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1.
Luo, Xu.
Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors.
Degree: PhD, Physics, 2015, Brown University
URL: https://repository.library.brown.edu/studio/item/bdr:419516/ 
► The nematic phase transition in Fe-based superconductors (FeSCs) has been a topic under intensive investigation. So far it is commonly accepted that the structural transition…
(more)
▼ The nematic phase transition in
Fe-
based
superconductors (FeSCs) has been a topic under intensive
investigation. So far it is commonly accepted that the structural
transition from tetragonal (C4) to orthorhombic (C2) symmetry in
FeSCs has an electronic nematic origin due to the unusual
anisotropy in resistivity, optical conductivity and orbital
occupancy observed above the structural transition. However, recent
studies of (Ba, Eu)Fe2(As1-xPx)2 by magnetic torque measurements
show the existence of a “true” nematic transition well above the
commonly accepted structural/nematic transition .Controversies
about this “true” nematic phase transition arise as residue strains
or external applied fields are known to break C4 symmetry and
render the structural transition merely a crossover. We performed
high resolution AC micro-calorimetry and SQUID magnetometry
measurements of BaFe2(As1-xPx)2 (x=0, 0.3) to investigate the
various phase transitions and to explore the “true” nematic phase
transition. The advantageous design of our membrane calorimeter
allows us to perform high resolution studies of the thermodynamic
phase transitions without any symmetry breaking fields. Our results
suggest that there is not a second order “true” nematic phase
transition in BaFe2(As1-xPx)2 even though the Ginzburg-Landau model
used to fit the magnetic torque data indicates that the expected
thermal anomaly should be within our experimental resolution. In
addition to the above, we present specific heat and magnetization
studies of Ba1-xNaxFe2As2 in search of the recent discovered
emergent reentrant C2 to C4 symmetry SDW transition in this series
of compound. Our results indeed locate a new phase transition in
Ba0.74Na0.26Fe2As2 at 45K. However, the absence of the conventional
SDW transition at around 80K in our data leaves doubt about the
exact nature of this new phase transition. We also systematically
studied the effects of heavy ion irradiation (HII) on the
anisotropy of YBCO single crystals by angular rotation specific
heat measurements. We found that the anisotropy of YBCO decreases
by approximately a factor of two with an irradiation dose of 6T
(matching field). The dependence of anisotropy on irradiation doses
agrees well with the prediction from a simple phenomenological
model that takes into account the anisotropic scattering caused by
columnar defects created in HII.
Advisors/Committee Members: Ling, Xinsheng (Director), Welp, Ulrich (Director), Marston, John (Reader), Mitrovic, Vesna (Reader).
Subjects/Keywords: Fe-based superconductor
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APA (6th Edition):
Luo, X. (2015). Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors. (Doctoral Dissertation). Brown University. Retrieved from https://repository.library.brown.edu/studio/item/bdr:419516/
Chicago Manual of Style (16th Edition):
Luo, Xu. “Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors.” 2015. Doctoral Dissertation, Brown University. Accessed January 26, 2021.
https://repository.library.brown.edu/studio/item/bdr:419516/.
MLA Handbook (7th Edition):
Luo, Xu. “Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors.” 2015. Web. 26 Jan 2021.
Vancouver:
Luo X. Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors. [Internet] [Doctoral dissertation]. Brown University; 2015. [cited 2021 Jan 26].
Available from: https://repository.library.brown.edu/studio/item/bdr:419516/.
Council of Science Editors:
Luo X. Thermodynamic Studies of Phase Transitions and Emerging
Orders in Unconventional Superconductors. [Doctoral Dissertation]. Brown University; 2015. Available from: https://repository.library.brown.edu/studio/item/bdr:419516/
University of Illinois – Urbana-Champaign
2.
Atkinson Mora, Juan.
Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect.
