Quantum Fields In Extreme Backgrounds, 2018 Washington University in St. Louis

#### Quantum Fields In Extreme Backgrounds, Leandro Medina De Oliveira

*Arts & Sciences Electronic Theses and Dissertations*

Quantum field theories behave in interesting and nontrivial ways in the presence of intense electric and/or magnetic fields. Describing such behavior correctly, particularly at finite (nonzero) temperature and density, is of importance for particle physics, nuclear physics, astrophysics, condensed matter physics, and cosmology. Incorporating these conditions as external parameters also provides useful probes into the nonperturbative structure of gauge theories.

In this work, formalism for describing matter in a variety of extreme conditions is developed and implemented. We develop several expansions of one-loop finite temperature effects for spinor particles in the presence of magnetic fields, including the effects of ...

Chiral Topological Insulator Of Magnons, 2018 University of Nebraska - Lincoln

#### Chiral Topological Insulator Of Magnons, Bo Li, Alexey Kovalev

*Faculty Publications, Department of Physics and Astronomy*

We propose a magnon realization of 3D topological insulator in the AIII (chiral symmetry) topological class. The topological magnon gap opens due to the presence of Dzyaloshinskii-Moriya interactions. The existence of the topological invariant is established by calculating the bulk winding number of the system. Within our model, the surface magnon Dirac cone is protected by the sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm that the backscattering is prohibited. By weakly breaking the chiral symmetry, we observe the magnon Hall response on the surface due to opening of the gap. Finally, we show that by changing ...

Standard And Anomalous Wave Transport Inside Random Media, 2018 The Graduate Center, City University of New York

#### Standard And Anomalous Wave Transport Inside Random Media, Xujun Ma

*All Dissertations, Theses, and Capstone Projects*

This thesis is a study of wave transport inside random media using random matrix theory. Anderson localization plays a central role in wave transport in random media. As a consequence of destructive interference in multiple scattering, the wave function decays exponentially inside random systems. Anderson localization is a wave effect that applies to both classical waves and quantum waves. Random matrix theory has been successfully applied to study the statistical properties of transport and localization of waves. Particularly, the solution of the Dorokhov-Mello-Pereyra-Kumar (DMPK) equation gives the distribution of transmission.

For wave transport in standard one dimensional random systems in ...

Properties Of Epitaxial Sr0.5ba0.5mno3 Films From First-Principles, 2018 University of Arkansas, Fayetteville

#### Properties Of Epitaxial Sr0.5ba0.5mno3 Films From First-Principles, Temuujin Bayaraa

*Theses and Dissertations*

Magnetoelectric multiferroics, that possess coupled magnetic and electric degrees of freedom, have been receiving ever renewed attention for more than 15 years since they hold promise for the design of novel devices exploiting their cross-coupling.

In this thesis, we present the results of first-principles studies on physical properties of multiferroic Sr0.5Ba0.5MnO3 films under epitaxial compressive and tensile strains, and chemical ordering. We start by reviewing multiferroic materials, a magnetoelectric coupling mechanism and then we give a brief introduction to the first-principles computational methods that are involved in this study.

Here, we report that Sr0.5Ba0.5MnO3 (SBM) films ...

Optimization Of Cuinxga1-Xse2 Solar Cells With Post Selenization, 2018 The Graduate Center, City University of New York

#### Optimization Of Cuinxga1-Xse2 Solar Cells With Post Selenization, Zehra Cevher

*All Dissertations, Theses, and Capstone Projects*

The chalcopyrite semiconductor CuIn_{x}Ga_{1-x}Se_{2} is considered as the most promising material for high efficiency thin film solar cells due to its exceptional radiation stability, tunable direct bandgap, high light absorption coefficient and low cost preparation methods. In this thesis, we present the systematic investigation of the deposition conditions to optimize the CuIn_{x}Ga_{1-x}Se_{2 }device performance using the two-step deposition method. Further, we utilized nonlinear optical methods to investigate the deposition parameters to optimize the bulk and interface properties of photovoltaic devices.

First, we investigated the deposition parameters to optimize the structural ...

