arXiv subjects

Yogesh Singh

Publications and source records attributed to Yogesh Singh.

88 records · Page 5Linked to original sources

arXiv2009

Magnetic, Transport, and Thermal Properties of Single Crystals of the Layered Arsenide BaMn2As2

Growth of BaMn2As2 crystals using both MnAs and Sn fluxes is reported. Room temperature crystallography, anisotropic isothermal magnetization M versus field H and magnetic susceptibility chi versus temperature T, electrical resistivity in the ab plane rho(T), and heat capacity C(T) measurements on the crystals were carried out. The tetragonal ThCr2Si2-type structure of BaMn2As2 is confirmed. After correction for traces of ferromagnetic MnAs impurity phase using M(H) isotherms, the inferred intrinsic chi(T) data of the crystals are anisotropic with chi_{ab}/chi_{c} \approx 7.5 at T = 2 K. The temperature dependences of the anisotropic chi data suggest that BaMn2As2 is a collinear antiferromagnet at room temperature with the easy axis along the c axis, and with an extrapolated Neel temperature T_N \sim 500 K. The rho(T) decreases with decreasing T below 310 K but then increases below \sim 50 K, suggesting that BaMn2As2 is a small band-gap semiconductor with an activation energy of order 0.03 eV. The C(T) data from 2 to 5 K are consistent with this insulating ground state, exhibiting a low temperature Sommerfeld coefficient gamma = 0.0(4) mJ/mol K^2. The Debye temperature is determined from these data to be theta_D = 246(4) K. BaMn2As2 is a potential parent compound for ThCr2Si2-type superconductors.

cond-mat.supr-con↗

arXiv2009

Magnetic ordering in EuRh2As2 studied by x-ray resonant magnetic scattering

Element-specific x-ray resonant magnetic scattering investigations were performed to determine the magnetic structure of Eu in EuRh2As2. In the temperature range from 46 K down to 6 K, an incommensurate antiferromagnetic (ICM)structure with a temperature dependent propagation vector (0 0 0.9) coexists with a commensurate antiferromagnetic (CM) structure. Angular-dependent measurements of the magnetic intensity indicate that the magnetic moments lie in the tetragonal basal plane and are ferromagnetically aligned within the a-b plane for both magnetic structures. The ICM structure is a spiral-like magnetic structure with a turn angle of 162 deg between adjacent Eu planes. In the CM structure, this angle is 180 deg. These results are consistent with band-structure calculations which indicate a strong sensitivity of the magnetic configuration on the Eu valence.

cond-mat.str-el↗

arXiv2009

Unusual Magnetic, Thermal, and Transport Behaviors of Single Crystal EuRh2As2

An antiferromagnetic transition is observed in single crystal EuRh2As2 at a high temperature T_N = 47 K compared to the ferromagnetic Weiss temperature theta = 12 K. We show that this is, surprisingly, consistent with mean field theory. A first-order field-induced magnetic transition is observed at T < T_N with an unusual temperature dependence of the transition field. A dramatic magnetic field-induced reduction of the electronic specific heat coefficient at 1.8-5.0 K by 38% at 9 T is observed. In addition, a strong positive magnetoresistance and a large change in the Hall coefficient occur below 25 K. Band structure calculations indicate that the Fermi energy lies on a steep edge of a narrow peak in the density of states.

cond-mat.str-el↗

arXiv2008

Low-dimensional magnetism in the mixed Cr valence spin-chain compound Sr_4Cr_3O_9

Sr_4Cr_3O_9 is the n = 2 member of a family of quasi-one-dimensional compounds A_{n+2}T_{n+1}O_{3n+3} (A = Ca, Sr, or Ba, T = transition metal, and n = 1 to \infty) having a crystal structure which consists of chains of n TO_6 octahedra alternated by one TO_6 trigonal prism running along the c-axis. The chains are arranged on a triangular lattice in the ab-plane. We report the synthesis, structure, magnetization M versus magnetic field H, magnetic susceptibility χversus temperature T and specific heat C versus T measurements on sintered and arc-melted polycrystalline samples of Sr_4Cr_3O_9. The χdata have a T dependence which is typical of low-dimensional magnetic systems with dominant antiferromagnetic (AF) exchange interactions. Specifically, χ(T) shows a broad maximum at T_{max} \approx 200K for the sintered pellet and T_{max}\approx 265K for the arc-melted sample, indicating the onset of short-range magnetic order. Below T = 15K we observe 2 anomalies in the χ(T) data for both samples suggesting the onset of long-range magnetic ordering. The corresponding anomalies in the C(T) data however are weak indicating that only a small amount of the expected magnetic entropy is recovered at the magnetic transitions and that strong short-range AF order exists above these temperatures.

