arXiv subjects

Haranath Ghosh

Publications and source records attributed to Haranath Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2014

Fermiology of 122 family of Fe-based superconductors: An ab initio study

Fermiology of various 122 systems are studied through first principles simulation. Electron doping causes expansion of electron and shrinkage of hole Fermi pockets. Isovalent Ru substitution (upto 35%) makes no visible modification in the electron and hole like FSs providing no clue regarding the nature of charge carrier doping. However, in case of 32% P doping there are considerable changes in the hole Fermi surfaces (FSs). From our calculations, it is very clear that two dimensionality of FSs may favour electron pair scattering between quasi-nested FSs which has important bearings in various orders (magnetic, orbital, superconducting) present in Fe-based superconductors.

cond-mat.supr-con↗

arXiv2014

Iron based superconductors: A brief overview

Fe-based superconductors were discovered in 2008. This discovery with T$_c$ values up to 56 K, generated a new belief in the field of superconductivity. Till its discovery, high temperature superconductivity in cuprates, created a prejudice that Cu-oxides are essential building blocks for a high temperature superconducting material. These Fe based superconductors do not contain Cu-O planes (some of the materials are even O free). It will be argued in this review, that these iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface together with an unusual Fermiology that can change rapidly with doping. This may lead to very different normal and superconducting state properties compared to those in standard electron-phonon coupled {\it conventional} superconductors. There are a large number of evidences showing that superconductivity, magnetism, orbital fluctuations are intimately related and coexist in these materials although the mechanism of superconductivity in these compounds is still unknown. The electronic specific heat, $\frac{2Δ}{k_B T_c}$ ratio, phase diagrams, isotope effect, crystal structures and there correlation to T$_c$ from various available experimental data are main inputs of this review to show the above.

cond-mat.supr-con↗

arXiv2014

Origin of structural and magnetic transitions in BaFe$_{2-x}$Ru$_x$As$_2$ materials

Using the experimentally measured temperature and doping dependent structural parameters on Ru doped BaFe$_2$As$_2$, orbital-dependent reconstruction of the electronic structure across the magnetostructural transition is found, through first principle simulations. Below structural transition there exists two distinct Fe - Fe bond distances which modifies the Fe-d$_{xy}$ orbital largely due to its planar spatial extension leading to Lifshitz transition, while the otherwise degenerate Fe-d$_{xz}$ and d$_{yz}$ orbitals become non-degenerate, giving rise to orbital order. The orbital order follows the temperature dependence of orthorhombocity and is also the cause of two distinct Fe - Fe bond distances. Doping dependent Fermi surfaces show nearly equal expansion of both the electron and hole like Fermi surfaces whereas the hole Fermi surface shrinks with temperature but the electron Fermi surface expands comparatively slowly. The observed structural transition in this compound is electronic in origin, occurs close to the Lifshitz transition whereas the suppression of the concurrent magnetic transition is due to loss of temperature dependent nesting of Fermi surface.

cond-mat.supr-con↗

arXiv2014

Intra-inter band pairing, order parameter symmetry in Fe-based superconductors : A model study

In the quest of why there should be a single transition temperature in a multi-gapped system like Fe-based materials we use two band model for simplicity. The model comprises of spin density wave (SDW), orbital density wave (ODW) arising due to nested pieces of the electron and hole like Fermi surfaces; together with superconductivity of different pairing symmetries around electron and hole like Fermi surfaces. We show that either only intra or only inter band pairing is insufficient to describe some of the experimental results like large to small gap ratio, thermal behaviour of electronic specific heat jump etc. It is shown that the inter-band pairing is essential in Fe-based materials having multiple gaps to produce a single global $T_c$. Some of our results in this scenario, matches with the earlier published work \cite{two-band-prb}, and also have differences. The origin of difference between the two is also discussed. Combined intra-inter band pairing mechanism produces the specific heat jump to superconducting transition temperature ratio proportional to square of the transition temperature, both in the electron and hole doped regime, for sign changing s$^{\pm}$ wave symmetry which takes the d+s pairing symmetry form. Our work thus demonstrates the importance of combined intra-inter band pairing irrespective of the pairing mechanism.

cond-mat.supr-con↗

arXiv2014

Effect of Synchrotron Polarization in Grazing Incidence X-ray Fluorescence analysis

