arXiv subjects

D. K. Singh

Publications and source records attributed to D. K. Singh.

At least 37 records · Page 2Linked to original sources

arXiv2017

Hydrodynamic signatures of stationary Marangoni-driven surfactant transport

We experimentally study steady Marangoni-driven surfactant transport on the interface of a deep water layer. Using hydrodynamic measurements, and without using any knowledge of the surfactant physico-chemical properties, we show that sodium dodecyl sulphate and Tergitol 15-S-9 introduced in low concentrations result in a flow driven by adsorbed surfactant. At higher surfactant concentration, the flow is dominated by the dissolved surfactant. Using Camphoric acid, whose properties are {\it a priori} unknown, we demonstrate this method's efficacy by showing its spreading is adsorption dominated.

physics.flu-dyn↗

arXiv2017

Magnetic diode at $T$ = 300 K

We report the finding of unidirectional electronic properties, analogous to a semiconductor diode, in two-dimensional artificial permalloy honeycomb lattice of ultra-small bond, with a typical length of ~ 12 nm. The unidirectional transport behavior, characterized by the asymmetric colossal enhancement in differential conductivity at a modest current application of ~ 10-15 $μ$A, persists to T = 300 K in honeycomb lattice of thickness ~ 6 nm. The asymmetric behavior arises without the application of magnetic field. A qualitative analysis of experimental data suggests the role of magnetic charge or monopoles in the unusual observations with strong implication for spintronics.

cond-mat.mes-hall↗

arXiv2017

Dynamics of a Camphoric Acid boat at the air-water interface

We report experiments on an agarose gel tablet loaded with camphoric acid (c-boat) set into self-motion by interfacial tension gradients at the air-water interface. We observe three distinct modes of c-boat motion: harmonic mode where the c-boat speed oscillates sinusoidally in time, a steady mode where the c-boat maintains constant speed, and a relaxation oscillation mode where the c-boat maintains near-zero speed between sudden jumps in speed and position at regular time intervals. Whereas all three modes have been separately reported before in different systems, we show they belong to a common description. Through control of the air-water surface tension with Sodium Dodecyl Sulfate (SDS), we experimentally deduce the three self-propulsive modes result from surface tension difference between Camphoric Acid (CA) and the ambient surroundings.

physics.flu-dyn↗

arXiv2016

Magnetic transitions in the topological magnon insulator Cu(1,3-bdc)

Topological magnon insulators are a new class of magnetic materials that possess topologically nontrivial magnon bands. As a result, magnons in these materials display properties analogous to those of electrons in topological insulators. Here, we present magnetization, specific heat, and neutron scattering measurements of the ferromagnetic kagome magnet Cu(1,3-bdc). Our measurements provide a detailed description of the magnetic structure and interactions in this material, and confirm that it is an ideal prototype for topological magnon physics in a system with a simple spin Hamiltonian.

cond-mat.str-el↗

arXiv2015

Electronic nature of the lock-in magnetic transition in CeXAl4Si2

We have investigated the underlying magnetism in newly discovered single crystal Kondo lattices CeXAl4Si2, where X = Rh, Ir. We show that the compound undergoes an incommensurate-to-commensurate magnetic transition at Tc = 9.19 K (10.75 K in Ir). The spin correlation in the incommensurate phase is described by a spin density wave configuration of Ce-ions, which locks-in to the long-range antiferromagnetic order at T = Tc. The qualitative analysis of the experimental data suggests the role of the Fermi surface nesting, instead of the lattice distortion causing the Umklapp correction or the soliton propagation, as the primary mechanism behind this phenomenon.

cond-mat.str-el↗

arXiv2015

A sum form functional equation on a closed domain and its role in information theory

This paper is devoted to finding the general solutions of the functional equation $\sumin \sumjm h(p_iq_j)=\sumin h(p_i)+\sumjm k_j(q_j)+λ\sumin h(p_i)\sumjm k_j(q_j)$ valid for all complete probability distributions $(p_1,\ldots,p_n)$, $(q_1,\ldots,q_m)$, $0\le p_i\le 1$, $0\le q_j\le 1$, $i=1,\ldots,n$; $j=1,\ldots,m$, $\sumin p_i=1$, $\sumjm q_j=1$; $n\ge 3$, $m\ge 3$ fixed integers; $λ\in\RR$, $λ\neq 0$ and the mappings $h:I\to\RR$, $k_j:I\to\RR$, $j=1,\ldots,m$; $I=[0,1]$, $\RR$ denoting the set of all real numbers. A special case of the above functional equation was treated earlier by L. Losonczi and Gy. Maksa.

cs.IT↗

arXiv2015

Magnetic fluctuations driven insulator-to-metal transition in Ca(Ir$_{1-x}$Ru$_{x}$)O$_{3}$