cond-mat.str-el↗

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

Crystal growth and ambient and high pressure study of the reentrant superconductor Tm_2Fe_3Si_5

We report single crystal growth of the reentrant superconductor Tm_2Fe_3Si_5, and measurements of the anisotropic static magnetic susceptibility χ(T) and isothermal magnetization M(H), ac susceptibility χ_ac(T), electrical resistivity ρ(T) and heat capacity C(T) at ambient pressure and χ_ac(T) at high pressure. The magnetic susceptibility along the c-axis χ_c(T) shows a small maximum around 250 K and does not follow the Curie-Weiss behavior while the magnetic susceptibility along the a-axis χ_a(T) follows a Curie-Weiss behavior between 130 K and 300 K with a Weiss temperature θand an effective magnetic moment μ_eff which depend on the temperature range of the fit. The easy axis of magnetization is perpendicular to the c-axis and χ_a/χ_c = 3.2 at 1.8 K. The ambient pressure χ_ac(T) and C(T) measurements confirm bulk antiferromagnetic ordering at T_N = 1.1 K. The sharp drop in χ_ac below T_N is suggestive of the existence of a spin-gap. We observe superconductivity only under applied pressures P\geq 2 kbar. The temperature-pressure phase diagram showing the non-monotonic dependence of the superconducting transition temperature T_c on pressure P is presented.

cond-mat.str-el↗

arXiv2007

Superconducting and normal state properties of the layered boride OsB2

OsB_2 crystallizes in an orthorhombic structure (Pmmn) which contains alternate boron and osmium layers stacked along the c-axis. The boron layers consist of puckered hexagons as opposed to the flat graphite-like boron layers in MgB_2. OsB_2 is reported to become superconducting below 2.1 K. We report results of the dynamic and static magnetic susceptibility, electrical resistivity, Hall effect, heat capacity and penetration depth measurements on arc-melted polycrystalline samples of OsB_2 to characterize its superconducting and normal state properties. These measurements confirmed that OsB_2 becomes a bulk superconductor below T_ c = 2.1 K. Our results indicate that OsB_2 is a moderate-coupling Type-II superconductor with an electron-phonon coupling constant λ_{ep} \approx 0.4 to 0.5, a small Ginzburg-Landau parameter κ\sim 1 to 2 and an upper critical magnetic field H_{c2}(0.5 K) \sim 420 Oe for an unannealed sample and H_{c2}(1 K) \sim 330 Oe for an annealed sample. The temperature dependence of the superfluid density n_{s}(T) for the unannealed sample is consistent with an s-wave superconductor with a slightly enhanced zero temperature gap Δ(0) = 1.9 k_{B}T_c and a zero temperature London penetration depth λ(0) = 0.38(2) μm. The n_{s}(T) data for the annealed sample shows deviations from the predictions of the single-band s-wave BCS model. The magnetic, transport and thermal properties in the normal state of isostructural and isoelectronic RuB_2, which is reported to become superconducting below 1.6 K, are also reported.

cond-mat.supr-con↗

arXiv2007

Magnetic and thermal properties of the S = 1/2 zig-zag spin-chain compound In2VO5

Static magnetic susceptibility χ, ac susceptibility χ_{ac} and specific heat C versus temperature T measurements on polycrystalline samples of In2VO5 and χand C versus T measurements on the isostructural, nonmagnetic compound In2TiO5 are reported. A Curie-Wiess fit to the χ(T) data above 175 K for In2VO5 indicates ferromagnetic exchange between V^{4+} (S = 1/2) moments. Below 150 K the χ(T) data deviate from the Curie-Weiss behavior but there is no signature of any long range magnetic order down to 1.8 K. There is a cusp at 2.8 K in the zero field cooled (ZFC) χ(T) data measured in a magnetic field of 100 Oe and the ZFC and field cooled (FC) data show a bifurcation below this temperature. The frequency dependence of the χ_{ac}(T) data indicate that below 3 K the system is in a spin-glass state. The difference ΔC between the heat capacity of In2VO5 and In2TiO5 shows a broad anomaly peaked at 130 K. The entropy upto 300 K is more than what is expected for S = 1/2 moments. The anomaly in ΔC and the extra entropy suggests that there may be a structural change below 130 K in In2VO5.