Total reflection x-ray fluorescence (TXRF) spectroscopy has seen a remarkable progress over the past years. Numerous applications in basic and applied sciences prove its importance. The large spectral background which is a major detrimental factor in the conventional x-ray fluorescence technique, limits the element detection sensitivities of the technique to ppm range. This spectral background reduces to a great extent in the TXRF technique due to the low extinction depth of the primary incident x-ray beam. In synchrotron radiation (SR) based TXRF measurements the spectral background reduces further because of the polarization of the synchrotron x-ray beam. Here, we discuss in detail the influence of synchrotron polarization on the spectral background in a fluorescence spectrum and its significance towards TXRF detection sensitivities. We provide a detailed theoretical description and show that how anisotropic scattering probability densities of the Compton and Elastic scattered x-rays depend on the scattering angle (θ) and azimuthal angle (ϕ) in the polarization plane of the SR beam.

cond-mat.mtrl-sci↗

arXiv2014

Structural investigations in BaFe2-xRuxAs2 as a function of Ru and temperature

We present Synchrotron XRD measurements on powdered single crystal samples of BaFe2-xRuxAs2 samples, as a function of Ru content at room temperature. The Rietveld refinements reveal that the a-axis increases with Ru substitution, while the c-axis decreases. The variation of positional co-ordinates of As (zAs), the Fe-As bond length and the As-Fe-As bond angles have been determined from the Rietveld refinements. In the sample with x=0.1, temperature dependent XRD measurements were carried out. The results indicate that while the orthorhombicity shows the characteristic increase with decrease in temperature, the As-Fe-As bond angles, Fe-As bond length and positional co-ordinate of the As show definite anomalies close to the structural transition. First principle ab-initio simulations are performed in order to understand the experimentally observed anomalies in structural parameters. The experimental observations are discussed in the context of the simulation results.

cond-mat.supr-con↗

arXiv2014

Coexistence of Co3+ and Co2+ in ceramic Co3TeO6; XANES, Magnetization and first principle study

Evidence of coexistence of Co3+ with Co2+ in ceramic Co3TeO6 through XANES, DC magnetization and first principal studies is provided. XANES along with linear combination fit provide relative concentrations of Co2+ and Co3+.Temperature dependent DC magnetization exhibits the same antiferromagnetic behavior as observed in single crystal. The presence of both Co2+ and Co3+ suggests that if the later is in high spin state, the effective magnetic moment is similar to that observed in single crystal studies. In contrast, if Co3+ is in low spin state effective magnetic moment is similar to that observed in Co3O4. It is further shown that both Co2+ and Co3+ in high spin states constitute a favorable ground state through first principle calculations where Rietveld refined Synchrotron X-ray diffraction data are inputs.

cond-mat.mtrl-sci↗

arXiv2014

Structural insight into $Co_{3-x}Mn_xTeO_6$; ($0 \lt x \le 2$) solid solutions using Synchrotron X-ray diffraction and XANES

Structural investigation on $Co_{3-x}Mn_xTeO_6$; ($0 \lt x \le 2$) solid solutions as a function of Mn concentration using Synchrotron X-ray diffraction (SXRD) and XANES measurements are presented. Phase diagram obtained from Rietveld Refinement on SXRD data as a function of Mn concentration indicates doping disproportionate mixing of both monoclinic (C2/c) and rhombohedral (R-3) structure (for $x \lt 0.5$) while only R-3 symmetry for $x \ge 0.5$. Furthermore, Rietveld analysis on SXRD data shows increase in lattice parameters and average transition metal oxygen (Co/Mn-O) bond lengths for $x \ge 0.5$. Co and Mn K-edge XANES spectra reveal that both Co and Mn are in mixed oxidation state of $Co^{2+}/Mn^{2+}$ and $Co^{3+}/Mn^{3+}$. Relative ratio of $Co^{3+}/Co^{2+}$ and $Mn^{3+}/Mn^{2+}$ using Linear combination fit (LCF) decrease with increasing $x$ for $x \ge 0.5$. These structural and spectroscopic evidences are used to provide possible interpretation of the reported observations of maximum Neel Temperature (TN) at $x ~\sim 0.5$ and enhancements of antiferro/ferro-magnetic interactions in $Co_{3-x}Mn_xTeO_6$ solid solutions.