Using First-Principles Calculations To Screen For Fragile Magnetism: Case Study Of Lacrge 3 And Lacrsb 3, 2018 Ames Laboratory

#### Using First-Principles Calculations To Screen For Fragile Magnetism: Case Study Of Lacrge 3 And Lacrsb 3, Manh Cuong Nguyen, Valentin Taufour, Sergey L. Bud’Ko, Paul C. Canfield, Vladimir P. Antropov, Cai-Zhuang Wang, Kai-Ming Ho

*Ames Laboratory Accepted Manuscripts*

In this paper, we present a coupled experimental/theoretical investigation of pressure effect on the ferromagnetism of LaCrGe3 and LaCrSb3 compounds. The magnetic, electronic, elastic, and mechanical properties of LaCrGe3 and LaCrSb3 at ambient condition are studied by first-principles density-functional theory calculations. The pressure dependences of the magnetic properties of LaCrGe3 and LaCrSb3 are also investigated. The ferromagnetism in LaCrGe3 is rather fragile, with a ferro- to paramagnetic transition at a relatively small pressure (around 7 GPa from our calculations, and 2 GPa in experiments). The key parameter controlling the magnetic properties of LaCrGe3 is found to be the proximity ...

Pressure-Temperature Phase Diagrams Of Cak(Fe1-Xnix)(4)As-4 Superconductors, 2018 Iowa State University and Ames Laboratory

#### Pressure-Temperature Phase Diagrams Of Cak(Fe1-Xnix)(4)As-4 Superconductors, Li Xiang, William R. Meier, Mingyu Xu, Udhara S. Kaluarachchi, Sergey L. Bud’Ko, Paul C. Canfield

*Ames Laboratory Accepted Manuscripts*

The pressure dependence of the magnetic and superconducting transitions and that of the superconducting upper critical field are reported for CaK(Fe1-xNix)(4)As-4, the first example of an Fe-based superconductor with spin-vortex-crystal-type magnetic ordering. Resistance measurements were performed on single crystals with two substitution levels (x = 0.033,0.050) under hydrostatic pressures up to 5.12 GPa and in magnetic fields up to 9 T. Our results show that, for both compositions, magnetic transition temperatures T-N are suppressed upon applying pressure; the superconducting transition temperatures T-c are suppressed by pressure as well, except for x = 0.050 in ...

Doping Evolution Of Spin Fluctuations And Their Peculiar Suppression At Low Temperatures In Ca(Fe1-Xcox)(2)As-2, 2018 Iowa State University and Ames Laboratory

#### Doping Evolution Of Spin Fluctuations And Their Peculiar Suppression At Low Temperatures In Ca(Fe1-Xcox)(2)As-2, A. Sapkota, Pinaki Das, A. E. Böhmer, Benjamin G. Ueland, D. L. Abernathy, Sergey L. Bud’Ko, Paul C. Canfield, Andreas Kreyssig, Alan I. Goldman, Robert J. Mcqueeney

*Ames Laboratory Accepted Manuscripts*

Results of inelastic neutron scattering measurements are reported for two annealed compositions of Ca(Fe1-xCox)(2)As-2, x = 0.026 and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for x = 0.026, similar to the parent CaFe2As2 compound. We conclude that the well-defined spin waves are likely to be present for all x corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in Ba(Fe1-xCox)(2)As-2, wherein the crossover corresponds to microscopically ...

Physics 516: Electromagnetic Phenomena (Spring 2018), 2018 University of Pennsylvania

#### Physics 516: Electromagnetic Phenomena (Spring 2018), Philip C. Nelson

*Department of Physics Papers*

These course notes are made publicly available in the hope that they will be useful. All reports of errata will be gratefully received. I will also be glad to hear from anyone who reads them, whether or not you find errors: pcn@upenn.edu.

Unconventional Pairing Symmetry Of Interacting Dirac Fermions On A Π -Flux Lattice, 2018 Beihang University

#### Unconventional Pairing Symmetry Of Interacting Dirac Fermions On A Π -Flux Lattice, Huaiming Guo, Ehsan Khatami, Yao Wang, Thomas Devereaux, Rajiv Singh, Richard Scalettar

*Faculty Publications*

The pairing symmetry of interacting Dirac fermions on the π-flux lattice is studied with the determinant quantum Monte Carlo and numerical linked-cluster expansion methods. The s∗- (i.e., extended s-) and d-wave pairing symmetries, which are distinct in the conventional square lattice, are degenerate under the Landau gauge. We demonstrate that the dominant pairing channel at strong interactions is an unconventional ds∗-wave phase consisting of alternating stripes of s∗- and d-wave phases. A complementary mean-field analysis shows that while the s∗- and d-wave symmetries individually have nodes in the energy spectrum, the ds∗ channel is fully gapped. The results ...