Degree: PhD, Physics, 2014, University of Illinois – Urbana-Champaign
URL: http://hdl.handle.net/2142/78286
► We report the results we obtained regarding the use of the Josephson effect to probe the symmetry of the superconducting order parameter of Co-doped Ba(FeAs)2…
(more)
▼ We report the results we obtained regarding the use of the Josephson effect to probe the symmetry of the superconducting order parameter of Co-doped Ba(FeAs)2 crystals (Ba122). This measurements were carried out to test the validity of the s± model, which predicts the pairing in the multiband Ba122 compounds being mediated by the exchange of spin excitations, with the condition that the electron and hole bands in the
superconductor's Fermi surface have opposite signs. Josephson interferometry applied using Pb/Cu/Ba122 SNS structures in corner/edge junction configurations shows two distinct Ic patterns that support an anisotropic s-wave superconducting gap. The use of IcRN data favors the hypothesis of the gap anisotropy being in direct connection to an electron gap having an opposite sign when compared to the hole gaps in the
material. Finally we considered the prediction of proximity-induced changes in the density of states of a thin film of a conventional
superconductor due to the presence of a Ba122 crystal. Our results found evidence of both a positive and a negative proximity effect, which in conjunction with other features identified in the density of states spectrum of the thin film of conventional
superconductor, points strongly towards the legitimacy of s± model.
Advisors/Committee Members: Van Harlingen, Dale J. (advisor), Giannetta, Russell W. (Committee Chair), Van Harlingen, Dale J. (committee member), Phillips, Philip W. (committee member), Pitts, Kevin T. (committee member).
Subjects/Keywords: proximity effect; Josephson; pairing symmetry; Ba122; Fe-based superconductor; Superconductor
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APA (6th Edition):
Atkinson Mora, J. (2014). Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect. (Doctoral Dissertation). University of Illinois – Urbana-Champaign. Retrieved from http://hdl.handle.net/2142/78286
Chicago Manual of Style (16th Edition):
Atkinson Mora, Juan. “Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect.” 2014. Doctoral Dissertation, University of Illinois – Urbana-Champaign. Accessed January 26, 2021.
http://hdl.handle.net/2142/78286.
MLA Handbook (7th Edition):
Atkinson Mora, Juan. “Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect.” 2014. Web. 26 Jan 2021.
Vancouver:
Atkinson Mora J. Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect. [Internet] [Doctoral dissertation]. University of Illinois – Urbana-Champaign; 2014. [cited 2021 Jan 26].
Available from: http://hdl.handle.net/2142/78286.
Council of Science Editors:
Atkinson Mora J. Probing the superconducting order parameter of Iron-based superconductors through the Josephson effect. [Doctoral Dissertation]. University of Illinois – Urbana-Champaign; 2014. Available from: http://hdl.handle.net/2142/78286
Louisiana State University
3.
Pan, Jiayun.
Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors.
Degree: PhD, Physical Sciences and Mathematics, 2016, Louisiana State University
URL: etd-04032017-150350
;
https://digitalcommons.lsu.edu/gradschool_dissertations/4428
► The discovery of Fe-based superconductors marked the beginning of a new era in unconventional superconductors. To date, the underlying superconducting mechanism has not yet been…
(more)
▼ The discovery of Fe-based superconductors marked the beginning of a new era in unconventional superconductors. To date, the underlying superconducting mechanism has not yet been fully understood. Among Fe-based superconductors, the relatively new members are Ca10PtnAs8(Fe2As2)5 (n = 3, 4) and CaFeAs2. They are unique because they contain special adjacent layers, i.e., metallic Pt4As8 in Ca10Pt4As8(Fe2As2)5, semiconducting Pt3As8 in Ca10Pt3As8(Fe2As2)5, and As chains in CaFeAs2. The focus of this research is to understand the electrical and magnetic properties of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) and CaFeAs2 in both the normal and superconducting state. Single crystals Ca10Pt4As8(Fe2As2)5 and Ca10Pt3As8(Fe2As2)5 with different dopings have been grown by the self-flux method. Using the same growth method, polycrystals CaFeAs2 and Ca0.85La0.15FeAs2 have been obtained. The experimental investigation of electrical resistivity, Hall effect, magnetoresistivity, magnetization, vortex imaging of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) single crystals has been proceeded. The electrical resistivity, magnetoresistivity and magnetization of CaFeAs2 have also been measured. We observed anisotropy in the superconducting and normal state of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) which is larger than many Fe-based superconductors. The results we obtained for Ca10Pt3As8(Fe2As2)5 with different doping not only reveal the doping effect on the in-plane and out-of-plane transport properties, but also indicate superconducting transition temperature Tc increases with increasing interlayer distance d. Among different doping, La is the most efficient in enhancing Tc and its normal state shows a total metallic behavior. Instead of having quadratic dependence to applied magnetic field like normal conductors, magnetoresistivity of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) is linear to H at fixed temperatures. It is rare in materials without strong disorder or Dirac-cone like electronic structure around the Fermi surface. Although its cause remains uncertain, we discussed the possibility that it may be related to the quantum interference effect. Through growth of CaFeAs2, we found La doping on the Ca site helps form stable crystal phase. Surprisingly, although CaFeAs2 has Dirac-cone like electronic structure around the Fermi surface which should lead to the linear H dependence of MR according to the quantum model developed by Abrikosov, magnetoresistivity of Ca0.85La0.15FeAs2 shows a quadratic dependence on H.
