*Rev Sci Instrum ; 91(1): 013102, 2020 Jan 01.*

**| MEDLINE**| ID: mdl-32012559

##### RESUMO

Characterizing and controlling electronic properties of quantum materials require direct measurements of nonequilibrium electronic band structures over large regions of momentum space. Here, we demonstrate an experimental apparatus for time- and angle-resolved photoemission spectroscopy using high-order harmonic probe pulses generated by a robust, moderately high power (20 W) Yb:KGW amplifier with a tunable repetition rate between 50 and 150 kHz. By driving high-order harmonic generation (HHG) with the second harmonic of the fundamental 1025 nm laser pulses, we show that single-harmonic probe pulses at 21.8 eV photon energy can be effectively isolated without the use of a monochromator. The on-target photon flux can reach 5 × 1010 photons/s at 50 kHz, and the time resolution is measured to be 320 fs. The relatively long pulse duration of the Yb-driven HHG source allows us to reach an excellent energy resolution of 21.5 meV, which is achieved by suppressing the space-charge broadening using a low photon flux of 1.5 × 108 photons/s at a higher repetition rate of 150 kHz. The capabilities of the setup are demonstrated through measurements in the topological semimetal ZrSiS and the topological insulator Sb2-xGdxTe3.

*Sci Rep ; 10(1): 2776, 2020 Feb 17.*

**| MEDLINE**| ID: mdl-32066748

##### RESUMO

The topological nodal-line semimetal (TNS) is a unique class of materials with a one dimensional line node accompanied by a nearly dispersionless two-dimensional surface state. However, a direct observation of the so called drumhead surface state within current nodal-line materials is still elusive. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with first-principles calculations, we report the observation of a topological nodal-loop (TNL) in SrAs3, whereas CaAs3 exhibits a topologically trivial state. Our data reveal that surface projections of the bulk nodal-points are connected by clear drumhead surface states in SrAs3. Furthermore, our magneto-transport and magnetization data clearly suggest the presence (absence) of surface states in SrAs3 (CaAs3). Notably, the observed topological states in SrAs3 are well separated from other bands in the vicinity of the Fermi level. RAs3 where R = Ca, Sr, thus, offers a unique opportunity to realize an archetype nodal-loop semimetal and establish a platform for obtaining a deeper understanding of the quantum phase transitions.

*Proc Natl Acad Sci U S A ; 116(41): 20333-20338, 2019 Oct 08.*

**| MEDLINE**| ID: mdl-31548424

##### RESUMO

Physics of the quantum critical point is one of the most perplexing topics in current condensed-matter physics. Its conclusive understanding is forestalled by the scarcity of experimental systems displaying novel aspects of quantum criticality. We present comprehensive experimental evidence of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic, and metamagnetic phases in the compound CePtIn4 Analyzing field variations of its magnetic susceptibility, magnetoresistance, and specific heat at very low temperatures, we trace modifications of the antiferromagnetic structure of the compound. Upon applying a magnetic field of increasing strength, the system undergoes metamagnetic transitions which persist down to the lowest temperature investigated, exhibiting first-order-like boundaries separating magnetic phases. This yields a unique phase diagram where the second-order phase transition line terminates at a tricritical point followed by 2 first-order lines reaching quantum critical end points as [Formula: see text] 0. Our findings demonstrate that CePtIn4 provides innovative perspective for studies of quantum criticality.

*Materials (Basel) ; 12(10)2019 May 27.*

**| MEDLINE**| ID: mdl-31137868

##### RESUMO

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F 4 ¯ 3m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2-950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 µV K-1 near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 µΩm at 350 K), resulting in a rather small magnitude of the power factor (less than 1 × 10-3 W m-1 K-2) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m-1 K-1 occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.

*Sci Rep ; 8(1): 16703, 2018 Nov 12.*

**| MEDLINE**| ID: mdl-30420684

##### RESUMO

We present the results of our comprehensive investigation on the antiferromagnetic heavy-fermion superconductor Ce3PtIn11 carried out by means of electrical transport, heat capacity and ac magnetic susceptibility measurements, performed on single-crystalline specimens down to 50 mK in external magnetic fields up to 9 T. Our experimental results elucidate a complex magnetic field - temperature phase diagram which contains both first- and second-order field-induced magnetic transitions and highlights the emergence of field stabilized phases. Remarkably, a prominent metamagnetic transition was found to occur at low temperatures and strong magnetic fields. In turn, the results obtained in the superconducting phase of Ce3PtIn11 corroborate an unconventional nature of Cooper pairs formed by heavy quasiparticles. The compound is an almost unique example of a heavy fermion system in which superconductivity may coexist microscopically with magnetically ordered state.

*Sci Rep ; 8(1): 13283, 2018 Sep 05.*

**| MEDLINE**| ID: mdl-30185891

##### RESUMO

Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. A majority of discovered topological semimetals are nonmagnetic and conserve time reversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodal-line semimetallic state in GdSbTe using angle-resolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the presence of multiple Fermi surface pockets including a diamond-shape, and small circular pockets around the zone center and high symmetry X points of the Brillouin zone (BZ), respectively. Furthermore, we observe the presence of a Dirac-like state at the X point of the BZ and the effect of magnetism along the nodal-line direction. Interestingly, our experimental data show a robust Dirac-like state both below and above the magnetic transition temperature (TN = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter.

*Dalton Trans ; 47(37): 12951-12963, 2018 Oct 07.*

**| MEDLINE**| ID: mdl-30151526

##### RESUMO

Phase relationship and structural behaviour in the substitutional series LaNi13-xGax and CeNi13-xGax have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi13-xGax resulting in five varieties of the NaZn13 structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi8.5Si4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi7In6 structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn13, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi13-xGax shows similar behaviour. The corresponding tI56-I âoI56 âoF112 âtI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi13-xGax samples show paramagnetic behavior, whereas the CeNi13-xGax series exhibits Curie-Weiss paramagnetism.

*Nat Commun ; 9(1): 3002, 2018 08 01.*

**| MEDLINE**| ID: mdl-30068909

##### RESUMO

Among the quantum materials that have recently gained interest are the topological insulators, wherein symmetry-protected surface states cross in reciprocal space, and the Dirac nodal-line semimetals, where bulk bands touch along a line in k-space. However, the existence of multiple fermion phases in a single material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic material Hf2Te2P and discover properties, which are unique in a single topological quantum material. We experimentally observe weak topological insulator surface states and our calculations suggest additional strong topological insulator surface states. Our first-principles calculations reveal a one-dimensional Dirac crossing-the surface Dirac-node arc-along a high-symmetry direction which is confirmed by our ARPES measurements. This novel state originates from the surface bands of a weak topological insulator and is therefore distinct from the well-known Fermi arcs in semimetals.

*Sci Rep ; 8(1): 11297, 2018 Jul 26.*

**| MEDLINE**| ID: mdl-30050089

##### RESUMO

Platinum ditelluride has recently been characterized, based on angle-resolved photoemission spectroscopy data and electronic band structure calculations, as a possible representative of type-II Dirac semimetals. Here, we report on the magnetotransport behavior (electrical resistivity, Hall effect) in this compound, investigated on high-quality single-crystalline specimens. The magnetoresistance (MR) of PtTe2 is large (over 3000% at T = 1.8 K in B = 9 T) and unsaturated in strong fields in the entire temperature range studied. The MR isotherms obey a Kohler's type scaling with the exponent m = 1.69, different from the case of ideal electron-hole compensation. In applied magnetic fields, the resistivity shows a low-temperature plateau, characteristic of topological semimetals. In strong fields, well-resolved Shubnikov - de Haas (SdH) oscillations with two principle frequencies were found, and their analysis yielded charge mobilities of the order of 103 cm2 V-1 s-1 and rather small effective masses of charge carriers, 0.11 me and 0.21 me. However, the extracted Berry phases point to trivial character of the electronic bands involved in the SdH oscillations. The Hall effect data corroborated a multi-band character of the electrical conductivity in PtTe2, with moderate charge compensation.

*Nano Lett ; 17(12): 7213-7217, 2017 12 13.*

**| MEDLINE**| ID: mdl-29110492

##### RESUMO

3D Dirac semimetals are an emerging class of materials that possess topological electronic states with a Dirac dispersion in their bulk. In nodal-line Dirac semimetals, the conductance and valence bands connect along a closed path in momentum space, leading to the prediction of pseudospin vortex rings and pseudospin skyrmions. Here, we use Fourier transform scanning tunneling spectroscopy (FT-STS) at 4.5 K to resolve quasiparticle interference (QPI) patterns at single defect centers on the surface of the line nodal semimetal zirconium silicon sulfide (ZrSiS). Our QPI measurements show pseudospin conservation at energies close to the line node. In addition, we determine the Fermi velocity to be âvF = 2.65 ± 0.10 eV Å in the Γ-M direction â¼300 meV above the Fermi energy EF and the line node to be â¼140 meV above EF. More importantly, we find that certain scatterers can introduce energy-dependent nonpreservation of pseudospin, giving rise to effective scattering between states with opposite pseudospin deep inside valence and conduction bands. Further investigations of quasiparticle interference at the atomic level will aid defect engineering at the synthesis level, needed for the development of lower-power electronics via dissipationless electronic transport in the future.

*Sci Rep ; 7(1): 12822, 2017 10 09.*

**| MEDLINE**| ID: mdl-28993691

##### RESUMO

Several rare-earth monopnictides were shown to exhibit extreme magnetoresistance and field-induced low-temperature plateau of electrical resistivity. These features are also hallmarks of topological semimetals, thus the family is intensively explored with respect to magneto-transport properties and possible hosting Dirac fermion states. We report a comprehensive investigation of Fermi surface and electrical transport properties of LuSb, another representative of this family. At low temperatures, the magnetoresistance of LuSb was found to exceed 3000% without saturation in fields up to 9 T. Analysis of the Hall effect and the Shubnikov-de Haas oscillations revealed that the Fermi surface of this compound consists of several pockets originating from fairly compensated multi-band electronic structure, in full accordance with our first-principles calculations. Observed magnetotransport properties of LuSb can be attributed to the topology of three-dimensional Fermi surface and a compensation of electron and hole contributions.

*Nat Commun ; 8: 15219, 2017 05 23.*

**| MEDLINE**| ID: mdl-28537261

##### RESUMO

Topological semimetals are systems in which conduction and valence bands cross each other and the crossings are protected by topological constraints. These materials provide intriguing tests for fundamental theories, while their unique physical properties promise a wide range of possible applications in low-power spintronics, optoelectronics, quantum computing and green energy harvesting. Here we report our study of the thermoelectric power of single-crystalline ZrSiS that is believed to be a topological nodal-line semimetal. We show that the thermoelectric power is an extremely sensitive probe of multiple quantum oscillations that are visible in ZrSiS at temperatures as high as 100 K. Two of these oscillations are shown to arise from three- and two-dimensional electronic bands, each with linear dispersion and the additional Berry phase predicted theoretically for materials with non-trivial topology. Our work not only provides further information on ZrSiS but also suggests a different route for studying other topological semimetals.

*Nat Commun ; 7: 13315, 2016 11 07.*

**| MEDLINE**| ID: mdl-27819655

##### RESUMO

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higher-binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements complemented by our first-principles calculations demonstrate the existence of the spin-polarized surface states at high-binding energy. The absence of such spin-polarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. Our direct experimental observation of spin-polarized surface states in BiPd provides critical information that will guide the future search for topological superconductivity in noncentrosymmetric materials.

*J Phys Condens Matter ; 28(43): 435602, 2016 11 02.*

**| MEDLINE**| ID: mdl-27603776

##### RESUMO

A polycrystalline sample of La2NiSi3 was investigated by means of heat capacity, magnetic susceptibility, magnetization, electrical resistivity and magnetoresistivity measurements. The compound was basically characterized as a Pauli paramagnet with metallic-like electrical conductivity, notably reduced in magnitude and weakly temperature dependent, as is usually observed for atomically disordered systems. Furthermore, the experimental data revealed the presence of a small amount of paramagnetic impurities. As a result, the low-temperature electrical resistivity in La2NiSi3 was found to be governed by both quantum corrections due to electron-electron interactions ([Formula: see text] contribution) and spin-flip Kondo scattering ([Formula: see text] contribution). The presence of paramagnetic impurities led to an increase in s-electron spin splitting due to the s-d interactions, manifested by a B (1/2) dependence of the magnetoresistivity, anomalously observed in the present study for thermal energy being larger than the Zeeman splitting energy [Formula: see text].

*Sci Rep ; 6: 18797, 2016 Jan 05.*

**| MEDLINE**| ID: mdl-26728755

##### RESUMO

We observed the coexistence of superconductivity and antiferromagnetic order in the single-crystalline ternary pnictide HoPdBi, a plausible topological semimetal. The compound orders antiferromagnetically at TN = 1.9 K and exhibits superconductivity below Tc = 0.7 K, which was confirmed by magnetic, electrical transport and specific heat measurements. The specific heat shows anomalies corresponding to antiferromagnetic ordering transition and crystalline field effect, but not to superconducting transition. Single-crystal neutron diffraction indicates that the antiferromagnetic structure is characterized by the propagation vector. Temperature variation of the electrical resistivity reveals two parallel conducting channels of semiconducting and metallic character. In weak magnetic fields, the magnetoresistance exhibits weak antilocalization effect, while in strong fields and temperatures below 50 K it is large and negative. At temperatures below 7 K Shubnikov-de Haas oscillations with two frequencies appear in the resistivity. These oscillations have non-trivial Berry phase, which is a distinguished feature of Dirac fermions.