Multimode Jahn-Teller Effect In Bulk Systems: A Case Of The Nv0 Center In Diamond, 2018 Ames Laboratory

#### Multimode Jahn-Teller Effect In Bulk Systems: A Case Of The Nv0 Center In Diamond, Jianhua Zhang, Cai-Zhuang Wang, Zizhong Zhu, Qing Huo Liu, Kai-Ming Ho

*Ames Laboratory Accepted Manuscripts*

The multimode Jahn-Teller (JT) effect in a bulk system of a neutral nitrogen-vacancy ( N V 0 ) center in diamond is investigated via first-principles density-functional-theory calculations and the intrinsic distortion path (IDP) method. The adiabatic potential energy surface of the electronic ground state of the N V 0 center is calculated based on the local spin-density approximation. Our calculations confirm the presence of the dynamic Jahn-Teller effect in the ground 2 E state of the N V 0 center. Within the harmonic approximation, the IDP method provides the reactive path of JT distortion from unstable high-symmetry geometry to stable low-symmetry energy ...

Reduction Of The Ordered Magnetic Moment And Its Relationship To Kondo Coherence In, 2018 Iowa State University and Ames Laboratory

#### Reduction Of The Ordered Magnetic Moment And Its Relationship To Kondo Coherence In, Benjamin G. Ueland, Na Hyun Jo, Aashish Sapkota, W. Tian, Morgan W. Masters, Halyna Hodovanets, Savannah S. Downing, Connor Schmidt, Robert Mcqueeney, Sergey L. Bud’Ko, Andreas Kreyssig, Paul C. Canfield, Alan Goldman

*Ames Laboratory Accepted Manuscripts*

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce1-&ITx&ITLa&ITx&ITCu2Ge2 due to nonmagnetic dilution by La are revealed through neutron diffraction results for x = 0.20, 0.40, 0.75, and 0.85. Magnetic Bragg peaks are found for 0.20 <= x <= 0.75, and both the Ned temperature T-N and the ordered magnetic moment per Ce mu linearly decrease with increasing x. The reduction in mu points to strong hybridization of the increasingly diluted Ce 4f electrons, and we find a remarkable quadratic dependence of mu on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory and show that Ce1-&ITx&ITLa&ITx&ITCu2Ge2 provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice.

Unconventional Pairing Symmetry Of Interacting Dirac Fermions On A Π -Flux Lattice, 2018 Beihang University

#### Unconventional Pairing Symmetry Of Interacting Dirac Fermions On A Π -Flux Lattice, Huaiming Guo, Ehsan Khatami, Yao Wang, Thomas P. Devereaux, Rajiv R.P. Singh, Richard T. Scalettar

*Ehsan Khatami*

Quasiprobability Behind The Out-Of-Time-Ordered Correlator, 2018 California Institute of Technology

#### Quasiprobability Behind The Out-Of-Time-Ordered Correlator, Nicole Yunger Halpern, Brian Swingle, Justin Dressel

*Mathematics, Physics, and Computer Science Faculty Articles and Research*

Two topics, evolving rapidly in separate fields, were combined recently: the out-of-time-ordered correlator (OTOC) signals quantum-information scrambling in many-body systems. The Kirkwood-Dirac (KD) quasiprobability represents operators in quantum optics. The OTOC was shown to equal a moment of a summed quasiprobability [Yunger Halpern, Phys. Rev. A 95, 012120 (2017)]. That quasiprobability, we argue, is an extension of the KD distribution. We explore the quasiprobability's structure from experimental, numerical, and theoretical perspectives. First, we simplify and analyze Yunger Halpern's weak-measurement and interference protocols for measuring the OTOC and its quasiprobability. We decrease, exponentially in system size, the number of ...

Beyond Triplet: Unconventional Superconductivity In A Spin-3/2 Topological Semimetal, 2018 University of Maryland, College Park and Ames Laboratory

#### Beyond Triplet: Unconventional Superconductivity In A Spin-3/2 Topological Semimetal, Hyunsoo Kim, Kefeng Wang, Yasuyuki Nakajima, Rongwei Hu, Steven Ziemak, Paul Syers, Limin Wang, Halyna Hodovanets, Jonathan D. Denlinger, Philip M. R. Brydon, Daniel F. Agterberg, Makariy A. Tanatar, Ruslan Prozorov, Johnpierre Paglione

*Ames Laboratory Accepted Manuscripts*

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The “spin” of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived and, in some cases, can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle electronic structure in the half-Heusler ...

In-Plane Magnetic Penetration Depth Of Superconducting Cakfe4as4, 2018 Paul Scherrer Institut

#### In-Plane Magnetic Penetration Depth Of Superconducting Cakfe4as4, Rustem Khasanov, William R. Meier, Yun Wu, Daixiang Mou, Sergey Bud’Ko, Ilya Eremin, Hubertus Luetkens, Adam Kaminski, Paul C. Canfield, Alex Amato

*Ames Laboratory Accepted Manuscripts*

The temperature dependence of the in-plane magnetic penetration depth (lambda(ab)) in an extensively characterized sample of superconducting CaKFe4As4 (T-c similar or equal to 35 K) was investigated using muon-spin rotation (mu SR). A comparison of lambda(-2)(ab) (T) measured by mu SR with the one inferred from angle-resolved photoemission spectroscopy (ARPES) data confirms the presence of multiple gaps at the Fermi level. An agreement between mu SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small superconducting gaps with an average zero-temperature value of Delta(0 ...

