arXiv subjects

Tomasz Klimczuk

Publications and source records attributed to Tomasz Klimczuk.

At least 55 records · Page 3Linked to original sources

arXiv2019

Observation of topological surface state in a superconducting material

The discovery of topological insulator phase has ignited massive research interests in novel quantum materials. Topological insulators with superconductivity further invigorate the importance of materials providing the platform to study the interplay between these two unique states. However, the candidates of such materials are rare. Here, we report a systematic angle-resolved photoemission spectroscopy (ARPES) study of a superconducting material CaBi2 [Tc = 2 K], corroborated by the first principles calculations. Our study reveals the presence of Dirac cones with a topological protection in this system. Systematic topological analysis based on symmetry indicator shows the presence of weak topological indices in this material. Furthermore, our transport measurements show the presence of large magnetoresistance in this compound. Our results indicate that CaBi2 could potentially provide a material platform to study the interplay between superconductivity and topology.

cond-mat.mes-hall↗

arXiv2019

Crossover from charge density wave stabilized antiferromagnetism to superconductivity in Nd$_{1-x}$La$_x$NiC$_2$ compounds

The path from the charge density wave antiferromagnet NdNiC$_2$ to a noncentrosymmetric superconductor LaNiC$_2$ has been studied by gradual replacement of Nd by La ions. The evolution of physical properties has been explored by structural, magnetic, transport, magnetoresistance and specific heat measurements. With the substitution of La for Nd, the Peierls temperature is gradually suppressed, which falls within the BCS mean-field relation for chemical pressure with a critical concentration of $x_c$ = 0.38. As long as charge density wave is maintained, the antiferromagnetic ground state remains robust against doping and despite of a Néel temperature reduction shows a rapid and sharp magnetic transition. Once the CDW is completely suppressed, intermediate compounds of the Nd$_{1-x}$La$_x$NiC$_2$ series reveal symptoms of a gradual softening of the features associated with AFM transition and increase of the spin disorder. Immediately after the antiferromagnetic transition is depressed to zero temperature, the further incorporation of La ions results in the emergence of superconductivity. This crossover in the Nd$_{1-x}$La$_x$NiC$_2$ is discussed in the terms of the possible quantum critical point.

cond-mat.str-el↗

arXiv2019

Charge density wave and large non-saturating magnetoresistance in YNiC$_2$ and LuNiC$_2$

We report a study of physical properties of two quasi-low dimensional metals YNiC$_2$ and LuNiC$_2$ including the investigation of transport, magnetotransport, galvanomagnetic and specific heat properties. In YNiC$_2$ we reveal two subsequent transitions associated with the formation of weakly coupled charge density wave at $T_{CDW}$ = 318 K, and its locking in with the lattice at $T_1$ = 275 K. These characteristic temperatures follow the previously proposed linear scaling with the unit cell volume, demonstrating its validity extended beyond the lanthanide-based $R$NiC$_2$. We also find that, in the absence of magnetic ordering able to interrupt the development of charge density wave, the Fermi surface nesting leads to opening of small pockets, containing high mobility carriers. This effect gives rise to substantial enhancement of magnetoresistance, reaching 470 % for YNiC$_2$ and 50 % for LuNiC$_2$ at $T$ = 1.9 K and $B$ = 9 T.

cond-mat.str-el↗

arXiv2018

Novel Superconducting SrSnP with Strong Sn-P Antibonding Interaction: Is the Sn Atom Single or Mixed Valent?

The large single crystals of SrSnP were prepared using Sn self-flux method. The superconductivity in the tetragonal SrSnP is observed with the critical temperature of ~2.3 K. The results of a crystallographic analysis, superconducting characterization, and theoretical assessment of tetragonal SrSnP are presented. The SrSnP crystallizes in the CaGaN structure type with space group P4/nmm (S.G.129, Pearson symbol tP6) according to the single crystal X-ray diffraction characterization. A combination of magnetic susceptibility, resistivity, and heat capacity measurements confirms the bulk superconductivity with Tc = 2.3(1) K in SrSnP. According to the X-ray photoelectron spectroscopy (XPS) measurement, the assignments of Sr2+ and P3- are consistent with the chemical valence electron balance principles. Moreover, it is highly likely that Sn atom has only one unusual oxidation state. First-principles calculations indicate the bands around Fermi level are hybridized among Sr-d, Sn-p, and P-p orbitals. The strong Sn-P and Sr-P interactions pose as keys to stabilize the crystallographic structure and induce the superconductivity, respectively. The physics-based electronic and phononic calculations are consistent with the molecular viewpoint. After including the spin-orbit coupling (SOC) into the calculation, the band degeneracies at gamma-point in the first Brillouin zone (BZ) split into two bands, which yield to the van Hove singularities around Fermi level.

