arXiv subjects

T. Klimczuk

Publications and source records attributed to T. Klimczuk.

At least 37 records · Page 2Linked to original sources

arXiv2009

Superconductivity at 2.2K in La3Ni4P4O2

We report the observation of superconductivity in La3Ni4P4O2 at 2.2 K. The layer stacking in this compound results in an asymmetric distribution of charge reservoir layers around the Ni2P2 planes. The estimated Wilson ratio, Rw ~ 5, indicates the presence of a strongly enhanced normal state susceptibility, but many of the basic superconducting characteristics are conventional. The estimated electronic contribution to the specific heat, gamma ~ 6.2 mJ mol-Ni^-1K^-2, is about 2/3 of that found in layered nickel borocarbide superconductors.

cond-mat.supr-con↗

arXiv2008

First order magnetic transition in single crystal CaFe$_2$As$_2$ detected by $^{75}$As NMR

We report $^{75}$As Nuclear Magnetic Resonance data in a single crystal of CaFe$_2$As$_2$. The Knight shift, electric field gradient, and spin-lattice relaxation rate are strongly temperature dependent in the paramagnetic state, and change discontinuously at the structural transition temperature, $T_S=T_N=167$ K. Immediately below, the NMR spectra reveal an internal field at the As site associated with the presence of a commensurate magnetic order. These results indicate that the structural and magnetic transitions in CaFe$_2$As$_2$ are first order and strongly coupled, and that the electron density in the FeAs plane is highly sensitive to the out-of-plane structure.

cond-mat.str-el↗

arXiv2008

Microscopic study of the effect of impurities on the first order spin density wave transition in BaFe$_2$As$_2$

We report an $^{75}$As NMR study of BaFe$_2$As$_2$ in both single crystals and polycrystal forms. We find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K. In contrast to Sn-free samples, we find that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order.

cond-mat.str-el↗

arXiv2008

First Order Phase Transition and Superconductivity in BaNi2As2 Single Crystals

We report the synthesis and physical properties of single crystals of stoichiometric BaNi2As2 that crystalizes in the ThCr2Si2 structure with lattice parameters a = 4.112(4) Åand c = 11.54(2) Å. Resistivity and heat capacity show a first order phase transition at T_0 = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4x10^{-10} Ω-cm/Oe. Resistivity, ac-susceptibility, and heat capacity find evidence for bulk superconductivity at T_c = 0.7 K. The Sommerfeld coefficient at T_c is 11.6 \pm 0.9 mJ/molK^2. The upper critical field is anisotropic with initial slopes of dH_{c2}^{c}/dT = -0.19 T/K and dH_{c2}^{ab}/dT = -0.40 T/K, as determined by resistivity.

cond-mat.supr-con↗

arXiv2008

Synthesis and Properties of CaFe$_2$As$_2$ Single Crystals

We report the synthesis and basic physical properties of single crystals of CaFe2As2, an isostructural compound to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystalizes in the ThCr2Si2 structure with lattice parameters a = 3.907(4) A and c = 11.69(2) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T_0 171 K. The magnetic susceptibility is nearly isotropic from 2 K to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 +- 0.3 mJ/molK2, and does not reveal any evidence for the presence of high frequency (> 300 K) optical phonon modes. The Hall coefficient is negative below the transition indicating dominant n-type carriers.

cond-mat.str-el↗

arXiv2008

Pressure-induced superconductivity in single crystal CaFe2As2

We report pressure-induced superconductivity in a single crystal of CaFe2As2. At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature Tc exceeding 10 K. The rate of Tc suppression with applied magnetic field is -0.7 K/T, giving an extrapolated zero-temperature upper critical field of 10-14T.

cond-mat.supr-con↗

arXiv2008

Physical properties of the new Uranium ternary compounds U3Bi4M3 (M=Ni, Rh)

We report the properties of two new isostructural compounds, U3Bi4Ni3 and U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based members of the larger 3-4-3 family. For U3Bi4Rh3, a logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material displaying ferromagnetic quantum criticality.