Influence Of Multiband Sign-Changing Superconductivity On Vortex Cores And Vortex Pinning In Stoichiometric High-Tc Cakfe4 As4, 2018 Universidad Autónoma de Madrid

#### Influence Of Multiband Sign-Changing Superconductivity On Vortex Cores And Vortex Pinning In Stoichiometric High-Tc Cakfe4 As4, Antón Fente, William R. Meier, Tai Kong, Vladimir G. Kogan, Sergey Bud’Ko, Paul C. Canfield, Isabel Guillamón, Hermann Suderow

*Ames Laboratory Accepted Manuscripts*

We use a scanning tunneling microscope to study the superconducting density of states and vortex lattice of single crystals of CaKFe 4 As 4 . This material has a critical temperature of T c = 35 K, one of the highest among stoichiometric iron based superconductors (FeBSCs), and is comparable to T c found near optimal doping in other FeBSCs. We observe quasiparticle scattering from defects with a pattern related to interband scattering between zone centered hole sheets. We measure the tunneling conductance in vortex cores and find a peak due to Caroli–de Gennes–Matricon bound states. The peak is located ...

Enhanced Moments Of Eu In Single Crystals Of The Metallic Helical Antiferromagnet Euco2-Yas2, 2018 Ames Laboratory

#### Enhanced Moments Of Eu In Single Crystals Of The Metallic Helical Antiferromagnet Euco2-Yas2, N. S. Sangeetha, V. K. Anand, Eduardo Cuervo-Reyes, V. Smetana, A.-V. Mudring, David C. Johnston

*Ames Laboratory Accepted Manuscripts*

The compound EuCo2-yAs2 with the tetragonal ThCr2Si2 structure is known to contain Eu+2 ions with spin S = 7/2 that order below a temperature T-N approximate to 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. We consistently find about 5% vacancies on the Co site from energy-dispersive x-ray analysis and x-ray diffraction refinements. Enhanced ordered and effective moments of ...

Pressure-Tuned Superconductivity And Normal-State Behavior In Ba(Fe0.943co0.057)2as2 Near The Antiferromagnetic Boundary, 2018 Center for High Pressure Science and Technology Advanced Research

#### Pressure-Tuned Superconductivity And Normal-State Behavior In Ba(Fe0.943co0.057)2as2 Near The Antiferromagnetic Boundary, W. Liu, Y. F. Wu, X. J. Li, Sergey L. Bud’Ko, Paul C. Canfield, C. Panagopoulos, P. G. Li, G. Mu, T. Hu, C. C. Almasan, H. Xiao

*Ames Laboratory Accepted Manuscripts*

Superconductivity in iron pnictides is unconventional and pairing may be mediated by magnetic fluctuations in the Fe sublattice. Pressure is a clean method to explore superconductivity in iron based superconductors by tuning the ground state continuously without introducing disorder. Here we present a systematic high pressure transport study in Ba ( Fe 1 − x Co x ) 2 As 2 single crystals with x = 0.057, which is near the antiferromagnetic instability. Resistivity ρ = ρ 0 + A T n was studied under applied pressure up to 7.90 GPa. The parameter n approaches a minimum value of n ≈ 1 at a critical ...

Robust S(+/-) Pairing In Cak(Fe1-Xnix)(4)As-4 (X=0 And 0.05) From The Response To Electron Irradiation, 2018 Iowa State University and Ames Laboratory

#### Robust S(+/-) Pairing In Cak(Fe1-Xnix)(4)As-4 (X=0 And 0.05) From The Response To Electron Irradiation, Serafim Teknowijoyo, Kyuil Cho, M. Konczykowski, Erik I. Timmons, Makariy A. Tanatar, William R. Meier, Mingyu Xu, Sergey L. Bud’Ko, Paul C. Canfield, Ruslan Prozorov

*Ames Laboratory Accepted Manuscripts*

Controlled pointlike disorder introduced by 2.5-MeV electron irradiation was used to probe the superconducting state of single crystals of CaK(Fe1-x Ni-x)(4)As-4 superconductor at x = 0 and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, T-c, at the rate of dT(c)/d rho(T-c)approximate to 0.19 K/(mu Omega cm) for x = 0 and 0.38 K/(mu Omega cm) for x = 0.05, respectively. In the Ni-doped compound (x = 0.05), the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed ...