R. C. Budhani

Council of Scientific and Industrial Research (CSIR), New Delhi, New Dilli, NCT, India

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Publications (204)539.68 Total impact

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    ABSTRACT: Magnetic properties of the ring array made with ultrathin (Pd-/Co-layer thickness ~0.5/0.25 nm) Co/Pd multilayers are reported. Systematic investigation of magnetization loops M(H) at varying temperatures (T = 3–300 K) and magnetic relaxations (at T = 300 K) were carried out in a plain film and ring arrays with different ring widths of 660, 500 and 340 nm. The M(H) loops of all the samples show anisotropy distribution, a reversal mechanism facilitated by nucleation of the reverse domain followed by pinning dependent domain wall motion, and a thermally stable remanant state with U b/kT > 250 (at T = 300 K, where U b is the activation barrier). Both anisotropy distribution and pinning are found to increase in the nanostructures along with a precipitous rise of the reversal field below ≈100 K. These results are discussed in terms of the presence of chemically sharp/disordered interface regions and inter-layer interactions in the Co/Pd multilayers.
    Journal of Physics D Applied Physics 07/2015; 48(29). DOI:10.1088/0022-3727/48/29/295005 · 2.72 Impact Factor
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    ABSTRACT: In the vicinity of a phase transition, the order parameter starts fluctuating before vanishing at the critical point. The fluctuation regime, i.e. the way the ordered phase disappears, is a characteristics of a transition, and determines the universality class it belongs to. This is valid for thermal transitions, but also for zero temperature Quantum Phase Transitions (QPT). In the case of superconductivity, the order parameter has an amplitude and a phase, which can both fluctuate according to well identified scenarios. The Ginzburg-Landau theory and its extensions describe the fluctuating regime of regular metallic superconductors, and the associated dynamics of the pair amplitude and the phase. When the system is two-dimensional and/or very disordered, phase fluctuations dominate. Here, we address the possibility that a new type of fluctuations occurs in superconductors with an anomalous dynamics. In particular we show that the superconducting to metal QPT that occurs upon changing the gate voltage in two-dimensional electron gases at LAO/STO and LTO/STO interfaces displays anomalous scaling properties, which can be explained by density driven superconducting critical fluctuations. A Finite Size Scaling (FSS) analysis reveals that the product z.nu (nu is the correlation length exponent and z the dynamical critical one) is z.nu = 3/2. We argue that critical superconducting fluctuations acquire an anomalous dynamics with z=3, since they couple to density ones in the vicinity of a spontaneous electronic phase separation, and that nu=1/2 corresponds to the mean-field value. This approach strongly departs from the conventional z=1 scenario in disordered 2D systems based on long-range Coulomb interactions with dominant phase fluctuations. This scenario can explain recent data in LSCO ultra-thin films, and apply to a whole class of two-dimensional superconductors.
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    Dushyant Kumar · Z. Hossain · R. C. Budhani
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    ABSTRACT: A metallic surface is realized on stoichiometric and insulating (100) SrTiO$_3$ by Ar$^+$ - ion irradiation. The sheet carrier density and Hall mobility of the layer are $\sim$$4.0$ $\times$ $10^{14}$ $/cm^2$ and $\sim$$2$ $\times$ $10^3$ $cm^2/Vs$ respectively at 15 K for the irradiation dose of $\sim$4.2 $\times 10^{18}$ $ions/cm^2$. These samples display ultraviolet light sensitive photoconductivity (PC) which is enhanced abruptly below the temperature ($\approx$100 K) where SrTiO$_3$ crystal undergoes an antiferrodistortive cubic-to-tetragonal ($O_h^1$ $\rightarrow$ $D_{4h}^{18}$) structural phase transition. This behaviour of PC maps well with the temperature dependence of dielectric function and electric field induced conductivity. The longevity of the PC-state also shows a distinct change below $\approx$100 K. At $T > 100$ K its decay is thermally activated with energy barrier of $\approx$36 meV, whereas at $T < 100$ K it becomes independent of temperature. We have examined the effect of electrostatic gating on the lifetime of the PC state. One non-trivial result is the ambient temperature quenching of the photo-conducting state by the negative gate field. This observation opens avenues for designing a solid state photo-electric switch. The origin and lifetime of the PC-state are understood in the light of field effect induced band bending, defect dynamics and thermal relaxation processes.