cond-mat.supr-con↗

arXiv2018

Correlation between charge density waves and antiferromagnetism in Nd$_{1-x}$Gd$_x$NiC$_2$ solid solution

We report a study on the evolution of a charge density wave and antiferromagnetism in the series of the polycrystalline solid solution Nd$_{1-x}$Gd$_x$NiC$_2$ (0 $\leq$ x $\leq$ 1) by means of magnetic and transport properties measurements. The experimental results reveal the violation of the de Gennes law and a strong correlation between the Peierls, Néel and Curie-Weiss temperatures, which strongly suggests a cooperative interaction between the charge density wave state and antiferromagnetism due to Fermi surface nesting enhancement of the RKKY interaction. We also find that, the obtained results for the Nd$_{1-x}$Gd$_x$NiC$_2$ (0 $\leq$ x $\leq$ 1) series overlap with the $T_{CDW}$ trend line in the phase diagram for RNiC$_2$ family.

cond-mat.str-el↗

arXiv2018

Superconductivity in the Superhard Boride WB$_{4.2}$

We show that the superhard boride WB$_{4.2}$ is a superconductor with a T$_c$ of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for WB$_{4.2}$ is 2.07(3) mJ mol$^{-1}$ K$^{-2}$ and the ΔC/γT$_c$ = 1.56, which is somewhat higher than what is expected for weakly coupled BCS type superconductors. The H$_{c2}$ vs T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero-temperature upper critical field (μ$_0$H$_{c2}$(0)) for WB$_{4.2}$ lies somewhere between the linear extrapolation of μ$_0$H$_{c2}$(T) to 0 K and expectations based on the WHH model.

cond-mat.supr-con↗

arXiv2018

A Family of Pb-based Superconductors with Variable Cubic to Hexagonal Packing

We describe three previously unreported superconductors, BaPb3, Ba0.89Sr0.11Pb3 and Ba0.5Sr0.5Pb3. These three materials, together with SrPb3, form a distinctive isoelectronic family of intermetallic superconductors based on the stacking of Pb planes, with crystal structures that display a hexagonal to cubic perovskite-like progression, as rarely seen in metals. The superconducting transition temperatures (Tc) are similar for all - 2.2 K for BaPb3, 2.7 K for Ba0.89Sr0.11Pb3 and 2.6 K for Ba0.5Sr0.5Pb3, and the previously reported Tc of SrPb3, ~ 2 K, is confirmed. The materials are moderate coupling superconductors, and calculations show that the electronic densities of states at the Fermi energy are primarily contributed by Pb. The observations suggest that the Pb-stacking variation has only a minor effect on the superconductivity.

cond-mat.supr-con↗

arXiv2018

Extended phase diagram of RNiC2 family: Linear scaling of the Peierls temperature

Physical properties for the late-lanthanide-based RNiC2 (R = Dy, Ho, Er, and Tm) ternary compounds are reported. All the compounds show antiferromagnetic ground state with the Néel temperature ranging from 3.4 K for HoNiC2 to 8.5 K for ErNiC2. The results of the transport and galvanomagnetic properties confirm a charge density wave state at and above room temperature with transition temperatures TCDW = 284, 335, 366, and 394 K for DyNiC2, HoNiC2, ErNiC2, and TmNiC2, respectively. The Peierls temperature TCDW scales linearly with the unit cell volume. A similar linear dependence has been observed for the temperature of the lock-in transition T1 as well. Beyond the intersection point of the trend lines, the lock-in transition is no longer observed. In this Rapid Communication we demonstrate an extended phase diagram for the RNiC2 family.

cond-mat.str-el↗

arXiv2018

The Chiral Non-Centrosymmetric Superconductors TaRh$_2$B$_2$ and NbRh$_2$B$_2$

The symmetry of a material's crystal structure has a significant effect on the energy states of its electrons. Inversion symmetry, for example, results in energetically degenerate electron energy bands for electrons with wave vectors $\textit{k}$ and -$\textit{k}$, but, when spatial inversion symmetry is absent, this equivalence is no longer possible. In a non-centrosymmetric superconductor, such inequivalence has an important effect: the standard superconducting state, where electrons with opposite momenta form pairs on the Fermi surface, is not possible. A handful of such materials is known; they display different degrees of influence of this lack of inversion symmetry on their superconducting properties. The effect of crystal structure chirality on the properties and applications of superconductors, on the other hand, is little discussed. Here we report the new isostructural non-centrosymmetric superconductors TaRh$_2$B$_2$ and NbRh$_2$B$_2$, which have a previously unreported crystal structure type. Not only do these materials lack inversion symmetry, but their crystal structure is also chiral; in other words, they can exist in right-handed or left-handed forms. Unlike most superconductors, their upper critical magnetic fields extrapolated to 0 K exceed the Pauli limit, which is often taken as the first indication that a superconducting material is anomalous. We propose that these materials represent a new kind of platform on which the effects of handedness on superconductors and their devices can be tested.