Subjects/Keywords: experimental investigation; magnetic property; transport property; Fe-based superconductor; single crystal
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APA (6th Edition):
Pan, J. (2016). Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors. (Doctoral Dissertation). Louisiana State University. Retrieved from etd-04032017-150350 ; https://digitalcommons.lsu.edu/gradschool_dissertations/4428
Chicago Manual of Style (16th Edition):
Pan, Jiayun. “Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors.” 2016. Doctoral Dissertation, Louisiana State University. Accessed January 26, 2021.
etd-04032017-150350 ; https://digitalcommons.lsu.edu/gradschool_dissertations/4428.
MLA Handbook (7th Edition):
Pan, Jiayun. “Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors.” 2016. Web. 26 Jan 2021.
Vancouver:
Pan J. Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors. [Internet] [Doctoral dissertation]. Louisiana State University; 2016. [cited 2021 Jan 26].
Available from: etd-04032017-150350 ; https://digitalcommons.lsu.edu/gradschool_dissertations/4428.
Council of Science Editors:
Pan J. Single Crystal Growth and Physical Property Investigations of Ca-FeAs-Based Superconductors. [Doctoral Dissertation]. Louisiana State University; 2016. Available from: etd-04032017-150350 ; https://digitalcommons.lsu.edu/gradschool_dissertations/4428
University of Tennessee – Knoxville
4.
Zhao, Jun.
Neutron scattering study of the high Tc superconductors.
Degree: 2010, University of Tennessee – Knoxville
URL: https://trace.tennessee.edu/utk_graddiss/766
► We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-Tc superconductivity, when the materials are tuned from…
(more)
▼ We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-Tc superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime.
We observed resonance mode in the electron doped cuprate Nd1.85Ce0.15CuO4, demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr0.88LaCe0.12CuO4, the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates.
Shortly after the discovery of high-Tc superconductivity in the Fe pnictides, we discovered that the magnetic phase diagram of CeFeAsO1-xFx is remarkably similar to that of the cuprates. Besides CeFeAsO, similar magnetic and lattice structures are also observed in PrFeAsO and SrFe2As2 systems. Neutron scattering measurements show that in SrFe2As2, the spectrum of magnetic excitations consists of a Bragg peak at the elastic position, a spin gap, and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimated the effective magnetic exchange coupling using a Heisenberg model.
In order to study the nature of the exchange interactions in the parent compound of Fe pnictides, we studied the high energy spin-wave excitations in CaFe2As2. Although the spin waves in the entire Brillouin zone can be described by an effective three-dimensional anisotropic Heisenberg Hamiltonian, the magnetism in this system is neither purely local nor purely itinerant; rather it is a complicated mix of the two.
When the Fe pnictide is tuned into superconducting regime with doping, the low energy spin fluctuation is dominated by a resonance mode. In the optimally electron doped BaFe1.9Ni0.1As2, application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of high Tc superconductivity.
Subjects/Keywords: neutron scattering; high Tc superconductors; Fe based superconductor; antiferromagnetism; spin fluctuations; Condensed Matter Physics
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APA (6th Edition):
Zhao, J. (2010). Neutron scattering study of the high Tc superconductors. (Doctoral Dissertation). University of Tennessee – Knoxville. Retrieved from https://trace.tennessee.edu/utk_graddiss/766
Chicago Manual of Style (16th Edition):
Zhao, Jun. “Neutron scattering study of the high Tc superconductors.” 2010. Doctoral Dissertation, University of Tennessee – Knoxville. Accessed January 26, 2021.
https://trace.tennessee.edu/utk_graddiss/766.
MLA Handbook (7th Edition):
Zhao, Jun. “Neutron scattering study of the high Tc superconductors.” 2010. Web. 26 Jan 2021.
Vancouver:
Zhao J. Neutron scattering study of the high Tc superconductors. [Internet] [Doctoral dissertation]. University of Tennessee – Knoxville; 2010. [cited 2021 Jan 26].
Available from: https://trace.tennessee.edu/utk_graddiss/766.
Council of Science Editors:
Zhao J. Neutron scattering study of the high Tc superconductors. [Doctoral Dissertation]. University of Tennessee – Knoxville; 2010. Available from: https://trace.tennessee.edu/utk_graddiss/766
McMaster University
5.
Ning, Fanlong.
75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2).
Degree: PhD, 2009, McMaster University
URL: http://hdl.handle.net/11375/17373
► We report a systematic investigation of the local electronic, magnetic, and superconducting properties of the new iron-based high temperature superconductor Ba(Fe(1-x)Co(x)As(2) (x = 0, 0.02…
(more)
▼ We report a systematic investigation of the local electronic, magnetic, and superconducting properties of the new iron-based high temperature superconductor Ba(Fe(1-x)Co(x)As(2) (x = 0, 0.02 , 0.04, 0.082) through the measurement of 75As and 59Co NMR (Nuclear Magnetic Resonance) lineshapes, Knight shift (K), and spinlattice relaxation rate ( 1/ T1) . The 75As NMR lineshape of the undoped parent compound splits into two sets due to discrete values of hyperfine magnetic field B(c)(hf)= ±1.32 Tesla below the magnetic ordering temperature to the SDW (Spin Density Wave) state, TSDW. In contrast, for lightly Co doped samples with x=0.02 and 0.04, the 75As and 59Co lineshapes become broad and featureless below Tsnw , indicating that the ground state is no longer the commensurate SDW ordered state. The
observed lineshape is consistent with an incommensurate SDW ordered state, or a commensurate state with large distribution of hyperfine field Bhf. In the optimally doped superconductor with x = 0.082 (Tc = 22 K) , we observe two types of As sites and three types of Co sites, respectively, as expected from a binomial distribution of Co dopants. We found no evidence for induced localized moments in the vicinity of Co dopants. This finding is in remarkable contrast with the case of Zn or Ni doped high Tc cuprates, and suggests that the fundamental physics of iron-based superconductors is different from that of cu prates. The temperature dependences of 75,59K and 75,59 (1/T1T) at both 75As and 59Co sites show that Ba(Fe(1-x)Co(x))(2)As(2) exhibits spin pseudo-gap like behavior down to ~100 K for a broad Co concentration range. Below ~100 K , we observe the enhancement of residual antiferromagnetic spin fluctuations associated with inter-band spin excitations between the hole and electron Fermi surfaces even for x= 0.082. This effect is suppressed in the overdoped sample with x= 0.099, and Tc decreases. Therefore, we suggest that antiferromagnetic spin fluctuations play a crucial role in the superconducting mechanism of Ba(Fe(1-x)Co(x))(2)As(2) . We also demonstrate that the superconductivity arises from a novel electronic state with spin susceptibility Xs ~ constant and in-plane resistivity P(ab)~T, which is not consistent with canonical Fermi-liquid behavior.
Thesis
Doctor of Philosophy (PhD)
Advisors/Committee Members: Imai, Takashi, Physics and Astronomy.
Subjects/Keywords: iron-based high temperature superconductor Ba(Fe(1-x)Co(x))2As(2); Nuclear magnetic resonance; Co dopants; doped superconductor; cuprates; antiferromagnetic spin fluctuations; spin susceptibility; in-plane resistivity
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APA ·
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MLA ·
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APA (6th Edition):
Ning, F. (2009). 75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2). (Doctoral Dissertation). McMaster University. Retrieved from http://hdl.handle.net/11375/17373
Chicago Manual of Style (16th Edition):
Ning, Fanlong. “75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2).” 2009. Doctoral Dissertation, McMaster University. Accessed January 26, 2021.
http://hdl.handle.net/11375/17373.
MLA Handbook (7th Edition):
Ning, Fanlong. “75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2).” 2009. Web. 26 Jan 2021.
Vancouver:
Ning F. 75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2). [Internet] [Doctoral dissertation]. McMaster University; 2009. [cited 2021 Jan 26].
Available from: http://hdl.handle.net/11375/17373.
Council of Science Editors:
Ning F. 75As and 59Co NMR Study of the Electron Doped Ba(Fe(1-x)Co(x))(2)As(2). [Doctoral Dissertation]. McMaster University; 2009. Available from: http://hdl.handle.net/11375/17373
. 
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