R-3 Repository :: Browsing by Author "Morosan, Emilia"

Browsing by Author "Morosan, Emilia"

Now showing 1 - 20 of 34

A Mott insulator continuously connected to iron pnictide superconductors
(Springer Nature, 2016) Song, Yu; Yamani, Zahra; Cao, Chongde; Li, Yu; Zhang, Chenglin; Chen, Justin S.; Huang, Qingzhen; Wu, Hui; Tao, Jing; Zhu, Yimei; Tian, Wei; Chi, Songxue; Cao, Huibo; Huang, Yao-Bo; Dantz, Marcus; Schmitt, Thorsten; Yu, Rong; Nevidomskyy, Andriy H.; Morosan, Emilia; Si, Qimiao; Dai, Pengcheng; Rice Center for Quantum Materials
Iron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.
An All Solid-State Laser System for Cooling and Trapping Lithium
(2013-09-16) Revelle, Melissa; Hulet, Randall G.; Killian, Thomas C.; Morosan, Emilia
Ultra-cold atoms have become an essential tool in studying unique phenomena in condensed matter systems such as superconductivity and quantum phase transitions. To accomplish these experiments we use an apparatus designed to trap and cool lithium atoms down to nano-Kelvin temperatures. Recently, significant upgrades to the laser system have been made to improve performance, increase stability, minimize maintenance and improve flexibility. We are working towards two exciting projects: proving the existence of an exotic superfluid state (FFLO) and probing the crossover between one and three-dimensions in a spin-1/2 Fermi gas with a spin-imbalance.
Chemical tuning of electrical and magnetic properties in the transition metal dichalcogenides
(2019-04-19) Choe, Jesse; Morosan, Emilia; Kelly, Kevin; Natelson, Douglas
Transition metal dichalcogenides are a diverse class of layered materials. Due to their quasi two-dimensional nature, they are a sandbox for investigating low dimensional physics, but can be doped in a variety of ways. Not only can substitutional doping occur on either the transition metal or chalcogen site, but intercalation between the layers can tune the system as well. Here I report on the results of three transition metal dichalcogenide systems with three drastically different results. In the Copper-Platinum-Selenium system, initial results suggest two new superconductors in the ternary phase diagram. Doping platinum into TiSe$_2$ results in an increase in the resistivity of several order of magnitude. Angle-resolved photo-emission spectroscopy shows that Platinum doping induces a pseudo gap in the system. Scanning tunneling microscopy measurements show domain wall formation. Power law fits to the resistivity suggests that the electrical transport is dominated by Luttinger liquid behavior. Finally, the intercalation of Iron into TiS$_2$ gives rise to several magnetic features. Large bowtie magnetoresistance arises showing an increase of up to 40\%. Hysteresis in magnetization shows sharp switching behavior coinciding with the bowtie in magnetoresistance. Ferromagnetic order is seen in conjunction with glassy behavior. These results are compared and contrasted to the results in Fe$_x$TiS$_2$. My results in these systems show that the depth and breadth of physical phenomena in the transition metal dichalcogenide system make it a fascinating system for investigating strongly correlated systems.
Correlating structural, electronic, and magnetic properties of epitaxial VSe2 thin films
(American Physical Society, 2020) Chen, Guannan; Howard, Sean T.; Maghirang, Aniceto B.; Nguyen Cong, Kien; Villaos, Rovi Angelo B.; Feng, Liang-Ying; Cai, Kehan; Ganguli, Somesh C.; Swiech, Waclaw; Morosan, Emilia; Oleynik, Ivan I.; Chuang, Feng-Chuan; Lin, Hsin; Madhavan, Vidya; Rice Center for Quantum Materials
The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer VSe2 films grown using molecular beam epitaxy. VSe2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200 °C and 450 °C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a×4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe2 film is weakly ferromagnetic, with ∼3.5 μB per unit similar to previous reports.
