arXiv subjects

S. Nandi

Publications and source records attributed to S. Nandi.

At least 127 records · Page 7Linked to original sources

arXiv2008

Suppression of antiferromagnetic spin fluctuations in the collapsed tetragonal phase of CaFe2As2

Inelastic neutron scattering measurements of CaFe2As2 under applied hydrostatic pressure show that the antiferromagnetic spin fluctuations observed in the ambient pressure, paramagnetic, tetragonal (T) phase are strongly suppressed, if not absent, in the collapsed tetragonal (cT) phase. These results are consistent with a quenched Fe moment in the cT phase and the strong decrease in resistivity observed upon crossing the boundary from the T to cT phase. The suppression or absence of static antiferromagnetic order and dynamic spin fluctuations in the non-superconducting cT phase supports the notion of a coupling between spin fluctuations and superconductivity in the iron arsenides.

cond-mat.supr-con↗

arXiv2008

A light scalar as the messenger of electroweak and flavor symmetry breaking

We propose a new framework for understanding the hierarchies of fermion masses and mixings. The masses and mixings of all Standard Model (SM) charged fermions other than top arise from higher dimensional operators involving a messenger scalar S and flavon scalars F_i. The flavons spontaneously break SM flavor symmetries at around the TeV scale. The SM singlet scalar S couples directly to the Higgs H and spontaneously breaks another U(1) at the electroweak scale. At the TeV scale, SM quarks and charged leptons have renormalizable couplings to S, but not to H or F_i. These couplings involve new heavy vectorlike fermions. Integrating out these fermions produces a pattern of higher dimensional operators that reproduce the observed hierarchies of the SM masses and mixings in terms of powers of the "little hierarchy": the ratio of the electroweak scale to the flavor-breaking scale. The framework has important phenomenological implications. Flavor-changing neutral currents are within experimental limits but D^0 mixing and B_s->mu+mu- could be close to current sensitivities. The neutral scalar s of the messenger field mixes with the light Higgs of the SM, which can have strong effects on Higgs decay branching fractions. The s mass eigenstate may be lighter than the Higgs, and could be detected at the Tevatron or the LHC.

hep-ph↗

arXiv2008

Lattice collapse and quenching of magnetism in CaFe2As2 under pressure: A single crystal neutron and x-ray diffraction investigation

Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the non-magnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase and we find no evidence of magnetic ordering in the cT phase. Band structure calculations show that the transition results in a strong decrease of the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment.

cond-mat.supr-con↗

arXiv2008

Absence of structural correlations of magnetic defects in heavy fermion LiV2O4

Magnetic defects have pronounced effects on the magnetic properties of the face-centered cubic compound LiV2O4. The magnetic defects arise from crystal defects present within the normal spinel structure. High-energy x-ray diffraction studies were performed on LiV2O4 single crystals to search for superstructure peaks or any other evidence of periodicity in the arrangement of the crystal defects present in the lattice. Entire reciprocal lattice planes are mapped out with help of synchrotron radiation. No noticeable differences in the x-ray diffraction data between a crystal with high magnetic defect concentration and a crystal with low magnetic defect concentration have been found. This indicates the absence of any long-range periodicity or short-range correlations in the arrangements of the crystal/magnetic defects.

cond-mat.str-el↗

arXiv2008

Inverted neutrino mass hierarchy and new signals of a chromophobic charged Higgs at the Large Hadron Collider

We explore the signals of a charged Higgs arising in a two Higgs doublet model respecting SU(2)_L times U(1) times Z_2 symmetry with three singlet right handed neutrinos, N_R. The charged Higgs in this model has negligible coupling with quarks, and has unsuppressed coupling to leptons and neutrinos. This leads to novel signatures of the charged Higgs at the LHC, especially in the case of an inverted neutrino mass hierarchy, in the form of electrons and muons with missing energy.

hep-ph↗

arXiv2008

Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

cond-mat.supr-con↗

arXiv2008

Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals

The London penetration depth, $λ(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $λ(T)$ follows a power-law, $Δλ(T) \sim T^n$ with $n=2.4 \pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $λ(T)$.

cond-mat.supr-con↗

arXiv2008

Anisotropic three-dimensional magnetism in CaFe2As2

Inelastic neutron scattering measurements on the low energy spin waves in CaFe2As2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data.