cond-mat.str-el↗

arXiv2007

Singlet ground state in the spin-1/2 dimer compound Sr3Cr2O8

Magnetic susceptibility and specific heat versus temperature measurements on polycrystalline samples of Sr3Cr2O8 and the isostructural, nonmagnetic compound Sr3V2O8 are reported. A Curie-Wiess fit to the high-temperature susceptibility data for Sr3Cr2O8 indicates that the Cr atoms are in the rare Cr^5+ (Spin S = 1/2) valence state as expected from the composition. The ground state was found to be a spin singlet with an excitation gap Delta/k_ B = 61.9(1) K to the magnetic triplet states, and a weak interdimer coupling of 6(2) K was inferred. The specific heat and susceptibility measurements on Sr3V2O8 reveal a phase transition at 110 K which is evidently a structural transition.

cond-mat.str-el↗

arXiv2005

Low temperature properties of a new Kondo Lattice compound Yb$_2$Ir$_3$Sn$_5$

We report the preparation and results of our magnetic and transport measurements on a new Yb-based ternary compound Yb$_2$Ir$_3$Sn$_5$. This compound forms in the orthorhombic Cmc21 structure. DC magnetic susceptibility between 1.8 K and 300 K reveal that Yb is in its magnetic trivalent state in this compound. Electrical resistivity measurements down to 1. 5K show a behavior typical of Kondo lattice compounds with a minimum in resistivity around 8K and a coherence peak near 3K. AC magnetic susceptibility down to 100mK reveals an anti-ferromagnetic transition below 1.35 K.

cond-mat.str-el↗

arXiv2005

Competition between Superconductivity and Charge Density Wave Ordering in the Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ Alloy System

We have performed bulk measurements such as dc magnetic susceptibility, electrical resistivity and heat capacity on the pseudo-ternary alloys Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ to study the interplay and competition between superconductivity and the charge density wave (CDW) ordering transition. We track the evolution of the superconducting transition temperature T$_{SC}$ and the CDW ordering temperature T$_{CDW}$ as a function of x (concentration of Ge) ($0.0 \leq x\leq~1.0$). We find that increasing x (increasing disorder) suppresses the T$_{CDW}$ rapidly with the concomitant increase in T$_{SC}$. We present a temperature-concentration (or volume) phase diagram for this system and compare our results with earlier work on substitution at the Lu or Ir site to show how dilution at the Si site presents a different situation from these other works. The heat capacity data in the vicinity of the CDW transition has been analyzed using a model of critical fluctuations in addition to a mean-field contribution and a smooth lattice background. We find that the critical exponents change appreciably with increasing disorder. This analysis suggests that the strong-coupling and non mean-field like CDW transition in the parent compound Lu$_5$Ir$_4$Si$_{10}$ changes to a mean-field like transition with increasing Ge concentration.

cond-mat.str-el↗

arXiv2003

Evolution of Superconductivity and Charge Density Wave Ordering in the Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ Alloy System

The compounds Lu$_5$Ir$_4$Si$_{10}$ and Lu$_5$Ir$_4$Ge$_{10}$ crystallize in the tetragonal Sc$_5$Co$_4$Si$_{10}$ type structure. Lu$_5$Ir$_4$Si$_{10}$ is known to superconduct below 3.9 K and it also exhibits a strongly coupled charge density wave (CDW) transition below 83 K. Lu$_5$Ir$_4$Ge$_{10}$ undergoes a transition into the superconducting state below about 2.4 K without any CDW transition at higher temperatures. Recent Si NMR measurements on polycrystalline samples of Lu$_5$Ir$_4$Si$_{10}$ suggest that there is no energy gap at the Si site across the CDW transition. Thus it is of interest to study the evolution of the superconductivity and the CDW transition when we dope at the Si site with small quantities of Ge. Here we present the evolution of T$_C$ and T$_{CDW}$ with concentration x of Ge in the alloy system Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ (x=0.0, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.4, 1.0) as estimated from dc susceptibility measurements. We find that the CDW is strongly suppressed with increasing x and there is a simultaneous enhancement of the superconducting transition temperature T$_C$ from 3.9 K for the undoped sample to almost 6.6 K for only 10% concentration of Ge in the alloy.

cond-mat.str-el↗

arXiv2003

Effect of Hydrostatic Pressure on the Kondo Effect in the Heavy Fermion System Yb$_2$Fe$_3$Si$_5$

We report the effect of hydrostatic pressure on the resistivity of the heavy fermion Kondo lattice compound Yb$_2$Fe$_3$Si$_5$ in the temperature range 2-300 K and in pressures upto 10 Kbar. At ambient pressure the resistivity shows a prominent Kondo effect. The Kondo minimum shifts from 22.5 K at ambient pressure to 19.2 K at a pressure of 10 Kbar. The depth of the Kondo minimum also progressively decreases with pressure. These results suggest a possible weakening of the Kondo effect with the application of pressure.