Correlations between magneto-transport properties and crystal structure in transition metal pnictides and chalcogenides
(2016-07-29) Chen, Chih-Wei; Morosan, Emilia
The interactions between electrons, phonons, and the lattice can result in one or more lattice, charge, spin, and orbital orders. These orders, and the competition between different states they generate, result in many interesting phenomena, such as magnetism, metal-to-insulator transition, giant magnetoresistance, superconductivity, heavy fermion behavior, etc. Here I report the research on three systems: Co$_2$As$_{1-x}$P$_x$, Fe$_x$TaS$_2$, and Sr$_2$Mn$_3$As$_2$O$_2$, and the corresponding phenomena. In Co$_2$As$_{1-x}$P$_x$, the magnetic properties are strongly correlated with their crystal structure. The P doping induces two structural phase transitions. The itinerant ferromagnetism in Co$_2$As is enhanced by the first structural phase transition ($x \sim$ 0.04) and quenched by the second structural phase transition ($x$ between 0.85 and 0.90). In Fe$_x$TaS$_2$, I studied the correlation between magneto-transport properties and the Fe concentration $x$. When x deviates from the two commensurate values 1/4 and 1/3 where Fe ions form superstructures, both the spin misalignment and the magnetoresistance increase. The largest magnetoresistance that has been observed so far is 140$\%$ in Fe$_{0.297}$TaS$_2$, where the Fe concentration is close to the average of two commensurate values. In Sr$_2$Mn$_3$As$_2$O$_2$, I grew the first Sr$_2$Mn$_3$As$_2$O$_2$ single crystals and performed single crystal neutron scattering, which reveals that the magnetic structure at one of its layers has a quasi two-dimensional antiferromagnetic order. The energy dispersion of this magnetic order has linear dependence with wave momentum at the low energy transfer region, which is consistent with the spin wave of antiferromagnetic order. Additionally, band structure calculations indicate that Sr$_2$Mn$_3$As$_2$O$_2$ is an Mott insulator and the Mott transition is both layer- and orbital-selective, where the $d_{x^2 - y^2}$ orbital in this layer dominates the Mott transition.
Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system
(Springer Nature, 2023) Gao, Bin; Chen, Tong; Wu, Xiao-Chuan; Flynn, Michael; Duan, Chunruo; Chen, Lebing; Huang, Chien-Lung; Liebman, Jesse; Li, Shuyi; Ye, Feng; Stone, Matthew B.; Podlesnyak, Andrey; Abernathy, Douglas L.; Adroja, Devashibhai T.; Duc Le, Manh; Huang, Qingzhen; Nevidomskyy, Andriy H.; Morosan, Emilia; Balents, Leon; Dai, Pengcheng
Magnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni2Mo3O8, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.
Doping induced quantum phase transition in the itinerant ferromagnet scandium indium
(2012) Svanidze, Eteri; Morosan, Emilia
Examination of quantum critical points of itinerant electron systems will aid with understanding of d-electron magnetism that exhibits both local and itinerant characteristics in different families of compounds. Doping-induced quantum phase transition of the itinerant ferromagnet Sc 3.1 In that is composed of non-magnetic elements is the focus of our work. Polycrystalline samples of (Sc 1-x Lu x ) 3.1 In with 0≤ x≤ 0.08 were prepared by arcmelting and then annealing for an extended period of time. Susceptibility measurements were performed in an applied magnetic field H = 0.1 T for temperatures T = 1.85 K to 300 K. Linearity of Arrott plots in low-field region was significantly improved by implementing the non-mean-field Arrott-Noakes technique where plotting M 1/β vs. ( H/M ) 1/γ is used to determine both the Curie temperature and composition. Modified Arrott plot approach was used in order to determine the new critical exponents β, γ and δ that better describe this compound. The Curie temperature of the Sc 3.1 In compound was found to be T C = 4.4 K and the critical composition x c = 0.02. This work was supported by NSF DMR 0847681.
