Strongly Correlated Systems Under High Magnetic Field: A Mixed Landau Levels Description For Fractional Quantum Hall Effect, 2019 Washington University in St. Louis

#### Strongly Correlated Systems Under High Magnetic Field: A Mixed Landau Levels Description For Fractional Quantum Hall Effect, Sumanta Bandyopadhyay

*Arts & Sciences Electronic Theses and Dissertations*

Strong correlation among electrons under high magnetic field gives rise to an entirely new arena of emergent physics, namely fractional quantum Hall effect. Such systems have entirely different elementary degrees of freedom and generally, demand non-perturbative approaches to develop a better understanding. In the literature, there are several analytical methodologies and numerical toolkits available to study such a system. Clustering of zeros, parent Hamiltonian, off-diagonal order parameter, parton construction, matrix product states are to be named among a few of those popular methodologies in the existing literature. Most of these methods work well in the lowest Landau level or holomorphic ...

Strongly Correlated Systems Under High Magnetic Field: A Mixed Landau Levels Description For Fractional Quantum Hall Effect, 2019 Washington University in St. Louis

#### Strongly Correlated Systems Under High Magnetic Field: A Mixed Landau Levels Description For Fractional Quantum Hall Effect, Sumanta Bandyopadhyay

*Arts & Sciences Electronic Theses and Dissertations*

Strong correlation among electrons under high magnetic field gives rise to an entirely new arena of emergent physics, namely fractional quantum Hall effect. Such systems have entirely different elementary degrees of freedom and generally, demand non-perturbative approaches to develop a better understanding. In the literature, there are several analytical methodologies and numerical toolkits available to study such a system. Clustering of zeros, parent Hamiltonian, off-diagonal order parameter, parton construction, matrix product states are to be named among a few of those popular methodologies in the existing literature. Most of these methods work well in the lowest Landau level or holomorphic ...

Enhanced Magnetic Ordering In Sm Metal And Search For Superconductivity In Cs And Rb Under Extreme Pressure, 2019 Washington University in St. Louis

#### Enhanced Magnetic Ordering In Sm Metal And Search For Superconductivity In Cs And Rb Under Extreme Pressure, Yuhang Deng

*Arts & Sciences Electronic Theses and Dissertations*

At ambient pressure Sm metal orders antiferromagnetically at 106 K and 14 K. The pressure-dependence of the magnetic ordering temperature To of Sm metal was determined through four-point electrical resistivity measurements in a diamond anvil cell to pressures as high as 150 GPa. A strong increase in To with pressure is observed above 85 GPa. In this pressure range Sm ions alloyed in dilute concentration with superconducting Y exhibit giant Kondo pair breaking. Taken together, these results suggest that for pressures above 85 GPa Sm is in a highly correlated electron state, like a Kondo lattice, with an unusually high ...

Tuning Phase-Stability And Short-Range Order Through Ai-Doping In (Cocrfemn)100-Xaix High Entropy Alloys, 2019 Ames Laboratory

#### Tuning Phase-Stability And Short-Range Order Through Ai-Doping In (Cocrfemn)100-Xaix High Entropy Alloys, Prashant Singh, Amalraj Marshal, Andrei V. Smirnov, Aayush Sharma, Ganesh Balasubramanian, K. G. Pradeep, Duane D. Johnson

*Ganesh Balasubramanian*

For (CoCrFeMn)100−xAlx high-entropy alloys, we investigate the phase evolution with increasing Al content (0≤x≤20 at.%). From first-principles theory, aluminum doping drives the alloy structurally from fcc to bcc separated by a narrow two-phase region (fcc+bcc), which is well supported by our experiments. Using KKR-CPA electronic-structure calculations, we highlight the effect of Al doping on the formation enthalpy (alloy stability) and electronic dispersion of (CoCrFeMn)100−xAlx alloys. As chemical short-range order indicates the nascent local order, and entropy changes, as well as expected low-temperature ordering behavior, we use KKR-CPA-based thermodynamic linear response to predict the ...