    Physical Review B 06/2015; 91(20). DOI:10.1103/PhysRevB.91.205117 · 3.74 Impact Factor
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    ABSTRACT: A fiber optic based probe is designed and developed for electrical transport measurements in presence of quasi-monochromatic (360–800 nm) light, varying temperature (T = 1.8–300 K), and magnetic field (B = 0–7 T). The probe is tested for the resistivity and Hall measurements performed on a LaAlO3–SrTiO3 heterointerface system with a conducting two dimensional electron gas.
    The Review of scientific instruments 05/2015; 86(5):056107. DOI:10.1063/1.4921486 · 1.58 Impact Factor
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    ABSTRACT: We report a simple experimental strategy for enhancing the figure-of-merit (ZT) of thermoelectric materials by introducing different kinds of defect features, including nano-porosity, in a range of nano to meso-scale dimensions, employing spark plasma assisted (SPS) reaction sintering of mechanically alloyed nanopowders. This strategy has been experimentally demonstrated in a well known thermoelectric compound, Cu2Se, which is shown to yield a high ZT of 2.1 at 973 K, which is among the highest reported value for this material and shows a ~ 40% increase over its bulk melt-processed counterpart. The main contribution to the enhanced ZT, despite moderate values of power factor, primarily originates from a very small value of thermal conductivity of 0.34 Wm-1K-1. This low thermal conductivity owes its origin primarily to the enhanced low-to-high wavelength phonon scattering by different kinds of defects, in a wide spectrum of nano to meso-scale dimensions, involving microstructural defects generated during high-energy ball milling, abundant nanocrystalline grain boundaries due to very low crystallite size, and more importantly the nano-to-meso scale residual porosity created due to SPS at optimized process parameters. The role of SPS in enhancing the ZT has been discussed and this strategy can also be applied to other thermoelectric materials
    Nano Energy 04/2015; 13:36-46. DOI:10.1016/j.nanoen.2015.02.008 · 10.21 Impact Factor
  • R. c. Budhani · Shubhankar Das · P. C. Joshi · A. Rastogi · Z. Hossain
    Americal Physical Society March Meeting, USA; 03/2015
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    Rajni Porwal · R P Pant · R C Budhani
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    ABSTRACT: Temperature (T) dependent microwave absorption measurements are performed on La0.67Ca0.33MnO3 (LCMO) epitaxial thin films of thickness 100 and 200 nm in an electron paramagnetic resonance spectrometer operating in X-band. The resonant absorption peak is monitored for out-of-plane ( H ⊥ ) and in-plane ( H ∥ ) dc magnetic field ( H ) as the system goes through magnetic ordering. These data suggest a resilient transformation to the ferromagnetic (FM) phase in the vicinity of the Curie temperature (TC ), indicative of a phase separation, which is dominant in the thinner film. The saturation magnetization is calculated from SQUID magnetometry on the same film. A pronounced zero-field absorption is seen in H ∥ geometry displaying anomalous growth in 100 nm film at T < TC . This feature is correlated with the magneto-conductivity of the manganite which is colossal in the vicinity of TC in the well-ordered film of thickness 200 nm. Signature of standing spin wave modes is seen in H ⊥ measurements which are analyzed to calculate the spin wave stiffness constant D(T) in the limit of zero temperature. The same is also inferred from the decay of equilibrium magnetization in the framework of Bloch law. These studies reveal that a bulk like LCMO is obtained in the fully relaxed thicker films.
