123 research outputs found
Isotope effect on electron-phonon interaction in the multiband superconductor MgB2
No full textWe investigate the effect of isotope substitution on the electron-phonon interaction in the multiband superconductor MgB2 using tunable laser-based angle-resolved photoemission spectroscopy. The kink structure around 70 meV in the σ band, which is caused by electron coupling to the E2g phonon mode, is shifted to higher binding energy by ∼3.5 meV in Mg10B2 and the shift is not affected by superconducting transition. These results serve as the benchmark for investigations of isotope effects in known, unconventional superconductors and newly discovered superconductors where the origin of pairing is unknown.This article is published as Mou, Daixiang, Soham Manni, Valentin Taufour, Yun Wu, Lunan Huang, S. L. Bud'ko, P. C. Canfield, and Adam Kaminski. "Isotope effect on electron-phonon interaction in the multiband superconductor MgB2." Physical Review B 93, no. 14 (2016): 144504. DOI: 10.1103/PhysRevB.93.144504. Posted with permission.</p
Tuning the Kondo effect in Yb (Fe1−x Cox)2 Zn20
No full textWe study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe1−xCox)2Zn20 (0≤x≤1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe2Zn20 can be well described by a Kondo model with degeneracy N = 8 and a TK∼30 K. The ground state of YbCo2Zn20 is close to a Kondo state with degeneracy N = 2 and a much lower TK∼ 2 K, even though the total crystalline electric field (CEF) splittings are similar for YbFe2Zn20 and YbCo2Zn20. Upon Co substitution, the coherence temperature of YbFe2Zn20 is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4≲x≲ 0.9, the ground state remains roughly the same, which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence in resistivity data down to 500 mK within this substitution range. The coherence reappears at around x≳ 0.9 and the coherence temperature increases with higher Co concentration levels.This article is published as Kong, Tai, Valentin Taufour, Sergey L. Bud'ko, and Paul C. Canfield. "Tuning the Kondo effect in Yb (Fe 1− x Co x) 2 Zn 20." Physical Review B 95, no. 15 (2017): 155103. DOI: 10.1103/PhysRevB.95.155103. Posted with permission.</p
A study of the physical properties of single crystalline Fe5B2P
No full textSingle crystals of Fe5B2P were grown by self-flux growth technique. Structural and electrical and magnetic anisotropic properties are studied. The Curie temperature of Fe5B2P is determined to be 655 +/- 2 K. The saturation magnetization is determined to be 1.72 mu(B)/Fe at 2 K. The temperature variation of the anisotropy constant K-1 is determined for the first time, reaching similar to 0.50 MJ/m(3) at 2 K, and it is comparable to that of hard ferrites. The saturation magnetization is found to be larger than the hard ferrites. The first principle calculations of saturation magnetization and anisotropy constant are found to be consistent with the experimental results. (C) 2015 Elsevier B.V. All rights reserved.This is a manuscript of an article published as Lamichhane, Tej N., Valentin Taufour, Srinivasa Thimmaiah, David S. Parker, Sergey L. Bud'ko, and Paul C. Canfield. "A study of the physical properties of single crystalline Fe5B2P." Journal of Magnetism and Magnetic Materials 401 (2016): 525-531. DOI: 10.1016/j.jmmm.2015.10.088. Posted with permission.</p
Momentum dependence of the superconducting gap and in-gap states in MgB2 multiband superconductor
No full textWe use tunable laser-based angle-resolved photoemission spectroscopy to study the electronic structure of the multiband superconductor MgB2. These results form the baseline for detailed studies of superconductivity in multiband systems. We find that the magnitude of the superconducting gap on both σ bands follows a BCS-like variation with temperature with Δ0∼7meV. The value of the gap is isotropic within experimental uncertainty and in agreement with a pure s-wave pairing symmetry. We also observe in-gap states confined to kF of the σ band that occur at some locations of the sample surface. The energy of this excitation, ∼3 meV, is somewhat larger than the previously reported gap on π Fermi sheet and therefore we cannot exclude the possibility of interband scattering as its origin.This article is published as Mou, Daixiang, Rui Jiang, Valentin Taufour, S. L. Bud'ko, P. C. Canfield, and Adam Kaminski. "Momentum dependence of the superconducting gap and in-gap states in MgB 2 multiband superconductor." Physical Review B 91, no. 21 (2015): 214519. DOI: 10.1103/PhysRevB.91.214519. Posted with permission.</p
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Experimental Study of Magnetic and Transport Properties of Antiferromagnet Y2Co3 and Superconductor LaNiGa2
No full textIn this dissertation, I present experimental investigations on two materials of interest to the condensed matter community: the antiferromagnetic properties of Y2Co3 and the superconductivity and normal state properties of LaNiGa2. Both studies involve the development of single crystal growth methods and bulk magnetic and physical transport property measurements and analysis.The interest in Y2Co3 originates from its relatively high Néel temperature among the Co-based antiferromagnetic compounds, despite its high cobalt content. I developed, for the first time, the single-crystal growth method and reported its crystal structure belongs to the Cmce (No. 64) orthorhombic space group. The magnetic structure was also determined for the first time with single-crystal neutron diffraction to be an A-type antiferromagnet with almost collinear magnetic moments alignment, despite a seemingly non-collinear behavior observed in bulk magnetization measurements. Such discrepancy is attributed to the considerable temperature dependence of itinerant