Effects of synthesis conditions on the transition metal dichalcogenide TiSe2
(2020-04-24) Moya, Jaime M; Morosan, Emilia
TiSe2 is part of a family of materials known as the transition metal dichalcogenides. Their quasi-two dimensional crystal structure sometimes gives rise to interesting phenomena, spanning a vast array physical and electronic properties including charge order or superconductivity when various intercalants or dopants are added. TiSe2 was shown to have charge ordering at a temperature of 200 K almost 45 years ago. Despite the time that has elapsed between this discovery and now, TiSe2 continues to be an intensely studied material because the nature of its charge ordering is still under debate. Some of the contradicting results are fueled by sample dependency related to growth method and conditions. Because of the small band gap or band overlap in TiSe2, it is not surprising that dilute impurities and growth conditions can drastically aect the transport properties of TiSe2. In this work, I systematically study the effect of variable growth conditions including post synthesis cooling rate, anneal time, and temperature, on the electrical resistivity of TiSe2. I find that slow cooling polycrystalline TiSe2 post synthesis drastically increases the low temperature resistivity, which is in stark contrast to the metallic low temperature resistivity observed in single crystalline TiSe2 grown by iodine vapor transport, where the iodine charge dopes the sample. Together, the logarithmic divergence of the resistivity and signatures in low temperature magnetoresistance point to signatures of the weak-localization effect. Annealing samples at low temperatures post synthesis also increase the low temperature resistivity, but with a less profound aect. Finally, quenching samples from high temperature, freezes in disorder, and decreases the low temperature resistivity.
Electronic nematic correlations in the stress-free tetragonal state of BaFe2−xNixAs2
(American Physical Society, 2015) Man, Haoran; Lu, Xingye; Chen, Justin S.; Zhang, Rui; Zhang, Wenliang; Luo, Huiqian; Kulda, J.; Ivanov, A.; Keller, T.; Morosan, Emilia; Si, Qimiao; Dai, Pengcheng
We use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe2−xNixAs2 iron pnictides in uniaxial-strained and external-stress-free detwinned states. Using a specially designed in situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial-strained tetragonal state of BaFe2−xNixAs2 below a temperature T∗, previously identified as a signature of the electronic nematic phase, is also present in the stress-free tetragonal phase below T**(
Impurity-Induced Plasmon Damping in Individual Cobalt-Doped Hollow Au Nanoshells
(American Chemical Society, 2014) Thibodeaux, Christyn A.; Kulkarni, Vikram; Chang, Wei-Shun; Neumann, Oara; Cao, Yang; Brinson, Bruce; Ayala-Orozco, Ciceron; Chen, Chih-Wei; Morosan, Emilia; Link, Stephan; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics; Rice Quantum Institute
The optical properties of plasmonic nanoparticles in the size range corresponding to the electrostatic, or dipole, limit have the potential to reveal effects otherwise masked by phase retardation. Here we examine the optical properties of individual, sub-50 nm hollow Au nanoshells (Co-HGNS), where Co is the initial sacrificial core nanoparticle, using single particle total internal reflection scattering (TIRS) spectroscopy. The residual Co present in the metallic shell induces a substantial broadening of the homogeneous plasmon resonance line width of the Co-HGNS, where the full width at half-maximum (fwhm) broadens proportionately with increasing Co content. This doping-induced line broadening provides a strategy for controlling plasmon line width independent of nanoparticle size, and has the potential to substantially modify the relative decay channels for localized nanoparticle surface plasmons.
Infrared Spectroscopy of Graphene in Ultrahigh Magnetic Fields
(2012-09-05) Booshehri, Layla; Kono, Junichiro; Morosan, Emilia; Mittleman, Daniel M.; Mielke, Charles H.
Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – possesses unusual zero-gap band structure with linear band dispersions, accommodating photon-like, massless electrons that have exhibited a variety of surprising phenomena, primarily in DC transport, in the last several years. In this thesis dissertation, we investigate graphene’s AC or infrared properties in the presence of an ultrahigh magnetic field, produced by a destructive pulsed method. The linear dispersions of graphene lead to unequally spaced Landau levels in a magnetic field, which we probe through cyclotron resonance (CR) spectroscopy in the magnetic quantum limit. Specifically, using magnetic fields up to 170 T and polarized midinfrared radiation with tunable wavelengths from 9.22 to 10.67 μm, we experimentally investigated CR in large-area graphene grown by chemical vapor deposition. Circular-polarization-dependent studies revealed strong p-type doping for as-grown graphene, and the dependence of the CR fields on the radiation wavelength allowed for an accurate determination of the Fermi energy. Upon annealing the sample to remove physisorbed molecules, which shifts the Fermi energy closer to the Dirac point, we made the unusual observation that hole and electron CR emerges in the magnetic quantum limit, even though the sample is still p-type. We theoretically show that this non-intuitive phenomenon is a direct consequence of the unusual Landau level structure of graphene. Namely, if the Fermi energy lies in the n = 0 Landau level, then CR is present for both electron-active and hole-active circular polarizations. Furthermore, if the Fermi level lies in the n = 0 Landau level, the ratio of CR absorption between the electron-active and hole-active peaks allows one to accurately determine the Fermi level and carrier density. Hence, high-field CR studies allow not only for fundamental studies but also for characterization of large-area, low-mobility graphene samples.
Intermediate valence to Kondo behavior in Yb3T4Ge13 and YbT3M7 compounds
(2017-12-01) Rai, Binod K; Morosan, Emilia
The f-electrons of Ce- and Yb-based compounds hybridize with conduction electrons, resulting in many interesting electronic properties such as heavy fermions, topological Kondo insulators, and superconductivity in the strongly correlated systems. This dissertation covers the discovery of new compounds in two families of compounds, R3T4Ge13 (3-4-13) and RT3M7 (1-3-7), which were poorly explored until recently. The objective of the work is to (i) synthesize high quality single crystals with the interest of understanding diverse electronic properties such as superconductivity, magnetism, low-carrier density in Kondo systems, and heavy fermions and (ii) characterize and explore quantum phenomena of these compounds using magnetization, specific heat, electrical resistivity, Hall effect, neutron, and optical measurements. These 3-4-13 germanides compounds show very diverse electronic properties such as superconductivity, magnetism, intermediate valence, and Kondo behavior. All seven Lu3T4Ge13-x (T = Co, Rh, Ir, Os) and Y3T4Ge13-x (T = Ir, Rh, Os) compounds are bulk superconductors with semimetallic normal state Other non-superconducting compounds change their electronic properties upon chemical substitution in the magnetic or transition metal site. Intermediate valence behavior upon Co doping into the Rh site in Yb3(Rh1-xCox)4Ge13 is persistent, while Ir doping into the Rh site in Yb3(Rh1-xIrx)4Ge13 drives the system into a heavy fermion state. Magnetic order T*mag = 0.9 K is observed for Yb3Ir4Ge13 and the quantum critical point is tuned around xc = 0.5 in Yb3(Rh1-xIrx)4Ge13. Furthermore, the low-carrier density nature is revealed in both magnetic Yb3Ir4Ge13 and non-magnetic reference Lu3Ir4Ge13 from electrical transport and optical measurements. In the 1-3-7 class of compounds, we discovered the first time YbRh3Si7, YbIr3Ge7, and CeIr3Ge7 compounds, including the first two Yb heavy fermion compounds in the series. YbRh3Si7 is the first Kondo system in this ScRh3Si7 structure type, which shows antiferromagnetic ordering below TN = 7.5 K with spins in the ab plane, and the metamagnetic phase transitions along the hard c axis. YbIr3Ge7 shows ferromagnetic ordering along the crystal electric field (CEF) hard direction with TC = 2.4 K. By contrast, CeIr3Ge7 does not show Kondo behavior, but it has strong CEF anisotropy with a surprisingly low TN = 0.63 K.
Investigating the use of directed magnetic assembly to create tunable colloidal fractal aggregates and DNA-linked bead-spring chains
(2014-09-12) Byrom, Julie Elizabeth; Biswal, Sibani L; Pasquali, Matteo; Morosan, Emilia
The purpose of this work is to develop techniques for building complex colloidal assemblies using an applied magnetic field. The greatest challenge in this field is creating sophisticated, dynamic structures from relatively simple building blocks and interactions. Particles with magnetic properties assemble via dipolar interactions into chains along the direction of the field. Additionally, it is possible to assemble nonmagnetic particles in a magnetic field if they are immersed in a magnetic fluid. This effect is known as negative magnetophoresis (the nonmagnetic particles develop a dipole in the direction antiparallel to the external field)—and the particles are effectively diamagnetic. However, the anisotropic dipolar interaction can be a limiting factor in producing complex structures, as many magnetic assemblies are one-dimensional. In this proposal, we combine paramagnetic and diamagnetic particles to create assemblies in two-dimensions (both parallel and perpendicular to the field) and demonstrate the ability to change the morphology of these assemblies simply by changing the magnetic susceptibility of the ferrofluid as well as the overall concentration and ratio of colloids in solution. These ramified aggregates may be used in the future to build gel-like networks, which can be used as novel magnetorheological fluids and will offer valuable insight into the process of gelation via dipolar interactions. Although they may lack the complexity of the fractal aggregates just described, chains of colloids are still an important analogue for studying polymer and biofilament behavior. One characteristic which can greatly affect the properties of these filaments is their flexibility. Previous attempts to create linked particle chains have been limited to persistence lengths in the rigid and semiflexible regimes, but here we describe the method we have devised to create chains that fall in the rigid, semiflexible, and flexible regimes. This involves linking the beads of the chain with long strands of DNA, of sizes varying up to the same order of the beads themselves. This creates a physical analogy of the “bead-spring” system proposed by Rouse and Zimm. We show that we can control the flexibility of the chains by either altering the length of the DNA to tune its spring constant, or alternatively we can also tune the magnetic field strength used to assemble the chains. The field strength controls the strength of the dipolar interactions between particles and regulates the interparticle spacing between beads. This can lead to greater or fewer numbers of DNA strands which are able to form bridges, and this changes the effective spring constant holding the beads together. We demonstrate that the flexibility changes predictably within a certain range of DNA sizes, and that when the DNA becomes of similar size to the particles the DNA becomes excluded from the chain and the chains subsequently become more rigid. Unlike chains linked with smaller molecules, these chains are very resilient to shear and torque, which will allow them to be used to study filament buckling and the development of bending instabilities in complex magnetic fields or flow patterns. Finally, we explore the use of shorter DNA linkers to create batches of chains which can exhibit different flexibilities at different temperatures. This is done by exploiting the melting behavior of DNA, because micron sized particles do not exhibit the sharp melting transitions of DNA-linked nanoparticles due to the lower density of DNA on the surface and their inability to melt cooperatively. Thus, as portions of the DNA linkers are melted we see a gradual increase in flexibility of the chains with a minimal amount of breakage along the chain. This process should be reversible and would allow us to create more versatile solutions of chains. Additionally, since the DNA on larger particles has a broader melting curve, we can use them to study the effects of the initial unbinding events, which would not be possible in a system where cooperative melting cascades create sharp melting transitions. Overall, this thesis provides novel insights into the use of directed magnetic assembly to create complex colloidal structures.
Kramers nodal lines and Weyl fermions in SmAlSi
(Springer Nature, 2023) Zhang, Yichen; Gao, Yuxiang; Gao, Xue-Jian; Lei, Shiming; Ni, Zhuoliang; Oh, Ji Seop; Huang, Jianwei; Yue, Ziqin; Zonno, Marta; Gorovikov, Sergey; Hashimoto, Makoto; Lu, Donghui; Denlinger, Jonathan D.; Birgeneau, Robert J.; Kono, Junichiro; Wu, Liang; Law, Kam Tuen; Morosan, Emilia; Yi, Ming
Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi
Layered transition metal pnictides investigated by experimental and computational methods
(2015-04-23) Wang, Jiakui; Morosan, Emilia; Dai, Pengcheng; Kelly, Kevin
Intensive research interest on transition metal pnictide compounds was stimulated by the discovery of unconventional superconductivity in Fe-pnictide compounds in 2008. A key observation in Fe-pnictide compunds is the intimate relationship between the structure, magnetism and superconductivity in those compunds. It is thus important to investigate a couple of transition metal pnictide compounds with similar structures to superconducting Fe-pnictide compounds, to elucidate how the magnetic and superconducting properties evolve with the type of crystallographic structure, lattice paramters and elements on a speci c site. In this thesis, R3T4As4O2 (R = La - Sm, T = Ni - Cu), which has a convolutional structure of 122 and 1111 Fe-pnictide superconductors, was investigated from the perspective of a couple of physical properties investigation as well as theoritical computation. The combination of experimental and theoretical methods reveals complex magnetic properties as the elements on rare earth and transition metal sites varies. Such observations would bene t a full understanding of the relationship between structure, magnetism and superconductivity in transition metal pnictide system as well the search for conventional superconductors with this speci c structure.
Magnetic and Electronic Correlations in Rare Earth Intermetallic Compounds
(2019-12-06) Stavinoha, Macy L; Morosan, Emilia
Magnetic and electronic correlations form the framework of physical properties observed in rare earth-based intermetallic compounds. Rare earth ions (La-Lu) can host a variety of behaviors including the Kondo effect, crystal electric field splitting, long-range magnetic order, and valence fluctuations whose combined effects determine the ground state magnetic and electronic properties of the host intermetallic compound. In solid state chemistry and condensed matter physics disciplines, physical properties can be traced through structural families where these behaviors are common to the crystallographic arrangement of the compound. When changes in the physical properties are correlated with small changes in the chemical composition or crystallographic structure of the compound, this provides a route to study and tune the underlying quantum mechanical interactions responsible for unusual or useful characteristics of these materials. In this thesis, I will report the discovery of two new intermetallic compounds YbIr3Si7 and YbIr3Ge7 and two new substitutional series Eu(Ga1-xAlx)4 and Eu1-xAxGa4. The isostructural analogs YbIr3Si7 and YbIr3Ge7 differ greatly in their physical properties, and each compound is unique in its own respect. YbIr3Si7 shows a combination of behaviors that has not been reported in any other compound, including Kondo correlations, insulator-like resistivity, long-range magnetic order, and the onset of conductive surface states at low temperature. YbIr3Ge7, however, is a rare Kondo lattice ferromagnet and the first of which to crystallize in a rhombohedral crystal structure. In contrast, EuGa4 is an antiferromagnetic compound with a ubiquitous tetragonal crystal structure. Small changes in the magnetic Eu sublattice or the nonmagnetic Ga sublattice can introduce unexpected changes in the observed physical properties of the system. Here, Al substitution in the Ga sublattice to produce the single crystal series Eu(Ga1-xAlx)4 results in nonlinear changes to the magnetic ordering temperature and the presence of ferromagnetic correlations, despite the similar size and electronic con figuration of Al and Ga. In contrast, substitution in the magnetic Eu sublattice to form Eu1-xAxGa4 with A = Ca, La, or Sr shows that introducing the smaller Ca ion causes structural distortion, hole doping with La suppresses the ordering temperature rapidly, and Sr substitution slowly suppresses the magnetic order without sacrificing crystal quality.
Magnetic field effects in an octupolar quantum spin liquid candidate
(American Physical Society, 2022) Gao, Bin; Chen, Tong; Yan, Han; Duan, Chunruo; Huang, Chien-Lung; Yao, Xu Ping; Ye, Feng; Balz, Christian; Stewart, J. Ross; Nakajima, Kenji; Ohira-Kawamura, Seiko; Xu, Guangyong; Xu, Xianghan; Cheong, Sang-Wook; Morosan, Emilia; Nevidomskyy, Andriy H.; Chen, Gang; Dai, Pengcheng
Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to Ce2Zr2O7, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in Ce2Sn2O7 pyrochlore, and its absence in Ce2Zr2O7. Therefore, our results identify Ce2Zr2O7 as a strong candidate for an octupolar U(1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.