cond-mat.mtrl-sci↗

arXiv2013

Spontaneous electric polarization and Magneto-dielectric coupling in ceramic multiferroic Co3TeO6

We report observation of magneto-electric and magneto-dielectric couplings in ceramic Co3TeO6. Temperature dependent DC magnetization and dielectric constant measurements together indicate coupling between magnetic order and electronic polarization. Strong anomaly in dielectric constant at ~ 18K in zero magnetic field indicates presence of spontaneous polarization. Observations like weak ferromagnetic order at lower temperature, field and temperature dependences of the ferroelectric transition provide experimental verification of the recent theoretical proposal by P. Toledano et al., Phys. Rev. B 85, 214439 (2012). We provide direct evidence of spin-phonon coupling as possible origin of magnetic order.

cond-mat.mtrl-sci↗

arXiv2003

Theory of excited state absorptions in phenylene-based $π$-conjugated polymers

Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon A$_g$ states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur A$_g$ states which are superpositions of one electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations, that are both comprised of the highest delocalized valence band and the lowest delocalized conduction band states only. The dominant PA is to one specific member of this class of states (the mA$_g$). In addition to the above class of A$_g$ states, PA can also occur to a higher energy kA$_g$ state whose 2e--2h component is {\em different} and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mA$_g$ and the kA$_g$ agree reasonably well to the experimentally observed low and high energy PAs in PPV. The calculated relative intensities of the two PAs are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work a new intense PA at an energy considerably lower than the region where PA have been observed currently. Based on earlier work that showed that efficient charge--carrier generation occurs upon excitation to odd--parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kA$_g$ that leads to charge generation subsequent to excitation to this state, as found experimentally.

cond-mat↗

arXiv2001

Stability of the doped antiferromagnetic state of the t-t'-Hubbard model

The next-nearest-neighbour hopping term t' is shown to stabilize the AF state of the doped Hubbard model with respect to transverse perturbations in the order- parameter by strongly suppressing the intraband particle-hole processes. For a fixed sign of t', this stabilization is found to be significantly different for electron and hole doping, which qualitatively explains the observed difference in the degree of robustness of the AF state in the electron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4}) cuprates. The t'-U phase diagram is obtained for both signs of the t' term, showing the different regions of stability and instability of the doped antiferromagnet. Doping is shown to suppress the t'-induced frustration due to the competing interaction J'. A study of transverse spin fluctuations in the metallic AF state reveals that the decay of magnons into particle-hole excitations yields an interesting low-energy result Γ\sim ωfor magnon damping.

cond-mat.str-el↗

arXiv2000

Electron correlation induced transverse delocalization and longitudinal confinement in excited states of phenyl-substituted polyacetylenes

Electron-electron interactions in general lead to both ground state and excited state confinement. We show, however, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups, which in turn leads to enhanced confinement in the chain direction. This co-operative delocalization--confinement lowers the energy of the one-photon state and raises the relative energy of the lowest two-photon state. The two-photon state is slightly below the optical state in mono-phenyl substituted polyacetylenes, but above the optical state in di-phenyl substituted polyacetylenes, thereby explaining the strong photoluminescence of the latter class of materials. We present a detailed mechanism of the crossover in the energies of the one- and two-photon states in these systems. In addition, we calculate the optical absorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low and high energy transitions that can be tested on oriented samples. Within existing theories of light emission from $π$-conjugated polymers, strong photoluminescence should be restricted to materials whose optical gaps are larger than that of trans-polyacetylene. The present work show that conceptually at least, it is possible to have light emission from systems with smaller optical gaps.

cond-mat.str-el↗

arXiv1999

Higher anisotropic d-wave symmetry in cuprate superconductors

We derive a pair potential from tight binding further neighbours attraction that leads to superconducting gap symmetry similar to that of the phenomenological spin fluctuation theory of high temperature superconductors (Monthoux, Balatsky, Pines, Phys. Rev. Lett. {\bf 67}, 3448). We show that higher anisotropic d-wave than the simpliest d-wave symmetry is one of the important ingredients responsible for higher BCS characteristic ratio.

cond-mat.supr-con↗

arXiv1999

Pairing symmetry and long range pair potential in a weak coupling theory of superconductivity