antiferromagnetic (AFM) exchange interactions, induced by thermal contraction along the b axis. Pressure study and high-field study revealed robust AMF ordering due to the compensating effect of lattice contraction on the ferromagnetic and AMF interactions. The high-field study revealed a spin-flop phase transition, offering further insight into the magnetocrystalline anisotropy.LaNiGa2 has attracted attention due to the evidence of time-reversal symmetry breaking in the superconducting state, alongside symmetry-enforced Dirac band crossings and anomalous pressure-dependent superconducting transition temperature. In this dissertation, I present a summary of the superconducting gap structure discussion based on recent works by our collaborators using our single-crystal samples, as well as the pressure studies. Furthermore, I discuss the normal state properties based on magnetization measurement and physical transport property measurement. The magnetization measurement indicates weak electron-electron interactions, making them unlikely to play a role in the unusual superconducting pairing. Transport measurements demonstrate multi-band behavior with both electron and hole carriers. The carrier densities remain nearly constant across the measured temperature range, with no evidence of sudden changes that might suggest electronic phase transitions. In particular, the magnetoresistance exhibits quadratic to linear field dependence, which deviates from conventional semiclassical transport behavior. Possible explanations including quantum transport effects resulting from small Fermi surface pockets and impeded circular motions are discussed. 
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Structural Characterization and Physical Properties of Superconducting LaNiGa2 and Antiferromagnetic CeIn3
No full textIn this dissertation, I will describe two experimental studies on two different condensed matter systems. The first study originates from our crystallographic findings on LaNiGa2. After developing a single crystal synthesis method, single-crystal X-ray diffraction results improve upon previous studies by showing that LaNiGa2 crystallizes in a Cmcm unit cell instead of a Cmmm one. As a result of uncovering nonsymmorphic symmetry operations, we show that these directly result in two topological features precisely at the Fermi level. With the previous knowledge that LaNiGa2 breaks time-reversal symmetry within its superconducting state and shows evidence for multi-gap superconducting behavior, we can then directly connect the new crystallographic results, the topological features, and the unique superconducting state. In the second study, we synthesize and then characterize single crystals of Nd substituted CeIn3. This well-studied heavy-fermion system has previously exhibited fascinating phenomena as its antiferromagnetic ordering is systematically suppressed. In Ce1-xNdxIn3 we reveal an interesting interplay between the Kondo lattice coherence and crystal electric field depopulation effects. Wherein we can separate the two features in electrical resistivity measurements in the most disordered substitution range of x=0.4-0.5. We also reveal a comprehensive phase diagram between the two antiferromagnetic ordering.In addition to these two studies, I also provide an overview of relevant concepts and experimental techniques. The purpose of this overview is to provide incoming graduate students a starting point to begin to understand what principles are relevant to the condensed matter systems discussed in this dissertation and how to perform specific experimental techniques to study said principles. Within the experimental section, I also provide some ``experimentalist's insights'' on the techniques I have used extensively throughout my graduate work
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Molten Hydroxide Flux Synthesis of KNi4S2 and its Magnetic and Physical Properties
No full textThis thesis describes how I used the molten-hydroxide flux method of single crystalsynthesis to synthesize some of the largest reported single crystals of KNi4S2 (142),
KNi2S2 (122), and K2Ni3S4 (234). This molten-hydroxide flux method had previously
never been implemented in our lab and, with my optimizations, could open up a previously inaccessible phase-space of meta-stable compounds for our group to explore. I also
report the first-known magnetization, resistivity, and heat-capacity data for KNi4S2. Although the KNi4S2 phase is isostructural to cuprate superconductors, we do not observe
signs of superconductivity nor antiferromagnetic order down to T = 2 K. Instead, our
magnetization measurements suggest that KNi4S2 may be a weak itinerant ferromagnet (TC ≈ 25 K). Growing large single crystals allowed us to study magnetic anisotropy,
and I found that with KNi4S2, the a and c appear to be isotropic while b seems to
be the easy magnetic direction. Resistivity measurements from T = 1.8 − 300 K show
the 142 phase has a resistivity similar to a bad metal. While I measure a relatively
high residual resistivity ratio (RRR) of 44 demonstrating good sample quality, no phase
transition is observed in either resistivity or heat capacity measurements. This may be
consistent with the weak itinerant nature of the magnetism, although we cannot exclude the possibility of an impurity origin of the ferromagnetic transition. In addition
to the 142 phase, I discuss the synthesis and present x-ray diffraction (XRD) patterns
and magnetization data for the 122 and 234 phases
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Experimental Studies of the Unusual and Fragile Ferromagnetism of LaCrGe3 and LaCrSb3