cond-mat.mtrl-sci↗

arXiv2008

Physical properties of the noncentrosymmetric superconductor Mg_10Ir_19B_16

Specific heat, electrical resistivity, and magnetic susceptibility measurements on a high quality sample of Mg10Ir19B16 provide a self-consistent determination of its superconducting properties. They indicate that Mg10Ir19B16 is a type-II superconductor (Tc=4.45K, k(0)=20), with an electron-phonon coupling constant l_ep=0.66. An analysis of the T-dependent specific heat shows that superconducting properties are dominated by an s-wave gap (D=0.7 meV). Point contact tunneling data provide evidence for multiple superconducting gaps, as expected from strong asymmetric spin-orbit coupling.

cond-mat.supr-con↗

arXiv2007

Magnetism and structure of LixCoO2 and comparison to NaxCoO2

The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0 are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and 1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling is observed, increasing in strength as more Co4+ is introduced. At an x value of about 0.65, the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. A comparison of the structures of the NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter. Further, the CoO6 octahedra in LixCoO2 are less distorted than those in NaxCoO2. We postulate that these structural differences strongly influence the physical properties in the two systems.

cond-mat.str-el↗

arXiv2007

Crystal Structure and Physical Properties of Mg6Cu16Si7-type M6Ni16Si7, for M = Mg, Sc, Ti, Nb, and Ta

Five compounds were investigated for magnetic character and superconductivity, all with non-magnetic nickel and band structures containing flat bands and steep bands. The syntheses and crystal structures, refined by powder X-ray diffraction, are reported for M6Ni16Si7, where M = Mg, Sc, Ti, Nb, and Ta. All compounds form in the Mg6Cu16Si7 structure type. Resistance measurements are also reported on M6Ni16Si7 (M = Mg, Sc, Ti, and Nb) down to 0.3 K, with all four showing metallic conductivity. No superconductivity is observed. Magnetization measurements for all compounds reveal essentially temperature independent paramagnetism, with a tendency toward more enhanced low temperature paramagnetism for the cases of Mg6Ni16Si7 and Sc6Ni16Si7.

cond-mat.supr-con↗

arXiv2007

The effect of Fe and Ru substitution for Ni on the thermopower in MgCNi_3

The intermetallic perovskite MgCNi3 is a superconductor with a Tc=7 K. Substitution of Fe and Ru for Ni decreases Tc monotonically as the doping concentration is increased. Here we report thermopower measurements, S(T), on MgCNi3, MgCNi3-xFex and MgCNi3-xRux. For MgCNi3, the thermopower is negative, - 12.5 mikroV/K, at 300 K. The absolute value of S decreases as x increases in MgCNi3-xFex and MgCNi3-xRux. The sign of S changes from negative to positive at low temperatures for values of x > 0.01. These data show that the carriers in MgCNi3 are electrons, and by increasing x and decreasing temperature, the participation of hole carriers clearly increases. The influence of the magnetic moments of the Fe atoms on the thermopower is not visible.

cond-mat.supr-con↗

arXiv2007

Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8

Mn3V2O8 is a magnetic system in which S = 5/2 Mn2+ is found in the kagome staircase lattice. Here we report the magnetic phase diagram for temperatures above 2 K and applied magnetic fields below 9 T, characterized by measurements of the magnetization and specific heat with field along the three unique lattice directions. At low applied magnetic fields, the system first orders magnetically below Tm1 ~ 21 K, and then shows a second magnetic phase transition at Tm2 ~ 15 K. In addition, a phase transition that is apparent in specific heat but not seen in magnetization is found for all three applied field orientations, converging towards Tm2 as H -> 0. The magnetic behavior is highly anisotropic, with critical fields for magnetic phase boundaries much higher when the field is applied perpendicular to the Kagome staircase plane than when applied in-plane. The field-temperature (H - T) phase diagrams are quite rich, with 7 distinct phases observed.

cond-mat.str-el↗

arXiv2007

Magnetic Structure and Properties of the S = 5/2 Triangular Antiferromagnet $α$-NaFeO$_2$