Magnetic fluctuations in transition metal oxides are a subject of intensive research because of the key role they are expected to play in the transition from the Mott insulator to the unconventional metallic phase of these materials, and also as drivers of superconductivity. Despite much effort, a clear link between magnetic fluctuations and the insulator-to-metal transition has not yet been established. Here we report the discovery of a compelling link between magnetic fluctuations and the insulator-to-metal transition in Ca(Ir$_{1-x}$Ru$_{x}$)O$_{3}$ perovskites as a function of the doping coefficient x. We show that when the material turns from insulator to metal, at a critical value of x$\sim$ 0.3, magnetic fluctuations change their character from antiferromagnetic, a Mott insulator phase, to ferromagnetic, an itinerant electron state with Hund's orbital coupling. These results are expected to have wide-ranging implications for our understanding of the unconventional properties of strongly correlated electrons systems

cond-mat.str-el↗

arXiv2015

Quasi-local critical nature of cooperative paramagnetic fluctuations in CaRuO$_{3}$ metal

We report new observation of cooperative paramagnetic fluctuations of Ru$^{4+}$ spins that coexist with the non-Fermi liquid state in CaRuO$_{3}$ perovskite below T $\simeq$ 22 K. Detailed electrical, magnetic and neutron scattering measurements reveal that the Ru$^{4+}$ ions reside in magnetic field independent random domains with dynamic properties that are reminiscent of the cooperative paramagnetic fluctuations. The linear ($E/T$) scaling of the dynamic susceptibilities and divergence of the mean relaxation time as $T$$\rightarrow$~0 K suggest quasi-local critical nature of the spin fluctuations. We argue that the non-Fermi liquid behavior arises due to the quantum critical nature of the cooperative paramagnetic fluctuations in CaRuO$_{3}$

cond-mat.str-el↗

arXiv2014

Mott insulator-to-metal transition in yttrium-doped CaIrO3

We report on the study of insulator-to-metal transition in post-perovskite compound CaIrO3. It is discovered that a gradual chemical substitution of calcium by yttrium leads to the onset of strong metallic behavior in this compound. This observation is in stark contrast to BaIrO3, which preserves its Mott insulating behavior despite excess of charge carriers due to yttrium doping. Magnetic measurements reveal that both compounds tend to exhibit magnetic character irrespective of the chemical substitution of Ca or Ba. We analyze these unusual observations in light of recent researches that suggest that unlike BaIrO3, CaIrO3 does not necessarily possess j = 1/2 ground state due to structural distortion. The insulator-to-metal transition in CaIrO3 will spur new researches to explore more exotic ground state, including superconductivity, in post-perovskite Mott insulators.

cond-mat.str-el↗

arXiv2014

Fragile antiferromagnetism in the heavy-fermion compound YbBiPt

We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, $τ_{\rm{AFM}}$ = ($\frac{1}{2} \frac{1}{2} \frac{1}{2}$), and ordered moments that align along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below $T_{\rm{N}}~\approx 0.4$ K and corresponds to a magnetic correlation length of $ξ_{\rm{n}} \approx$ 80 $\rmÅ$, and a broad component that persists up to $T^*\approx$ 0.7 K and corresponds to antiferromagnetic correlations extending over $ξ_{\rm{b}} \approx$ 20 $\rmÅ$. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound.

cond-mat.str-el↗

arXiv2013

Nonconventional Spin Glass Transition in a Chemically Ordered Pyrochlore

We report on the study of unusual spin glass properties in the geometrically frustrated pyrochlore Tb$_{2}$Mo$_{2}$O$_{7}$, $T$$_{g}$$\simeq$24 K. The analysis of the nonlinear part of dc and complex susceptibilities, near the glass transition regime, suggests the existence of a statistical distribution of relaxation times in short-range ordered ferromagnetic clusters. In addition, the magnetic spins are not sufficiently frozen below the glass transition temperature; apparently, responsible for the non-equilibrium scaling behavior of the static critical exponents of nonlinear susceptibilities. Our report is expected to shed new light in understanding the freezing properties of frustrated pyrochlores with short range ferromagnetic interactions.

cond-mat.str-el↗

arXiv2013

Interplay of spin gap and nanoscopic quantum fluctuations in intermetallic CeCu2Ge2

Neutron scattering measurements are used to elucidate interplay of the field-induced spin gap and the quantum fluctuations in archetypal heavy electron system CeCu$_{2}$Ge$_{2}$. A spin gap of $Δ$ = 0.56 meV is found to develop near the field-induced quantum critical state at $H$$_{c}$$\simeq$ 8 T, driven by fluctuations of the long range antiferromagnetic order parameter. The superconducting transition temperature in many magnetic superconductors is found to exhibit one-to-one correspondence with the spin gap. In this regard, the demonstration of an interplay of the spin gap with the nanoscopic dynamic correlation provides new arena to explore direct relation between the quantum critical behavior and the unconventional superconductivity, especially in heavy electrons systems.