Dependence Of The Absolute Value Of The Penetration Depth In (Ba1–X Kx) Fe2 As2 On Doping, 2019 Technion–Israel Institute of Technology

#### Dependence Of The Absolute Value Of The Penetration Depth In (Ba1–X Kx) Fe2 As2 On Doping, Avior Almoalem, Alon Yagil, Kyuil Cho, Serafim Teknowijoyo, Makariy A. Tanatar, Ruslan Prozorov, Yong Liu, Thomas A. Lograsso, Ophir M. Auslaender

*Thomas A. Lograsso*

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor Ba1−xKxFe2As2. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth (λab) in underdoped, optimally doped, and overdoped samples. The results suggest an abrupt increase of λab as doping is increased from xopt, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for x=0.19 and 0.58 is compatible with previously observed structural symmetries at those doping levels.

Self-Consistent Two-Gap Description Of Mgb2 Superconductor, 2019 Iowa State University and Ames Laboratory

#### Self-Consistent Two-Gap Description Of Mgb2 Superconductor, Hyunsoo Kim, Kyuil Cho, Makariy A. Tanatar, Valentin Taufour, Stella K. Kim, Sergey L. Bud’Ko, Paul C. Canfield, Vladimir G. Kogan, Ruslan Prozorov

*Ames Laboratory Accepted Manuscripts*

A self-consistent two-gap γ -model is used to quantitatively describe several thermodynamic properties of MgB 2 superconductor. The superconducting coupling matrix, νij , was obtained from the fitting of the superfluid density in the entire superconducting temperature range. Using this input, temperature-dependent superconducting gaps, specific heat, and upper critical fields were calculated with no adjustable parameters and compared with the experimental data as well as with the first-principles calculations. The observed agreement between fit and data shows that γ -model provides adequate quantitative description of the two-gap superconductivity in MgB 2 and may serve as a relatively simple and versatile self-consistent ...

Computational Studies Of Thermal Properties And Desalination Performance Of Low-Dimensional Materials, 2019 University of Nebraska - Lincoln

#### Computational Studies Of Thermal Properties And Desalination Performance Of Low-Dimensional Materials, Yang Hong

*Student Research Projects, Dissertations, and Theses - Chemistry Department*

During the last 30 years, microelectronic devices have been continuously designed and developed with smaller size and yet more functionalities. Today, hundreds of millions of transistors and complementary metal-oxide-semiconductor cells can be designed and integrated on a single microchip through 3D packaging and chip stacking technology. A large amount of heat will be generated in a limited space during the operation of microchips. Moreover, there is a high possibility of hot spots due to non-uniform integrated circuit design patterns as some core parts of a microchip work harder than other memory parts. This issue becomes acute as stacked microchips get ...

Impact Of Damping On The Superconducting Gap Dynamics Induced By Intense Terahertz Pulses, 2019 University of Minnesota

#### Impact Of Damping On The Superconducting Gap Dynamics Induced By Intense Terahertz Pulses, Tianbai Cui, Xu Yang, Chirag Vaswani, Jigang Wang, Rafael M. Fernandes, Peter P. Orth

*Ames Laboratory Accepted Manuscripts*

We investigate the interplay between coherent gap dynamics and damping in superconductors taken out of equilibrium by strong optical pulses with subgap terahertz frequencies. A semiphenomenological formalism is developed to include the damping within the electronic subsystem that arises from effects beyond Bardeen-Cooper-Schrieffer mean-field theory, such as interactions between Bogoliubov quasiparticles and decay of the Higgs mode. These processes, conveniently expressed as longitudinal T1 and transverse T2 relaxation times in the standard pseudospin language for superconductors, cause the gap amplitude to be suppressed after the pulse is turned off, but before the timescale where thermalization occurs due to coupling to ...