cond-mat.str-el↗

arXiv2003

Magnetic Ordering and Superconductivity in the RE$_2$Ir$_3$Ge$_5$ (RE = Y, La-Tm, Lu) System

We find that the compounds for RE = Y, La-Dy, crystallize in the tetragonal Ibam (U$_2$Co$_3$Si$_5$ type) structure whereas the compounds for RE = Er-Lu, crystallize in a new orthorhombic structure with a space group Pmmn. Samples of Ho$_2$Ir$_3$Ge$_5$ were always found to be multiphase. The compounds for RE = Y to Dy which adopt the Ibam type structure show a metallic resistivity whereas the compounds with RE = Er, Tm and Lu show an anomalous behavior in the resistivity with a semiconducting increase in $ρ$ as we go down in temperature from 300K. Interestingly we had earlier found a positive temperature coefficient of resistivity for the Yb sample in the same temperature range. We will compare this behavior with similar observations in the compounds RE$_3$Ru$_4$Ge$_{13}$ and REBiPt. La$_2$Ir$_3$Ge$_5$ and Y$_2$Ir$_3$Ge$_5$ show bulk superconductivity below 1.8K and 2.5K respectively. Our results confirm that Ce$_2$Ir$_3$Ge$_5$ shows a Kondo lattice behavior and undergoes antiferromagnetic ordering below 8.5K. Most of the other compounds containing magnetic rare-earth elements undergo a single antiferromagnetic transition at low temperatures (T$\leq$12K) while Gd$_2$Ir$_3$Ge$_5$, Dy$_2$Ir$_3$Ge$_5$ and Nd$_2$Ir$_3$Ge$_5$ show multiple transitions. The T$_N$'s for most of the compounds roughly scale with the de Gennes factor. which suggests that the chief mechanism of interaction leading to the magnetic ordering of the magnetic moments may be the RKKY interaction.

cond-mat.str-el↗

arXiv2003

Antiferromagnetic ordering in the Kondo lattice system Yb$_2$Fe$_3$Si$_5$

Compounds belonging to the R$_2$Fe$_3$Si$_5$ series exhibit unusual superconducting and magnetic properties. Although a number of studies have been made on the first reentrant antiferromagnet superconductor Tm$_2$Fe$_3$Si$_5$, the physical properties of Yb$_2$Fe$_3$Si$_5$ are largely unexplored. In this work, we attempt to provide a comprehensive study of bulk properties such as, resistivity, susceptibility and heat-capacity of a well characterized polycrystalline Yb$_2$Fe$_3$Si$_5$. Our measurements indicate that Yb$^{3+}$ moments order antiferromagnetically below 1.7 K. Moreover, the system behaves as a Kondo lattice with large Sommerfeld coefficient ($γ$) of 0.5~J/Yb mol K$^{2}$ at 0.3 K, which is well below T$_N$. The absence of superconductivity in Yb$_2$Fe$_3$Si$_5$ down to 0.3 K at ambient pressure is attributed to the presence of the Kondo effect.

cond-mat.str-el↗

arXiv2003

Unusual Ground State Properties of the Kondo-Lattice Compound Yb2Ir3Ge5

We report sample preparation, structure, electrical resistivity, magnetic susceptibility and heat capacity studies of a new compound Yb$_2$Ir$_3$Ge$_5$. We find that this compound crystallizes in an orthorhombic structure with a space group PMMN unlike the compound Ce$_2$Ir$_3$Ge$_5$ which crystallizes in the tetragonal IBAM (U$_2$Co$_3$Si$_5$ type) structure. Our resistivity measurements indicate that the compound Yb$_2$Ir$_3$Ge$_5$ behaves like a typical Kondo lattice system with no ordering down to 0.4 K. However, a Curie-Weiss fit of the inverse magnetic susceptibility above 100 K gives an effective moment of only 3.66 $μ$$_B$ which is considerably less than the theoretical value of 4.54 $μ$$_B$ for magnetic Yb$^3+$ ions. The value of $θ_{P}$ = -15.19 K is also considerably higher indicating the presence of strong hybridization. An upturn in the low temperature heat capacity gives an indication that the system may order magnetically just below the lowest temperature of our heat capacity measurements (0.4 K). The structure contains two sites for Yb ions and the present investigation suggests that Yb may be trivalent in one site while it may be significantly lower (close to divalent) in the other.

cond-mat.str-el↗