We study the superconducting phase with two component order parameter scenario, such as, $d_{x^2-y^2} + e^{iθ}s_α$, where $α= xy, x^2+y^2$. We show, that in absence of orthorhombocity, the usual $d_{x^2-y^2}$ does not mix with usual $s_{x^2+y^2}$ symmetry gap in an anisotropic band structure. But the $s_{xy}$ symmetry does mix with the usual d-wave for $θ=0$. The d-wave symmetry with higher harmonics present in it also mixes with higher order extended $s$ wave symmetry. The required pair potential to obtain higher anisotropic $d_{x^2-y^2}$ and extended s-wave symmetries, is derived by considering longer ranged two-body attractive potential in the spirit of tight binding lattice. We demonstrate that the dominant pairing symmetry changes drastically from $d$ to $s$ like as the attractive pair potential is obtained from longer ranged interaction. More specifically, a typical length scale of interaction $ξ$, which could be even/odd multiples of lattice spacing leads to predominant $s/d$ wave symmetry. The role of long range interaction on pairing symmetry has further been emphasized by studying the typical interplay in the temperature dependencies of these higher order $d$ and $s$ wave pairing symmetries.

cond-mat.supr-con↗

arXiv1999

Superconductors of mixed order parameter symmetry in a Zeeman magnetic field

We study the effect of Zeeman magnetic field in the superconducting phase with two component order parameter scenario, such as, $d_{x^2-y^2} + e^{iθ}α$, where $α= d_{xy}, s$. This scenario is equivalent to applying magnetic field parrllel to Cu-O planes. In a weak magnetic field, which does not cause much change to the predominant d-wave, supresses the minor $α$ component leading to pure d-wave phase. This observation is in contrary to the effect of magnetic field applied in the c-direction to the Cu-O planes which is believed to induce a minor component $α$ to d-wave superconductors. We also show that the response of such superconductors to a weak Zeeman magnetic field can be quite different depending on the phase $θ$ of the minor component ($α$).

cond-mat.supr-con↗

arXiv1999

Interplay of spin density wave and superconductivity with different pairing symmetry

A model study for the coexistence of the spin density wave and superconductivity is presented. With reference to the recent angle resolved photo emmission experimental data in high T_c cuprates, presence of the nested pieces of bands is assumed. The single band Hubbard model, therefore, when treated within the Hatree-Fock mean field theory leads to a spin density wave (SDW) ground state. The superconductivity (SC) is assumed to be due to a generalised attractive potential with a separable form without specifying to any particular origin. It therefore allows a comparative study of the coexistence of superconductivity of different order parameter symmetry with the spin density wave state. We find that the phase diagram, comprising of the amplitudes of the respective gaps (SC and SDW) Vs. band filling resembles to that of the high T_c cuprates only when the order parameter of the superconducting phase has d-wave symmetry. Thermal variation of different order parameters (e.g, SC and SDW) also show interesting coexistence and reentrance behaviors that are consistent with experimental observations, specially for the borocarbides.

cond-mat.supr-con↗

arXiv1998

Time reversal symmetry breaking superconductivity

We study time reversal symmetry breaking superconductivity with $Δ_k = Δ_{x^2-y^2} (k) +e^{iθ} Δ_α$ ($α= s$ or $d_{xy}$) symmetries. It is shown that the behavior of such superconductors could be {\em qualitatively} different depending on the minor components ($α$) and its phase at lower temperatures. It is argued that such {\em qualitatively different} behaviors in thermal as well as in angular dependencies could be a {\em source} of consequences in transport and Josephson physics. Orthorhombicity is found to be a strong mechanism for mixed phase (in case of $α= s$). We show that due to electron correlation the order parameter is more like a pure $d_{x^2-y^2}$ symmetry near optimum doping.

cond-mat.str-el↗

arXiv1998

Complex order parameter symmetry and thermal conductivity

Thermal behaviour of superconductors with complex order parameter symmetry is studied within a weak coupling theory. It is shown numerically, that the thermal nature of the different components of complex order parametrs are qualitatively different. Within the complex order parameter scenario, the recent experimental observations by Krishna {\it et al.}, [Science {\bf 277}, 83 (1997)] on magnetothermal conductivity and by J. Ma {\it et al.}, [Science {\bf 267}, 862 (1995)] on temperature dependent gap anisotropy for high temperature superconductors can have natural explanation.

cond-mat.supr-con↗