cond-mat.str-el↗

arXiv2012

Anomalous magnetoresistance effect in nanoengineered material

The periodic response of magnetoresistance to an externally tunable parameter, such as magnetic field or chemical composition, in the bulk or an artificially designed material has been exploited for technological applications as well as to advance our understanding of many novel effects of solid state physics. Some notable examples are the giant magnetoresistance effect in layered materials, the quantum hall effect in semiconductor heterostructure and the phase coherence of electronic wave function in disordered metals. In recent years, the ability to engineer materials at the nanoscale has played a key role in exploring new phenomenon. Using a system involving periodic Co dots array in direct contact with a surrounding polycrystalline Cu film, we report the observation of giant thermal hysteresis and an anomalous oscillatory magnetoresistance behavior. The unusual aspects of oscillatory magnetoresistance include its observation along only one field scan direction in an intermediate temperature range of 100 K < T < 200 K. Reducing the thickness of the Cu film weakens the magnetoresistance oscillation. These properties suggest a new phenomenon, which could be harnessed for future technological applications.

cond-mat.mtrl-sci↗

arXiv2012

Spin-stripe density varies linearly with hole content in single-layer Bi2201 cuprate

We have performed inelastic neutron scattering measurements on the single-layer cuprate Bi2+xSr2-xCuO6+y (Bi2201) with x=0.2, 0.3, 0.4 and 0.5, a doping range that spans the spin-glass (SG) to superconducting (SC) phase boundary. The doping evolution of low energy spin fluctuations was found to be characterized by a change of incommensurate modulation wave vector from the tetragonal [110] to [100]/[010] directions, while maintaining a linear relation between the incommensurability and the hole concentration, delta p. In the SC regime, the spectral weight is strongly suppressed below 4 meV. Similarities and differences in the spin correlations between Bi2201 and the prototypical single-layer system La2-xSrxCuO4 are discussed.

cond-mat.str-el↗

arXiv2012

Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2

Quantum-mechanical fluctuations in strongly correlated electron systems cause unconventional phenomena such as non-Fermi liquid behavior, and arguably high temperature superconductivity. Here we report the discovery of a field-tuned quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave ordered heavy fermion metal that exhibits unconventional superconductivity under ~ 10 GPa of applied pressure. Our finding of the associated quantum critical spin fluctuations of the antiferromagnetic spin density wave order, dominating the local fluctuations due to single-site Kondo effect, provide new information about the underlying mechanism that can be important in understanding superconductivity in this novel compound.

cond-mat.str-el↗

arXiv2012

Multiple Magnetic Structures of Correlated Ce-ions in Intermetallic CeAu2Ge2

Neutron diffraction measurements on a high quality single crystal of CeAu2Ge2 reveal two new magnetic structures that coexist with commensurate long range antiferromagnetic order below the Neel temperature T_N ~ 14 K. The new magnetic structures, which exhibit distinct temperature and field dependencies, are described by both commensurate and incommensurate magnetic structures and introduces new region in the H-T phase diagram. These new experimental observations provide essential information for developing a universal understanding of the magnetic properties of this class of Ce-based CeX2M2 (X = Cu, Ag, Au; M = Ge, Si) compounds, which are prototypes for exploring quantum phase transitions and their interplay with unconventional superconductivity.

cond-mat.str-el↗

arXiv2012

Probing the connections between superconductivity, stripe order, and structure in La1.905Ba0.095Cu1-yZnyO4

The superconducting system La2-xBaxCuO4 is known to show a minimum in the transition temperature, Tc, at x = 1/8 where maximal stripe order is pinned by the anisotropy within the CuO2 planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x = 0.095, where Tc reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1% Zn substituted for Cu, which reduces Tc to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO2 layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.

cond-mat.str-el↗

arXiv2012

Uniaxial linear resistivity of superconducting La(1.905)Ba(0.095)CuO(4) induced by an external magnetic field

We present an experimental study of the anisotropic resistivity of superconducting La(2-x)Ba(x)CuO(4) with x=0.095 and transition temperature Tc=32 K. In a magnetic field perpendicular to the CuO(2) layers, H(perp), we observe that the resistivity perpendicular to the layers, ρ(perp), becomes finite at a temperature consistent with previous studies on very similar materials; however, the onset of finite parallel resistivity, ρ(par), occurs at a much higher temperature. This behavior contradicts conventional theory, which predicts that ρ(perp) and ρ(par) should become finite at the same temperature. Voltage vs. current measurements near the threshold of voltage detectability indicate linear behavior perpendicular to the layers, becoming nonlinear at higher currents, while the behavior is nonlinear from the onset parallel to the layers. These results, in the presence of moderate H(perp), appear consistent with superconducting order parallel to the layers with voltage fluctuations between the layers due to thermal noise. In search of uncommon effects that might help to explain this behavior, we have performed diffraction measurements that provide evidence for H(perp)-induced charge and spin stripe order. The field-induced decoupling of superconducting layers is similar to the decoupled phase observed previously in La(2-x)Ba(x)CuO(4) with x=1/8 in zero field.

cond-mat.supr-con↗