cond-mat.supr-con↗

arXiv2008

Single crystal growth and physical properties of the layered arsenide BaRh_2As_2

Single crystals of BaRh_2As_2 have been synthesized from a Pb flux. We present the room temperature crystal structure, single crystal x-ray diffraction measurements as a function of temperature T, anisotropic magnetic susceptibility χversus T, electrical resistivity in the ab-plane ρversus T, Hall coefficient versus T and magnetic field H, and heat capacity C versus T measurements on the crystals. The single crystal structure determination confirms that BaRh_2As_2 forms in the tetragonal ThCr_2Si_2 type structure (space group I4/mmm) with lattice parameters a = b = 4.0564(6)Åand c = 12.797(4) Å. Band structure calculations show that BaRh_2As_2 should be metallic with a small density of states at the Fermi energy N(E_ F) = 3.49 states/eV f.u. (where f.u. \equiv formula unit) for both spin directions. ρ(T) data in the ab-plane confirm that the material is indeed metallic with a residual resistivity ρ(2K) = 29 μΩcm, and with a residual resistivity ratio ρ(310K)/ρ(2K) = 5.3. The observed χ(T) is small (\sim 10^{-5} cm^3/mol) and weakly anisotropic with χ_{ab}/χ_ c \approx 2. The C(T) data indicate a small density of states at the Fermi energy with the low temperature Sommerfeld coefficient γ= 4.7(9) mJ/mol K^2. There are no indications of superconductivity, spin density wave, or structural transitions between 2K and 300K. We compare the calculated density of states versus energy of BaRh_2As_2 with that of BaFe_2As_2.

cond-mat.supr-con↗

arXiv2008

Lattice and magnetic instabilities in CaFe2As2: A single crystal neutron diffraction study

Neutron diffraction measurements of a high quality single crystal of CaFe2As2 are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at TS = 172.5K (on cooling) and 173.5K (on warming). Coincident with the structural transition we observe a rapid, but continuous, ordering of the Fe moments, in a commensurate antiferromagnetic structure is observed, with a saturated moment of 0.80(5)muB/Fe directed along the orthorhombic a-axis. The hysteresis of the structural transition is 1K between cooling and warming and is consistent with previous thermodynamic, transport and single crystal x-ray studies. The temperature onset of magnetic ordering shifts rigidly with the structural transition providing the clearest evidence to date of the coupling between the structural and magnetic transitions in this material and the broader class of iron arsenides.

cond-mat.supr-con↗

arXiv2008

Anisotropic thermodynamic and transport properties of single crystalline (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (x = 0 and 0.45)

Single crystals of BaFe$_2$As$_2$ and (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated and can have dimensions as large as 3 x 3 x 0.2 mm$^3$. The BaFe$_2$As$_2$ single crystals manifest a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat and the anisotropic magnetic susceptibility. In the (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near $T_c$ is small, but resolvable, giving $ΔC_p/γT_c \approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by only $\sim 4$ K when applied along the c-axis and by $\sim 2$ K when applied perpendicular to the c-axis. The ratio of the anisotropic upper critical fields, $γ= H_{c2}^{\perp c} / H_{c2}^{\| c}$, varies between 2.5 and 3.5 for temperatures down to $\sim 2$ K below $T_c$.

cond-mat.supr-con↗

arXiv2008

First order structural phase transition in CaFe$_2$As$_2$

CaFe$_2$As$_2$ has been synthesized and found to form in the tetragonal, ThCr$_2$Si$_2$ structure with lattice parameters $a = 3.912(68) Å$ and $c = 11.667(45) Å$. Upon cooling through 170 K, CaFe$_2$As$_2$ undergoes a first order, structural phase transition to a low temperature, orthorhombic phase with a $2 - 3$ K range of hysteresis and coexistence. This transition is clearly evident in microscopic, thermodynamic and transport measurements. CaFe$_2$As$_2$ is the third member of the AFe$_2$As$_2$ (A = Ba, Sr, Ca) family to exhibit such a dramatic phase transition and is a promising candidate for studies of doping induced superconductivity.

cond-mat.str-el↗

arXiv2008

Structural transition and anisotropic properties of single crystalline SrFe2As2

Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.

cond-mat.supr-con↗

arXiv2008

Universal Doublet-Singlet Higgs Couplings and phenomenology at the CERN Large Hadron Collider

We consider a minimal extension of the standard model where a real, gauge singlet scalar field is added to the standard spectrum. Introducing the Ansatz of universality of scalar couplings, we are led to a scenario which has a set of very distinctive and testable predictions: (i) the mixing between the standard model Higgs and the new state is near maximal, (ii) the ratio of the two Higgs mass eigenstates is fixed ($\sim \sqrt{3}$), (iii) the decay modes of each of the two eigenstates are standard model like. We also study how electroweak precision tests constrain this scenario. We predict the lighter Higgs to lie in the range of 114 and 145 GeV, and hence the heavier one between 198 and 250 GeV. The predictions of the model can be tested at the upcoming LHC.

hep-ph↗

arXiv2008

The nature of Ho magnetism in multiferroic HoMnO3

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho3+ in multiferroic HoMnO3. In zero field, Ho3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1x10^7 V/m, the magnetic structure of Ho3+ remains unchanged.

cond-mat.other↗

arXiv2008

New Prediction For Leptonic Theta_13

An extension of the neutrino sector with two right handed singlet neutrinos responsible for Dirac neutrino masses is discussed. We show that this setup with flavor symmetry can give large solar and atmospheric neutrino mixings and suppressed Theta_13. The flavor symmetry U(1) x S_4 is shown to lead to Theta_23 = π/4 and a new predictive formula for the Theta_13(\simeq 0.015).

hep-ph↗

arXiv2007

A Mechanism for Kaluza-Klein Number Violation in UED and Implications for LHC

If the Higgs does not propagate into an otherwise universal extra dimension, then tree-level Kaluza-Klein number is not conserved in the Yukawa interactions. This leads to mixing between zero-mode fermions and their associated Kaluza-Klein excitations. The effect can be quite large for the top quark, since its mass (171.4 plus or minus 2.1 GeV) is approximately one-half the current Tevatron mass bound (about 350-400 GeV) for Kaluza-Klein excitations of quarks with one universal extra dimension. In contrast to the SM, the bi-unitary transformation that diagonalizes the mass matrix does not diagonalize the effective 4D Yukawa coupling matrix because of the presence of the 5D kinetic terms. This leads to violation of tree-level Kaluza-Klein number conservation in the gauge sector due to a unique feature of the 5D theory. This allows Kaluza-Klein excitations of the fermions to be produced singly, and opens up new decay channels in the gauge sector. We compute the cross section for the production of the lowest-lying Kaluza-Klein excitations of the top and bottom quarks in the non-universal Higgs model at the Large Hadron Collider, and consider their subsequent decays. The effect is quite large: For example, pair production of Kaluza-Klein excitations of the bottom quark will lead to a pair of top quarks plus a pair of charged weak gauge bosons. Since the production is via the strong interaction, such final states stand out well against the Standard Model background, and are observable at the LHC for a wide range of compactification scales. Finally, we discuss the effects that such a model has on the T- parameter, and propose a model with maximal mixing where the compactification scale may be as large as a universal extra dimension.

hep-ph↗

arXiv2007

Collider Implications of a Non-Universal Higgs

We consider in detail the possibility that the Higgs is wholly or partially excluded from propagating into one otherwise universal extra dimension. This exclusion of the Higgs from propagating into an otherwise universal extra dimension violates tree-level Kaluza-Klein number conservation in the Yukawa interactions. As a consequence, there is inter-mode mixing between fermions. For example, zero-mode fermions mix with their associated Kaluza-Klein excitations. This is in contrast to the original universal extra dimensions scenario, in which conservation of Kaluza-Klein number prohibits such inter-mode mixing. Inter-mode mixing is especially significant for the top quark, since its mass (171.4 +/- 2.1 GeV) is approximately one-half the current Tevatron mass bound (~350-400 GeV) for Kaluza-Klein excitations of quarks propagating into universal extra dimensions. We compute the effects that mixing among the zero-modes and lowest-lying Kaluza-Klein excitations has on the lightest third-generation charge 2/3 quark mass eigenvalue in the non-universal Higgs model with one otherwise universal extra dimension. Another consequence of the inter-mode mixing is that the Kaluza-Klein excitations of the fermions can decay to a zero-mode Higgs and a corresponding zero-mode fermion. As a result, the pair production of Kaluza-Klein excitations of the top quark would lead to two zero-mode Higgs bosons plus a zero-mode top quark/anti-quark pair. We compute the cross section that the non-universal Higgs model contributes to Higgs production at the Large Hadron Collider. The effect is quite large: For example, the Kaluza-Klein contribution to Higgs production is comparable to or larger than the Standard Model contribution, depending on the Higgs mass, for compactification scales up to about 600 GeV.

hep-ph↗