cond-mat.supr-con↗

arXiv2017

Ternary Bismuthide SrPtBi2: Computation and Experiment in Synergism to Explore Solid-State Materials

A combination of theoretical calculation and the experimental synthesis to explore the new ternary compound is demonstrated in the Sr-Pt-Bi system. Since Pt-Bi is considered as a new critical charge-transfer pair for superconductivity, it inspired us to investigate the Sr-Pt-Bi system. With a thorough calculation of all the known stable/metastable compounds in the Sr-Pt-Bi system and crystal structure predictions, the thermodynamic stability of hypothetical stoichiometry, SrPtBi2, is determined. Followed by the high-temperature synthesis and crystallographic analysis, the first ternary bismuthide in Sr-Pt-Bi, SrPtBi2 was prepared and the stoichiometry was confirmed experimentally. SrPtBi2 crystallizes in the space group Pnma (S.G. 62, Pearson Symbol oP48), which matches well with theoretical prediction using an adaptive genetic algorithm (AGA). Using first-principles calculations, we demonstrate that the orthorhombic structure has lower formation energies than other 112 structure types, such as tetragonal BaMnBi2 (CuSmP2) and LaAuBi2 (CuHfSi2) structure types. The bonding analysis indicates the Pt-Bi interactions play a critical role in structural stability. The physical properties measurements show the metallic properties with low electron-phonon interactions at the low temperature, which agrees with the electronic structure assessment.

cond-mat.mtrl-sci↗

arXiv2017

Superconductivity in the Nb-Ru-Ge $σ$-Phase

We show that the previously unreported ternary $σ$-phase material Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ is a superconductor with a critical temperature of 2.2 K. Temperature-dependent magnetic susceptibility, resistance, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant $γ$ for Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ is 91 mJ mol-f.u.$^{-1}$K$^{-2}$ and the specific heat anomaly at the superconducting transition, $Δ$C/$γ$T$_c$, is approximately 1.38. The zero-temperature upper critical field ($μ_0$H$_{c2}$(0)) was estimated to be 2 T by resistance data. Field-dependent magnetization data analysis estimated $μ_0$H$_{c1}$(0) to be 5.5 mT. Thus, the characterization shows Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ to be a type II BCS superconductor. This material appears to be the first reported ternary phase in the Nb-Ru-Ge system, and the fact that there are no previously reported binary Nb-Ru, Nb-Ge, or Ru-Ge $σ$-phases shows that all three elements are necessary to stabilize the material. A $σ$-phase in the Ta-Ru-Ge system was synthesized but did not display superconductivity above 1.7 K, which suggests that electron count cannot govern the superconductivity observed. Preliminary characterization of a possible superconducting $σ$-phase in the Nb-Ru-Ga system is also reported.

cond-mat.supr-con↗

arXiv2017

Magnetism and charge density waves in RNiC$_2$ (R = Ce, Pr, Nd)

We have compared the magnetic, transport, galvanomagnetic and specific heat properties of CeNiC$_2$, PrNiC$_2$ and NdNiC$_2$ to study the interplay between charge density waves and magnetism in these compounds. The negative magnetoresistance in NdNiC$_2$ is discussed in terms of the partial destruction of charge density waves and an irreversible phase transition stabilized by the field induced ferromagnetic transformation is reported. For PrNiC$_2$ we demonstrate that the magnetic field initially weakens the CDW state, due to the Zeeman splitting of conduction bands. However, the Fermi surface nesting is enhanced at a temperature related to the magnetic anomaly.

cond-mat.str-el↗

arXiv2017

Growth, Crystal Structure and Magnetic Characterization of Zn-Stabilized CePtIn4

The growth and characterization of CePtIn4, stabilized by 10% Zn substitution for In, is reported. The new material is orthorhombic, space group Cmcm (No. 63), with lattice parameters a = 4.51751(4) Å, b = 16.7570(2) Å, and c = 7.36682(8) Å, and the refined crystal composition has 10% of Zn substituted for In, i.e. the crystals are CePt(In3.6Zn0.1)4. Crystals were grown using a self-flux method: only growths containing Zn yielded CePtIn4 crystals, while Ce3Pt4In13 crystals formed when Zn was not present. Anisotropic temperature-dependent magnetic susceptibilities for single crystals show that Zn-stabilized CePtIn4 orders magnetically at ~1.9 K. High-temperature Curie-Weiss fits indicate an effective moment of ~2.30 muB/ Ce and a directionally averaged Weiss-temperature of approximately - 9 K. Specific heat data shows a peak consistent with the ordering temperature seen in the magnetic susceptibility data. Zn-stabilized CePtIn4 is metallic and displays no superconducting transition down to 0.14 K.

cond-mat.mtrl-sci↗

arXiv2017

S-shaped suppression of the superconducting transition temperature in Cu-intercalated NbSe2

2H-NbSe2 is the prototype and most frequently studied of the well-known transition metal dichalcogenide (TMDC) superconductors. Widely acknowledged to be a conventional superconductor, its transition temperature to the superconducting state (Tc) is 7.3 K - a Tc that is substantially higher than those seen for the majority of TMDCs, where Tcs between 2 and 4 K are the norm. Here we report the intercalation of Cu into 2H-NbSe2 to make CuxNbSe2. As is typically found when chemically altering an optimal superconductor, Tc decreases with increasing x, but the way that Tc is suppressed in this case is unusual - an S-shaped character is observed, with an inflection point near x = 0.03 and, at higher x, a leveling off of the Tc near 3 K - down to the usual value for a layered TMDC. Electronic characterization reveals corresponding S-like behavior for many of the materials parameters that influence Tc. To illustrate its character, the superconducting phase diagram for CuxNbSe2 is contrasted to those of FexNbSe2 and NbSe2-xSx.

cond-mat.supr-con↗

arXiv2016

Field induced suppression of charge density wave in GdNiC$_2$

We report the specific heat, magnetic, magnetotransport and galvanomagnetic properties of polycrystalline GdNiC$_2$. In the intermediate temperature region above $T_N$ = 20 K, we observe large negative magnetoresistance due to Zeeman splitting of the electronic bands and partial destruction of a charge density wave ground state. Our magnetoresistance and Hall measurements show that at low temperatures a magnetic field induced transformation from antiferromagnetic order to a metamagnetic phase results in the partial suppression of the CDW.

cond-mat.str-el↗

arXiv2016

Effect of Electron Count and Chemical Complexity in the Ta-Nb-Hf-Zr-Ti High-Entropy Alloy Superconductor

High-entropy alloys are made from random mixtures of principal elements on simple lattices, stabilized by a high mixing entropy. The recently discovered BCC Ta-Nb-Hf-Zr-Ti high entropy alloy superconductor appears to display properties of both simple crystalline intermetallics and amorphous materials, e.g. it has a well defined superconducting transition along with an exceptional robustness against disorder. Here we show that the valence-electron count dependence of the superconducting transition temperature in the high entropy alloy falls between those of analogous simple solid solutions and amorphous materials, and test the effect of alloy complexity on the superconductivity. We propose high-entropy alloys as excellent intermediate systems for studying superconductivity as it evolves between crystalline and amorphous materials.

cond-mat.supr-con↗

arXiv2016

Crystal structure and low-energy Einstein mode in ErV$_2$Al$_{20}$ intermetallic cage compound

Single crystals of a new ternary aluminide ErV$_2$Al$_{20}$ were grown using a self-flux method. The crystal structure was determined by powder X-ray diffraction measurements and Rietveld refinement, and physical properties were studied by means of electrical resistivity, magnetic susceptibility and specific heat measurements. These measurements reveal that ErV$_2$Al$_{20}$ is a Curie-Weiss paramagnet down to 1.95 K with an effective magnetic moment $μ_{eff}$ = 9.27(1) $μ_B$ and Curie-Weiss temperature $Θ_{CW}$ = -0.55(4) K. The heat capacity measurements show a broad anomaly at low temperatures that is attributed to the presence of a low-energy Einstein mode with characteristic temperature $Θ_E$ = 44 K, approximately twice as high as in the isostructural 'Einstein solid' VAl$_{10.1}$.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in CaBi$_{2}$

Superconductivity is observed with critical temperature $T_{c}$ = 2.0 K in self-flux-grown single crystals of $CaBi_{2}$. This material adopts the $ZrSi_{2}$ structure type with lattice parameters a = 4.696(1) $Å$, b = 7.081(2) $Å$ and c = 4.611(1) $Å$. The crystals of $CaBi_{2}$ were studied by means of magnetic susceptibility, specific heat and electrical resistivity measurements. The heat capacity jump at $T_{c}$ is $ΔC/γT_{c}$ = 1.41, confirming bulk superconductivity; the Sommerfeld coefficient $γ$ = 4.1 $mJ\: mol^{-1}\, K^{-2}$ and the Debye temperature $Θ_{D}$ = 157 K. The electron-phonon coupling strength is $λ_{el-ph}$ = 0.59, and the thermodynamic critical field $H_{c}$ is low, between 111 and 124 Oe $CaBi_{2}$ is a moderate coupling type-I superconductor. Results of electronic structure calculations are reported and charge densities, electronic bands, densities of states and Fermi surfaces are discussed, focusing on the effects of spin\textendash orbit coupling and electronic property anisotropy. We find a mixed quasi-2D + 3D character in the electronic structure, which reflects the layered crystal structure of the material.

cond-mat.supr-con↗