The magnetic properties of $α$-NaFeO$_2$ are studied by neutron diffraction and magnetization measurements. An ordered phase with spins aligned along the b$_hex$ axis exists at low temperatures (T < 4 K). At intermediate temperatures (4 K < T < 11 K), the system passes through an incommensurate ordered phase before transforming into a short range ordered state at higher temperatures that persists up to at least 50 K. Although the short range ordering does not persist to room temperature according to neutron diffraction, the magnetic susceptibility does not follow Curie-Weiss behavior, even up to 320 K. This rich magnetic behavior can be understood qualitatively as a competition between different magnetic exchange interactions that are similar in magnitude. The delicate balance between these interactions makes $α$-NaFeO$_2$ a candidate for more detailed theoretical work to understand magnetic behavior in frustrated magnetic systems.

cond-mat.str-el↗

arXiv2006

Superconductivity in Mg10Ir19B16

Mg10Ir19B16, a previously unreported compound in the Mg-Ir-B chemical system, is found to be superconducting at temperatures near 5 K. The fact that the compound exhibits a range of superconducting temperatures between 4 and 5 K suggests that a range of stoichiometries is allowed, though no structural evidence for this is observed. The compound has a large, noncentrosymmetric, body centered cubic unit cell with a = 10.568 Angstrom, displaying a structure type for which no previous superconductors have been reported.

cond-mat.supr-con↗

arXiv2006

Superconductivity in Cu_xTiSe_2

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids. They are collective states that arise from intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well-studied examples, with TiSe_2 one of the first CDW-bearing materials known. The competition between CDW and superconducting collective electronic states at low temperatures has long been held and explored, and yet no chemical system has been previously reported where finely controlled chemical tuning allows this competition to be studied in detail. Here we report how, upon controlled intercalation of TiSe_2 with Cu to yield Cu_xTiSe_2, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum T_c of 4.15 K found at x = 0.08. Cu_xTiSe_2 thus provides the first opportunity to study the CDW to Superconductivity transition in detail through an easily-controllable chemical parameter, and will provide new insights into the behavior of correlated electron systems.

cond-mat.supr-con↗

arXiv2005

Ca_25Co_22O_56(OH)_28: a layered misfit compound

The high pressure synthesis, structure and magnetic properties of Ca_25Co_22O_56(OH)_28 are reported. The compound has a misfit structure, consisting of double, square calcium oxide hydroxide rock-salt-like layers between hexagonal CoO_2 layers. The misfit compound crystallizes in the monoclinic space group C2/m, and can be characterized by the coexistence of two subsystems with common a=4.893(5)A, c=8.825(9)A and b=95.745(8) parameters, and different b parameters: b_RS=4.894(5)A, and b_HEX=2.809(3)A, for the rock-salt and hexagonal type planes respectively. The compound shows Curie-Weiss paramagnetism with an antiferromagnetic Weiss temperature of -43K and a reduced Co moment. Substantial deviations from Curie-Weiss behavior are seen below 50K with no indication of magnetic ordering. No superconductivity was observed down to a temperature of 2K.

cond-mat.mtrl-sci↗

arXiv2004

The effect of {11}^B substitution on the superconductivity in MgCNi3

The crystal structure of boron doped superconducting MgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported. The solubility limit of boron is determined to be approximately x=0.16. The unit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases from x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from 7.09K (x=0) to 6.44K (x=0.155).

cond-mat.supr-con↗

arXiv2004

Hydration phase diagram for sodium cobalt oxide Na0.3CoO2*yH2O

The hydration phase diagram for sodium cobalt oxyhydrate, Na0.3CoO2*yH2O (y=0, 0.6, 1.3), was determined as a function of relative humidity at 298K. It is found that greater than 75% relative humidity is needed for complete hydration of anhydrous Na0.3CoO2 to the superconducting phase Na0.3CoO2*1.3H2O. Dehydration studies show that a minimum of 43% relative humidity is needed to maintain the stability of the fully hydrated superconducting phase. The intermediate hydrate, Na0.3CoO2*0.6H2O, is stable between 10% and 50% relative humidity on hydration, and 35% to 0% relative humidity on dehydration.

cond-mat.supr-con↗