Role Of The Fermi Surface For The Pressure-Tuned Nematic Transition In The Bafe2 As2 Family, 2019 Iowa State University and Ames Laboratory

#### Role Of The Fermi Surface For The Pressure-Tuned Nematic Transition In The Bafe2 As2 Family, Elena Gati, Li Xiang, Sergey L. Bud’Ko, Paul C. Canfield

*Ames Laboratory Accepted Manuscripts*

The tetragonal-to-orthorhombic phase transition at Ts, which precedes the antiferromagnetic phase transition at TN in many iron-based superconductors, is considered one of the manifestations of electronic *nematic* order. By constructing temperature-pressure phase diagrams of pure and Co-doped BaFe2As2, we study the relation of Ts and TN under pressure p. Our data disclose two qualitatively different regimes in which ΔT=Ts−TN either increases or decreases with p. We provide experimental evidence that the transition between the two regimes may be associated with sudden changes of the Fermi surface topology. Therefore, our results not only support the electronic origin of the ...

Distinct Pressure Evolution Of Coupled Nematic And Magnetic Orders In Fese, 2019 Ames Laboratory and Karlsruhe Institute of Technology

#### Distinct Pressure Evolution Of Coupled Nematic And Magnetic Orders In Fese, Anna E. Böhmer, Karunakar Kothapalli, Wageesha T. Jayasekara, John M. Wilde, Bing Li, Aashish Sapkota, Benjamin G. Ueland, Pinaki Das, Yumin Xiao, Wenli Bi, Jiyong Zhao, E. Ercan Alp, Sergey L. Bud’Ko, Paul C. Canfield, Alan I. Goldman, Andreas Kreyssig

*Ames Laboratory Accepted Manuscripts*

We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain Mössbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to ∼ 6 GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at p = 6.8 GPa , the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that ...

Skyrmions And Spirals In Mnsi Under Hydrostatic Pressure, 2019 Delft University of Technology

#### Skyrmions And Spirals In Mnsi Under Hydrostatic Pressure, L. J. Bannenberg, R. Sadykov, R. M. Dalgliesh, C. Goodway, Deborah L. Schlagel, Thomas A. Lograsso, P. Falus, E. Lelièvre-Berna, A. O. Leonov, C. Pappas

*Ames Laboratory Accepted Manuscripts*

The archetype cubic chiral magnet MnSi is home to some of the most fascinating states in condensed matter, such as skyrmions and a non-Fermi-liquid behavior in conjunction with a topological Hall effect under hydrostatic pressure. Using small angle neutron scattering, we study the evolution of the helimagnetic, conical, and skyrmionic correlations with increasing hydrostatic pressure. We show that the helical propagation vector smoothly reorients from ⟨111⟩ to ⟨100⟩ at intermediate pressures. At higher pressures, above the critical pressure, the long-range helimagnetic order disappears at zero magnetic field. Nevertheless, skyrmion lattices and conical spirals form under magnetic fields, in a part ...

Competing Magnetic Phases And Itinerant Magnetic Frustration In Srco2 As2, 2019 Iowa State University and Ames Laboratory

#### Competing Magnetic Phases And Itinerant Magnetic Frustration In Srco2 As2, Bing Li, Benjamin G. Ueland, W. T. Jayasekara, D. L. Abernathy, N. S. Sangeetha, David C. Johnston, Qing-Ping Ding, Yuji Furukawa, Peter P. Orth, Andreas Kreyssig, Alan I. Goldman, Robert J. Mcqueeney

*Ames Laboratory Accepted Manuscripts*

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo2As2 below T approximate to 100 K centered at the stripe-type AF propagation vector of (1/2, 1/2), and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability toward ferromagnetism ordering to ...

Effect Of Ni Doping On Vortex Pinning In Cak (Fe1−X Nix)4as4 Single Crystals, 2019 Ames Laboratory and Centro Atómico Bariloche and Instituto Balseiro, CNEA

#### Effect Of Ni Doping On Vortex Pinning In Cak (Fe1−X Nix)4as4 Single Crystals, N. Haberkorn, Mingyu Xu, William R. Meier, J. Schmidt, Sergey L. Bud’Ko, Paul C. Canfield

*Ames Laboratory Accepted Manuscripts*

We study the correlation between chemical composition and vortex dynamics in Ni-doped CaK(Fe1−xNix)4As4 (x=0, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities Jc and the flux creep rates S. The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy (U0<100K). The undoped crystals display modest Jc values at low temperatures and high magnetic fields applied along the c axis. Jc(T) dependences at high fields display an unusual peak. The enhancement in Jc(T) matches with an increase in U0 and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in Tc there is a nonmonotonic change in Jc. Initially Jc increases, reaching a maximum value for x=0.015, and then Jc decreases for x≥0.025. This change in Jc(x) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of Jc(H) also manifests a change in behavior between these x values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, x. This implies that there is no appreciable change in the mechanism that determines the vortex relaxation.

Suppression Of Ferromagnetic Spin Fluctuations In The Filled Skutterudite Superconductor Sros4 As12 Revealed By 75as Nmr-Nqr Measurements, 2019 Iowa State University and Ames Laboratory

#### Suppression Of Ferromagnetic Spin Fluctuations In The Filled Skutterudite Superconductor Sros4 As12 Revealed By 75as Nmr-Nqr Measurements, Qing-Ping Ding, K. Nishine, Y. Kawamura, J. Hayashi, C. Sekine, Yuji Furukawa

*Ames Laboratory Accepted Manuscripts*

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe4As12 [Q.-P. Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out As-75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in the newly discovered isostructural superconductor SrOs4As12 with a superconducting transition temperature of T-c similar to 4.8 K. Knight shift K determined by the NQR spectrum under a small magnetic field (<= 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, is nearly independent of temperature above similar to 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe4As12, we found that the values of vertical bar K vertical bar and 1/T1T in SrOs4As12 are smaller than those in SrFe4As12, indicating no obvious ferromagnetic spin correlations in SrOs4As12. From the temperature dependence of 1/T-1 in the superconducting state, an s-wave superconductivity is realized.

Non-Fermi-Liquid Behaviors Associated With A Magnetic Quantum-Critical Point In Sr(Co{1-X}Ni{X})2as2 Single Crystals, 2019 Ames Laboratory

#### Non-Fermi-Liquid Behaviors Associated With A Magnetic Quantum-Critical Point In Sr(Co{1-X}Ni{X})2as2 Single Crystals, Nediadath S. Sangeetha, Lin-Lin Wang, Andrei V. Smirnov, Volodymyr Smetana, A.-V. Mudring, Duane D. Johnson, Makariy A. Tanatar, Ruslan Prozorov, David C. Johnston

*Duane D. Johnson*

Electron-doped Sr(Co{1-x}Ni{x})2As2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD), magnetic susceptibility chi(H,T), isothermal magnetization M(H,T), heat capacity Cp(H,T), and electrical resistivity ho(H,T) measurements versus applied magnetic field H and temperature T. The chi(T) data show that the crystals exhibit an antiferromagnetic (AFM) ground state almost immediately upon Ni doping on the Co site. Ab-initio electronic-structure calculations for x = 0 and x = 0 ...

Hole Doping And Antiferromagnetic Correlations Above The Néel Temperature Of The Topological Semimetal (Sr1-X Kx) Mnsb2, 2019 Ames Laboratory

#### Hole Doping And Antiferromagnetic Correlations Above The Néel Temperature Of The Topological Semimetal (Sr1-X Kx) Mnsb2, Yong Liu, Farhan Islam, Kevin W. Dennis, Wei Tian, Benjamin G. Ueland, Robert J. Mcqueeney, David Vaknin

*Ames Laboratory Accepted Manuscripts*

Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal (Sr0.97K0.03)MnSb2 confirm the three-dimensional (3D) C-type antiferromagnetic (AFM) ordering of the Mn2+ moments at TN=305±3 K, which is slightly higher than that of the parent SrMnSb2 with TN=297±3 K. Susceptibility measurements of the K-doped and parent crystals above TN are characteristic of 2D AFM systems. This is consistent with high-temperature neutron diffraction of the parent compound that displays persisting 2D AFM correlations well above TN to at least ∼560 K with no evidence of a ferromagnetic phase. Analysis of the de Haas–van ...

Single Pair Of Weyl Fermions In The Half-Metallic Semimetal Eucd2as2, 2019 Ames Laboratory

#### Single Pair Of Weyl Fermions In The Half-Metallic Semimetal Eucd2as2, Lin-Lin Wang, Na Hyun Jo, Brinda Kuthanazhi, Yun Wu, Robert J. Mcqueeney, Adam Kaminski, Paul C. Canfield

*Paul C. Canfield*

Materials with the ideal case of a single pair of Weyl points (WPs) are highly desirable for elucidating the unique properties of Weyl fermions. EuC d 2 A s 2 is an antiferromagnetic topological insulator or Dirac semimetal depending on the different magnetic configurations. Using first-principles band-structure calculations, we show that inducing ferromagnetism in EuC d 2 A s 2 can generate a single pair of WPs from splitting the single pair of antiferromagnetic Dirac points due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that a single pair of WPs is obtained in EuC d 2 ...

Temperature-Dependent Anisotropies Of Upper Critical Field And London Penetration Depth, 2019 Iowa State University and Ames Laboratory

#### Temperature-Dependent Anisotropies Of Upper Critical Field And London Penetration Depth, V. G. Kogan, Ruslan Prozorov, A. E. Koshelev

*Ames Laboratory Accepted Manuscripts*

We show on a few examples of one-band materials with spheroidal Fermi surfaces and anisotropic order parameters that anisotropies γH of the upper critical field and γλ of the London penetration depth depend on temperature, a feature commonly attributed to multiband superconductors. The parameters γH and γλ may have opposite temperature dependences or may change in the same direction depending on the Fermi-surface shape and on the character of the gap nodes. For two-band systems, the behavior of anisotropies is affected by the ratios of bands densities of states, Fermi velocities, anisotropies, and order parameters. We investigate in detail the ...

Electronic Structure Of The Topological Superconductor Candidate Au2pb, 2019 Iowa State University and Ames Laboratory

#### Electronic Structure Of The Topological Superconductor Candidate Au2pb, Yun Wu, Gil Drachuck, Lin-Lin Wang, Duane D. Johnson, Przemyslaw Swatek, Benjamin Schrunk, Daixiang Mou, Lunan Huang, Sergey L. Bud’Ko, Paul C. Canfield, Adam Kaminski

*Duane D. Johnson*

We use magnetization measurements, high-resolution angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations to study the electronic properties of Au2Pb, a topological superconductor candidate. The magnetization measurements reveal three discontinuities at 40, 51, and 99 K that agree well with reported structural phase transitions. To measure the band structure along desired crystal orientations, we utilized polishing, sputtering, and annealing to obtain clean flat sample surfaces. ARPES measurements of the Au2Pb (111) surface at 110 K shows a shallow hole pocket at the center and flower-petal-like surface states at the corners of the Brillouin zone. These observations match the ...

Electronic Structure Of The Topological Superconductor Candidate Au2pb, 2019 Iowa State University and Ames Laboratory

#### Electronic Structure Of The Topological Superconductor Candidate Au2pb, Yun Wu, Gil Drachuck, Lin-Lin Wang, Duane D. Johnson, Przemyslaw Swatek, Benjamin Schrunk, Daixiang Mou, Lunan Huang, Sergey L. Bud’Ko, Paul C. Canfield, Adam Kaminski

*Paul C. Canfield*

We use magnetization measurements, high-resolution angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations to study the electronic properties of Au2Pb, a topological superconductor candidate. The magnetization measurements reveal three discontinuities at 40, 51, and 99 K that agree well with reported structural phase transitions. To measure the band structure along desired crystal orientations, we utilized polishing, sputtering, and annealing to obtain clean flat sample surfaces. ARPES measurements of the Au2Pb (111) surface at 110 K shows a shallow hole pocket at the center and flower-petal-like surface states at the corners of the Brillouin zone. These observations match the ...