*Inorg Chem ; 54(7): 3439-45, 2015 Apr 06.*

**| MEDLINE**| ID: mdl-25789825

##### RESUMO

The aluminides CePd2Al2, CePd3Al3, and CePd4Al4 were synthesized and their properties studied by X-ray diffraction, magnetic, heat capacity, and electrical transport measurements. The crystal structures of CePd2Al2 and CePd4Al4 were determined and refined from the single-crystal X-ray diffraction data, while that of CePd3Al3 was designed by the trial-and-error method on the basis of crystal chemistry considerations and refined by the Rietveld method from the X-ray powder diffraction data. All three compounds were found to crystallize in the tetragonal space group P4/nmm with Z = 2. The lattice parameters of CePd2Al2 are a = 4.3974(9) Å and c = 9.871(4) Å. Those of CePd3Al3 are a = 4.3045(7) Å and c = 13.4426(18) Å, while those of CePd4Al4 are a = 4.325(2) Å and c = 16.230(5) Å. The structures represent a new homologous series built of structural units of the CaBe2Ge2- and CsCl-type. The three compounds were established to order antiferromagnetically at 2.5(1) K, 3.5(1) K, and 2.6(1) K for CePd2Al2, CePd3Al3, and CePd4Al4, respectively. All of them are Kondo lattices with the characteristic energy scale of 3-7 K.

*Sci Rep ; 5: 9158, 2015 Mar 17.*

**| MEDLINE**| ID: mdl-25778789

##### RESUMO

We present electronic transport and magnetic properties of single crystals of semimetallic half-Heusler phase LuPdBi, having theoretically predicted band inversion requisite for nontrivial topological properties. The compound exhibits superconductivity below a critical temperature Tc = 1.8 K, with a zero-temperature upper critical field Bc2 ≈ 2.3 T. Although superconducting state is clearly reflected in the electrical resistivity and magnetic susceptibility data, no corresponding anomaly can be seen in the specific heat. Temperature dependence of the electrical resistivity suggests existence of two parallel conduction channels: metallic and semiconducting, with the latter making negligible contribution at low temperatures. The magnetoresistance is huge and clearly shows a weak antilocalization effect in small magnetic fields. Above about 1.5 T, the magnetoresistance becomes linear and does not saturate in fields up to 9 T. The linear magnetoresistance is observed up to room temperature. Below 10 K, it is accompanied by Shubnikov-de Haas oscillations. Their analysis reveals charge carriers with effective mass of 0.06 me and a Berry phase very close to π, expected for Dirac-fermion surface states, thus corroborating topological nature of the material.

*J Phys Condens Matter ; 25(12): 126001, 2013 Mar 27.*

**| MEDLINE**| ID: mdl-23449086

##### RESUMO

Metamagnetic transitions in CePd5Ge3 were investigated by means of low-temperature magnetization, magnetic susceptibility, electrical resistivity and magnetoresistivity measurements. In transverse magnetic fields applied in a direction close to the b-axis the antiferromagnetic structure of the compound undergoes two successive transitions, first to a spin-flop phase and then to a paramagnetic phase with field-induced ferromagnetic-like alignment of the Ce magnetic moments. In contrast, a single anomaly occurs in the magnetic field dependences of the resistivity in a transverse magnetic field applied close to the c-axis, which reflects a direct transition from antiferromagnetic to paramagnetic state. Such behavior is in agreement with theoretical descriptions of metamagnetic transitions in uniaxial antiferromagnets. The experimental magnetic and electrical transport data indicate that the b-axis is the easy axis of magnetization in CePd5Ge3.

*J Phys Condens Matter ; 25(14): 145601, 2013 Apr 10.*

**| MEDLINE**| ID: mdl-23478251

##### RESUMO

Single crystals of Ce2PdGa12 were studied by means of specific heat and electrical transport measurements, performed in wide ranges of temperature and magnetic field strength. The results corroborate the antiferromagnetic ordering reported in the literature, yet seem to exclude the heavy-fermion nature of the compound that has been previously suggested. The derived magnetic phase diagram involves a tricritical point that separates the phase boundaries of second- and first-order character. The transition from the antiferromagnetic state to a magnetic field-induced paramagnetic state realized in strong fields seems to be governed by a spin-flip mechanism without involving any intermediate magnetic phases.

*Proc Natl Acad Sci U S A ; 110(9): 3293-7, 2013 Feb 26.*

**| MEDLINE**| ID: mdl-23404698

##### RESUMO

When a second-order magnetic phase transition is tuned to zero temperature by a nonthermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these "quantum critical" superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature T(c) often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below T(c) is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points, showing that the superfluid density in these nodal superconductors universally exhibits, unlike the expected T-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this noninteger power law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta k close to the nodes in the superconducting energy gap Δ(k). We suggest that such "nodal criticality" may have an impact on low-energy properties of quantum critical superconductors.