    Journal of Applied Physics 01/2015; 117(84):13904-1467. DOI:10.1063/1.4905262 · 2.19 Impact Factor
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    Rajni Porwal · R. P. Pant · R. C. Budhani
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    ABSTRACT: Temperature (T) dependent microwave absorption measurements are performed on La0.67Ca0.33MnO3 (LCMO) epitaxial thin films of thickness 100 and 200 nm in an electron paramagnetic resonance spectrometer operating in X-band. The resonant absorption peak is monitored for out-of-plane (H?) and in-plane (Hk) dc magnetic field (H) as the system goes through magnetic ordering. These data suggest a resilient transformation to the ferromagnetic (FM) phase in the vicinity of the Curie temperature (TC), indicative of a phase separation, which is dominant in the thinner film. The saturation magnetization is calculated from SQUID magnetometry on the same film. A pronounced zero-field absorption is seen in Hk geometry displaying anomalous growth in 100 nm film at T<TC. This feature is correlated with the magneto-conductivity of the manganite which is colossal in the vicinity of TC in the well-ordered film of thickness 200 nm. Signature of standing spin wave modes is seen in H? measurements which are analyzed to calculate the spin wave stiffness constant D(T) in the limit of zero temperature. The same is also inferred from the decay of equilibrium magnetization in the framework of Bloch law. These studies reveal that a bulk like LCMO is obtained in the fully relaxed thicker films.
    Journal of Applied Physics 01/2015; 117(2015):013904. · 2.19 Impact Factor
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    ABSTRACT: The coexistence of superconductivity and ferromagnetism in the conducting quasi two-dimensional electron gas formed at oxide heterostructures has raised hopes of observing novel phenomenon not seen in conventional semiconductor devices. In this article, we report the observation of a novel magnetic-field assisted transient superconducting state at the interface of LaAlO3 and SrTiO3 at 245 mK. The transient superconducting state appears concomitantly with a Lifshitz transition in the system as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of in-plane magnetization.
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    ABSTRACT: In LaTiO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures, the bending of the SrTiO3 conduction band at the interface forms a quantum well that contains a superconducting two-dimensional electron gas (2-DEG). Its carrier density and electronic properties, such as superconductivity and Rashba spin-orbit coupling can be controlled by electrostatic gating. In this article we show that the Fermi energy lies intrinsically near the top of the quantum well. Beyond a filling threshold, electrons added by electrostatic gating escape from the well, hence limiting the possibility to reach a highly-doped regime. This leads to an irreversible doping regime where all the electronic properties of the 2-DEG, such as its resistivity and its superconducting transition temperature, saturate. The escape mechanism can be described by the simple analytical model we propose.
    Scientific Reports 10/2014; 4:6788. DOI:10.1038/srep06788 · 5.58 Impact Factor
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    ABSTRACT: We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (n(square)) drops monotonically from similar to 2.2 x 10(14) cm(-2) to similar to 2.5 x 10(13) cm(-2) on replacing approximate to 60% of the Al sites by Cr and the sheet resistance (R-square) exceeds the quantum limit for localization (h/2e(2)) in the concentrating range 40-60% of Cr. The samples with Cr <= 40% show a distinct minimum (T-m) in metallic R-square (T) whose position shifts to higher temperatures on increasing the substitution. Distinct signatures of Rashba spin-orbit interaction (SOI) induced magnetoresistance (MR) are seen in R-square measured in out of plane field (H-perpendicular to) geometry at T <= 8K. Analysis of these data in the framework of Maekawa-Fukuyama theory allows extraction of the SOI critical field (H-SO) and time scale (tau(SO)) whose evolution with Cr concentration is similar as with the increasing negative gate voltage in LAO/STO interface. The MR in the temperature range 8K <= T <= T-m is quadratic in the field with a +ve sign for H-perpendicular to and -ve sign for H-parallel to. The behaviour of H-parallel to MR is consistent with Kondo theory which in the present case is renormalized by the strong Rashba SOI atT < 8K.
    Journal of Physics Condensed Matter 10/2014; 27(12). DOI:10.1088/0953-8984/27/12/125007 · 2.35 Impact Factor
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    Shubhankar Das · P. C. Joshi · A. Rastogi · Z. Hossain · R. C. Budhani
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    ABSTRACT: Measurements of magneto-thermopower (S(H, T)) of interfacial delta doped LaTiO$_3$/SrTiO$_3$ (LTO/STO) heterostructure by an iso-structural antiferromagnetic perovskite LaCrO$_3$ are reported. The thermoelectric power of the pure LTO/STO interface at 300 K is $\approx$ 118 $\mu$V/K, but increases dramatically on $\delta$-doping. The observed linear temperature dependence of S(T) over the temperature range 100 K to 300 K is in agreement with the theory of diffusion thermopower of a two-dimensional electron gas. The S(T) displays a distinct enhancement in the temperature range (T $<$ 100 K) where the sheet resistance shows a Kondo-type minimum. We attributed this maximum in S(T) to Kondo scattering of conduction electron by localized impurity spins at the interface. The suppression of S by a magnetic field, and the isotropic nature of the suppression in out-of-plane and in-plane field geometries further strengthen the Kondo model based interpretation of S(H, T).
    Physical Review B 08/2014; 90(7). DOI:10.1103/PhysRevB.90.075133 · 3.74 Impact Factor
  • Himanshu Pandey · R. Prasad · R. C. Budhani
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • P. K. Rout · R. C. Budhani
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The dynamics of magnetization (M) reversal and relaxation as a function of temperature (T) are reported in three non-interacting NiFe ring arrays having fixed ring outer diameter and varying widths. Additionally, the dependence of M(H) loop on the angle (θ) between magnetic field (H) and the plane of the rings is addressed. The M(H) loops show a double step transition from onion state (OS) to vortex state (VS) at all temperatures (T = 3 to 300 K) and angles (θ = 0 to 90°). The critical reversal fields HC1 (OS to VS) and HC2 (VS to OS) show a pronounced dependence on T, ring width, and θ. Estimation of the transverse and vortex domain wall energies reveals that the latter is favored in the OS. The OS is also the remanent state in the smallest rings and decays with the effective energy scale (U0/T) of 50 and 32 meV/K at 10 and 300 K, respectively. The robust in-plane anisotropy of magnetization of ring assemblies is established by scaling the M(H) with θ.
    Journal of Applied Physics 03/2014; 115(16). DOI:10.1063/1.4872139 · 2.19 Impact Factor
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    Ankur Rastogi · J. J. Pulikkotil · R. C. Budhani
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    ABSTRACT: We report the effect of δ doping at the LaAlO3/SrTiO3 interface with LaMnO3 monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broadband optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO3/SrTiO3 interface from metal-to-insulator with increasing LaMnO3 sub-monolayer thickness, we also observe an enhancement in the photoresponse and relaxation time constant. A possible scenario for the PC based on defect clusters, random potential fluctuations, and large lattice relaxation models, along with the role of structural phase transition in SrTiO3, is discussed. For pure LaAlO3/SrTiO3, the photoconductivity appears to originate from interband transitions between Ti-derived 3d bands which are eg in character and O 2p–Ti t2g hybridized bands. The band structure changes significantly when fractional layers of LaMnO3 are introduced. Here the Mn eg bands (≈1.5 eV above the Fermi energy) within the photoconducting gap lead to a reduction in the photoexcitation energy and a gain in overall photoconductivity.
    Physical Review B 03/2014; 89(12):125127. DOI:10.1103/PhysRevB.89.125127 · 3.74 Impact Factor
  • P. K. Rout · R. K. Rakshit · R. C. Budhani
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    ABSTRACT: We report the Josephson coupling between optimally doped superconducting La1.85Sr0.15CuO4 through the normal metal barrier of La1.85Sr0.15Cu0.97Co0.03O4 (LSCCO). The critical current (Ic) across the junction exists even up to a barrier thickness of 50 nm. The barrier thickness dependence study of Ic yields a large coherence length in LSCCO of (15.2±1.4) nm at 5 K, which indicates the presence of giant proximity effect in such systems.
