with holes at the cost of the bulk, without changing the size of these clusters. This implies that Al substitution provides sufficient disorder to quench the length scales of the striped phases.

cond-mat.str-el↗

arXiv2003

Dilution of 2D antiferromagnetism by Mn site substitution in La{_1}Sr{_2}Mn{_{2-x}}Al{_x}O{_7}

We report the effect of Al substitution on the Mn site of the bilayered half doped manganite La{_1}Sr{_2}Mn{_2}O{_7}. This substitution dilutes the magnetically active Mn-O-Mn network without introducing an appreciable distortion in the lattice and ionic considerations leads to a predominant reduction of Mn{^{4+}} with increasing Al. The rate of fall of the long range antiferromagnetic transition temperature as a function of substitution is seen to match well with established quasi 2D Heisenberg systems indicating that the nature of magnetic interactions in this quasi 2-D system is of the short range Heisenberg type. The magnetic contribution of the specific heat estimated using Fisher's relation could be fitted with a function incorporating the presence of a gapped Fermi surface appropriate for this type of systems. The resistivity was seen to increase as a function of substitution due to the weakening of the double exchange within the ferromagnetic layers of these A type of antiferromagnets and justifiably, in the paramagnetic region the data could be fitted to Mott's equation for the variable range hopping of polarons in 2 dimensions.

cond-mat.str-el↗

arXiv2003

Effect of Al substitution on the charge ordered Pr{_0.5}Ca{_0.5}MnO{_3}: Structure, Magnetism and Transport

We report the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr{_0.5}Ca{_0.5}MnO{_3}. This substitution introduces impurities in the Mn-O-Mn network, causes the Mn{^3+}/Mn{^4+} ratio to deviate away from unity and is seen to predominantly replace Mn{^4+}. The strength of charge ordering is supressed with increasing Al doping, as is seen from ac susceptibility, the magnetocaloric effect and dc resistivity. The observation of traces of charge ordering over an extended range of impurity doping of 10% besides being indicative of the robust nature of charge ordering in Pr{_0.5}Ca{_0.5}MnO{_3}, also underscores the relatively insignificant effect that Al substitution has on the lattice distortions and the importance of non-magnetic substitutions. For all the samples, the conductivity in the temperature range above the charge ordering transition is observed to be due to the adiabatic hopping of small polarons, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid.

cond-mat↗

arXiv2003

Probing a ferromagnetic critical regime using nonlinear susceptibility

The second order para-ferromagnetic phase transition in a series of amorphous alloys (Fe{_5}Co{_{50}}Ni{_{17-x}}Cr{_x}B{_{16}}Si{_{12}}) is investigated using nonlinear susceptibility. A simple molecular field treatment for the critical region shows that the third order suceptibility (chi{_3}) diverges on both sides of the transition temperature, and changes sign at T{_C}. This critical behaviour is observed experimentally in this series of amorphous ferromagnets, and the related assymptotic critical exponents are calculated. It is shown that using the proper scaling equations, all the exponents necessary for a complete characterization of the phase transition can be determined using linear and nonlinear susceptiblity measurements alone. Using meticulous nonlinear susceptibility measurements, it is shown that at times chi{_3} can be more sensitive than the linear susceptibility (chi{_1}) in unravelling the magnetism of ferromagnetic spin systems. A new technique for accurately determining T{_C} is discussed, which makes use of the functional form of chi{_3} in the critical region.

cond-mat.mtrl-sci↗

arXiv2003

Observation of 3D Heisenberg-like ferromagnetism in single crystal La_{0.875}Sr_{0.125}MnO_3

We report measurements and analysis of the magnetic critical phenomena in a single crystal of La${_{0.875}}$Sr${_{0.125}}$MnO${_3}$. The critical exponents associated with the ferromagnetic transition have been determined from ac susceptibility and dc magnetisation data. Techniques like the Kouvel$-$Fischer plots, modified Arrots Plots and the critical isotherm analysis were used for this purpose. The values of the the exponents $γ$, $β$ and $δ$ obtained are found to match very well with those predicted for the 3D Heisenberg model. Our results are consistent with recent numerical calculations and suggest that though the double exchange interaction is driven by the motion of conduction electrons, the effective magnetic interaction near the transition is renormalized to a short range one.

cond-mat.str-el↗

arXiv1997

Geometric Residue Theorems for Bundle Maps

In this paper we prove geometric residue theorems for bundle maps over a compact manifold. The theory developed associates residues to the singularity submanifolds of the map for any invariant polynomial. The theory is then applied to a variety of settings: smooth maps between equidimensional manifolds, CR-singularities, finite singularities and singularities of odd forms as spinor bundle maps.

dg-ga↗