Magnetization and magnetoresistance in iron intercalated transition metal dichalcogenides
(2016-04-26) Choe, Jesse; Morosan, Emilia; Kelly, Kevin
The understanding of magnetism in strongly correlated electronic systems is a vital area of research. Not only is it linked to other phenomena like high temperature superconductivity in the cuprates and iron pnictides, but magnetic materials have been used in electronics since before the computer. As it becomes harder to prop up Moore's law by increasing the density of transistors, mankind must look towards new methods to improve technology or risk stagnation. Research into alternative materials for technology, such as transition metal dichalcogenides, is a promising direction of research to maintain the rate of technological improvement. Our work focuses on the effect of iron intercalation in TiS$_2$. Single crystals of Fe$_x$TiS$_2$ (0 $\le x \le$ 1) were grown using vapor transport. Anisotropic susceptibility and magnetization measurements of the samples were measured, showing ferromagnetism and sharp switching behavior in the magnetization. Finally electrical transport measurements were taken, both with and without field. Measurements of magnetoresistance for $x$ = 0.2 and 0.3 show large magnetoresistance (up to $\sim$ 60\%) and an atypical `bowtie' shape.
Metal-to-insulator transition in Pt-doped TiSe 2 driven by emergent network of narrow transport channels
(Springer Nature, 2021) Lee, Kyungmin; Choe, Jesse; Iaia, Davide; Li, Juqiang; Zhao, Junjing; Shi, Ming; Ma, Junzhang; Yao, Mengyu; Wang, Zhenyu; Huang, Chien-Lung; Ochi, Masayuki; Arita, Ryotaro; Chatterjee, Utpal; Morosan, Emilia; Madhavan, Vidya; Trivedi, Nandini
Metal-to-insulator transitions (MIT) can be driven by a number of different mechanisms, each resulting in a different type of insulator—Change in chemical potential can induce a transition from a metal to a band insulator; strong correlations can drive a metal into a Mott insulator with an energy gap; an Anderson transition, on the other hand, due to disorder leads to a localized insulator without a gap in the spectrum. Here, we report the discovery of an alternative route for MIT driven by the creation of a network of narrow channels. Transport data on Pt substituted for Ti in 1T-TiSe2 shows a dramatic increase of resistivity by five orders of magnitude for few % of Pt substitution, with a power-law dependence of the temperature-dependent resistivity ρ(T). Our scanning tunneling microscopy data show that Pt induces an irregular network of nanometer-thick domain walls (DWs) of charge density wave (CDW) order, which pull charge carriers out of the bulk and into the DWs. While the CDW domains are gapped, the charges confined to the narrow DWs interact strongly, with pseudogap-like suppression in the local density of states, even when they were weakly interacting in the bulk, and scatter at the DW network interconnects thereby generating the highly resistive state. Angle-resolved photoemission spectroscopy spectra exhibit pseudogap behavior corroborating the spatial coexistence of gapped domains and narrow domain walls with excess charge carriers.
Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4
(Springer Nature, 2022) Wu, Han; Hallas, Alannah M.; Cai, Xiaochan; Huang, Jianwei; Oh, Ji Seop; Loganathan, Vaideesh; Weiland, Ashley; McCandless, Gregory T.; Chan, Julia Y.; Mo, Sung-Kwan; Lu, Donghui; Hashimoto, Makoto; Denlinger, Jonathan; Birgeneau, Robert J.; Nevidomskyy, Andriy H.; Li, Gang; Morosan, Emilia; Yi, Ming; Rice Center for Quantum Materials
Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.

Browsing by Author "Morosan, Emilia"

Results Per Page

Sort Options