    03/2014; 1591(1). DOI:10.1063/1.4873019
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    Pramod Kumar · Anjana Dogra · R. C. Budhani
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    ABSTRACT: We report the growth of ultrathin film of Mn doped LaTiO3 on TiO2 terminated SrTiO3 (001) substrate by pulsed laser deposition (PLD) and their electrical transport characteristics including magnetoresistance (MR). Though the replacement of Mn in LaTiO3 at the Ti site in dilute limit does not affect the metallic behaviour of films but variation in resistance is observed. Normalised resistance behaviour is explained on the basis of variation in charge carriers and increased interaction between Mn atoms in the system under investigation.
    03/2014; 1591(1). DOI:10.1063/1.4872961
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    ABSTRACT: The 2D electron gas (2DEG) formed at the LaXO3/SrTiO3 (X 1⁄4 Al, Ti) oxide interface appears inhomogeneous in several experiments. In particular, we discuss evidences of electron inhomo- geneities provided by the phenomenology of the superconducting (SC) phase, which occurs when the carrier density is tuned above a critical value by means of gating, and of the superconductor- to-metal transition driven by gate voltage or magnetic ̄eld. The measured resistance and super°uid density result from the percolative connection of superconducting \puddles" with randomly distributed critical temperatures, embedded in a weakly localizing metallic matrix. We propose a possible intrinsic origin of the electron inhomogeneity, resulting from the strong Rashba spin–orbit coupling (RSOC) measured at these oxide interfaces.
    03/2014; 04(01):1440004. DOI:10.1142/S2010324714400049
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    ABSTRACT: The breaking of inversion symmetry combined with polar discontinuity and the presence of strong electronic correlations due to partially filled 3d electronic shell of Ti ions at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) and LaTiO$_3$/SrTiO$_3$ (LTO/STO) interfaces conspire to produce exotic electronic phases. These include macroscopic superconducting and ferromagnetic orders and electronic transport renormalized by strong spin-orbit (S-O) interaction originating to the built-in electrostatic fields at the interface and scattering by localized moments. An important ingredient in this exotic soup of causes and effects is also the quantum paraelectric nature of STO. In order to separate out the contributions of these phenomena from electronic behavior of oxide interfaces, new approaches need to be developed. Here, we present a study of delta doping at LTO/STO interface with iso-structural perovskite of LaCrO$_3$ (LCO) that dramatically alters the properties of the two dimensional electron gas (2DEG) at the interface. The effects include a reduction in sheet-carrier density, prominence of the low temperature resistivity minimum in LAO/STO and LTO/STO 2DEG, enhancement of weak antilocalization below 10 K and observation of a strong anisotropic magnetoresistance (MR). The positive and negative MR for out-of-plane and in-plane field respectively and the field and temperature dependencies of MR suggest Kondo scattering by localized Ti$^{3+}$ moments renormalized by S-O at T < 10 K, with the increase of Cr$^{3+}$ concentration at the interface.

Publication Stats

2k Citations
539.68 Total Impact Points

Institutions

  • 2012–2015
    • Council of Scientific and Industrial Research (CSIR), New Delhi
      New Dilli, NCT, India
  • 2011–2015
    • National Physical Laboratory - India
      • Division of Materials Physics and Engineering (NPL)
      Old Delhi, NCT, India
  • 1997–2014
    • Uttar Pradesh Textile Technology Institute
      Cawnpore, Uttar Pradesh, India
  • 1994–2014
    • Indian Institute of Technology Kanpur
      • Department of Physics
      Cawnpore, Uttar Pradesh, India
  • 1991–2014
    • Brookhaven National Laboratory
      • Physics Department
      New York City, New York, United States
  • 2001
    • University of Maryland, College Park
      • Department of Physics
      Maryland, United States
  • 1993
    • University of Nebraska at Lincoln
      Lincoln, Nebraska, United States
  • 1990
    • Emory University
      • Department of Physics
      Atlanta, Georgia, United States