No full textThis dissertation details my exploration of two compounds: LaCrGe3 and LaCrSb3. While their names differ by only two letters, and therefore their compositions by only one element, they are quite different. The relevant question to ask about these two ferromagnetic systems is: “what happens when their ferromagnetism is suppressed?" Although the magnetic phase diagram of LaCrGe3 under pressure has previously been charted, there are aspects of its rich temperature-pressure-magnetic field phase space that are contested. In particular, through a meticulous study of its magnetic domain behavior, I provide evidence in favor of the existence of multiple ferromagnetic states in LaCrGe3. We find that investigating domain behavior can lead to a more accurate way of characterizing new materials and perhaps a method of probing crossovers between ferromagnetic states. On the other hand, the ferromagnetism of LaCrSb3 is relatively robust to pressure. Therefore, I use Fe substitution to suppress its Curie temperature and discover the first reported magnetic phase diagram of its kind—one with an avoided quantum tricritical point. The LaCr1-xFexSb3 system has a temperature-chemical substitution-magnetic field phase diagram that is ripe with magnetic features for closer examination. Further study of both materials will likely lead to additional discoveries in the world of magnetism
Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe
No full textThe pressure dependence of the upper critical field, Hc2,c, of single crystalline FeSe was studied using measurements of the interplane resistivity, ρc, in magnetic fields parallel to tetragonal c axis. Hc2,c(T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, Tc, of FeSe exhibits a nonmonotonic dependence with local maximum at p1≈0.8 GPa and local minimum at p2≈1.2 GPa. The slope of the upper critical field at Tc,(dHc2,c/dT)Tc, also exhibits a nonmonotonic pressure dependence with distinct changes at p1 and p2. For pp2 the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov–de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of Tc(p) at p2≈1.2 GPa.This article is published as Kaluarachchi, Udhara S., Valentin Taufour, Anna E. Böhmer, Makariy A. Tanatar, Sergey L. Bud'ko, Vladimir G. Kogan, Ruslan Prozorov, and Paul C. Canfield. "Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe." Physical Review B 93, no. 6 (2016): 064503. DOI: 10.1103/PhysRevB.93.064503. Posted with permission.</p
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Magnetic and Electric Properties of Topological materials Co3Sn2S2 and CeCoGe3
No full textTopology is a study of continuous deformation of objects. Of particular interest to the physicists is the topological invariant, a quantity which is preserved under a continuous deformation. For example, a coffee cup (with a handle) can be deformed continuously into a donut, and the number of holes (which is called genus in mathematical language) remains untouched during the process. It would require a discontinuous deformation to create or remove a hole, i.e., to deform a solid sphere to a donut. In physics, certain physical observables can be represented mathematically by topological invariants. These topological invariants are preserved under continuous changes in the Hamiltonian, unless the system undergoes a topological phase transition that causes a discontinuous change of the corresponding topological invariant, which is the so-called ``topological protection''.Topological materials represent a class of materials with unique electronic properties due to the unusual surface and edge states. Those states are protected by the topological invariants, making them robust against impurities and defects in the materials. The study of topological materials traces back to 1980s where people found quantum Hall effect, where the Hall conductance is quantized in units of e^2/h, regardless of the details of the Hamiltonian, such as strength of magnetic field and random disorder potential. Now it is well known that the quantized Hall conductance is related to a topological invariant called Chern number, which is the intrinsic nature of the electronic band structures. Later in 2000s, the discovery of topological insulator Bi2Se3, which exhibits dissipationless conduction of electricity on the surfuce while being insulating in the bulk, verifies the early theory of topological materials. Nowadays, the field of topological materials expands to topological superconductors, Weyl semimetals and Dirac semimetals. The topological nature in these materials gives rise to the exotic phenomena, for example, Weyl semimetals are characterized with Fermi arcs on the surface and exhibit unique electric transport properties such as Chiral anomaly and large anomalous Hall effect. Such unusual electronic properties offer promising applications in quantum computing and energy-efficient electronics.This dissertation describes my research on two topological materials---Weyl semimetal Co3Sn2S2 and Kondo-Weyl compound CeCoGe3. Previous work on Co3Sn2S2 has revealed its interesting electric properties such as Chiral anomaly, anomalous Hall effect and a breaking-down of Ohm's Law. Our work shows it also exhibits unusual domain wall motion due to its magnetic structure. The second project is about CeCoGe3, which is a potential Weyl metal with the interplay of Kondo effect. Our detailed study on the electric properties of this compound reveals a giant intrinsic anomalous Hall conductance, which is the largest among all compounds according to our best knowledge. This manuscript is currently under review for publication.This dissertation starts with an overview of fundamental physical principles related these compounds in chapter I, followed by a description of experimental techniques in chapter II. The detailed studies of Co3Sn2S2 and CeCoGe3 will be discussed in chapter III and chapter IV respectively
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