## Condensed Matter Seminars This Term

Condensed Matter
Thursday, January 18, 2018
11:00 AM
Physics Building, Room 313

"Available"
Special Nuclear Seminar

Thursday, January 25, 2018
3:30 PM
Physics Building, Room 204
Note special time.
Note special room.
Cunfeng Feng [Host: Xiaochao Zheng]
Shandong University
"Searching the origin of the knee of cosmic ray spectrum -LHAASO experiment"
ABSTRACT:

The curve of cosmic ray energy spectrum is turned around PeV region, which is called "the knee" of the cosmic ray spectrum. The origin of the PeV knee of cosmic ray spectrum remains a puzzle since its first discovery near 70 years ago. Searching for the origin of the knee is one of main aims of LHAASO experiment, a hybrid cosmic ray observatory, which is building on Haizi mountain, south of China. In this talk, the puzzle of the knee of cosmic ray spectrum will be introduced briefly together with the LHAASO experiment introduction. Then I will focus on the technique of scintillator detector of LHAASO and the photomultiplier tube used in this detector. The PMT test bench of Shandong University will also will be introduced.

Condensed Matter
Thursday, February 1, 2018
11:00 AM
Physics Building, Room 313
Sergey Vakhrushev [Host: Despina Louca]
Ioffe Institute, St. Petersburg, Russia
"Phase transitions and mode coupling in PbZrO3 and Zr-rich PbZr1-xTixO3 solid solutions"
ABSTRACT:

(PbZrO3)1-x-(PbTiO3)x (PZT) solid solutions probably represent the most studied group of functional dielectrics materials. For many years the main efforts were devote to the study of the PZT compounds at the morphotropic boundary region (x≈0.5). However recently Zr-rich (x<0.06) compounds attracted new attention due to their potential for the electric energy storage and electricaloric application. Beside possible application related interest these crystals demonstrate extremely reach phase diagram including antiferroelectric, ferroelectric and incommensurate phases.

In my presentation I would like to concentrate on the dynamical features related to the phase transitions in the Zr-rich PZT (including PbZrO3 itself) and in the PbHfO3. In the papers [1,2] we demonstrated that the antiferroelectric phase transition In PbZrO3 with order parameter described by the wavevector qAFE=(¼ ¼ 0) can be considered as a missed incommensurate transition with some arbitrary wavevector, corresponding to the flat part of the dispersion curve of the TA mode. Later in PbHfO3 and PbZrO3 at high pressure the minima at the TA dispersion curves were found [3,4], resulting in the realization of the incommensurate phases.

Extremely complicated diffraction pattern is observed in the PZr1-xTixO3 crystals with x<0.06. In the intermediate phase between the paraelectric and antiferroelectric phases incommensurate phase is sometime observed similar to that in the PbZrO3 under high presuure. And in addition complicated system of the satellite peaks in the vicinity of the qM=(½ ½ 0) including first order and second order satellites exists. In addition to the satellite peaks near the M-points we found second order satellites near the main Bragg peaks.

Observed diffraction pattern can be fully described by the incommensurate structure determined by 2 wavevectors from the same star: q1=(0.5+δ 0.5-δ -δ)  and q2 =(0.5-δ δ 0.5+ δ). Combination of the q1 and q2 describes all observed superstructure peaks.

Creation of the true incommensurate phase can be attributed to the mode softening not at qM, but at a position shifted from the zone boundary. Such unusual soft mode can be described in terms of the coupling of 2 modes in the vicinity of M-point, namely TA mode and oxygen tilt mode. Such coupling is forbidden at qM but became allowed aside of it. Proposed model provides qualitative agreement with the results of the inelastic and diffuse X-ray scattering measurements

# References [1] A. K. Tagantsev et al., Nat. Commun., 4, 2229 (2013) [2]R. G. Burkovsky, et al. Phys. Rev. B 90, 144301 (2014) [3] R.G. Burkovsky, et al.. J. Phys.: Condens. Matter,  27, 335901 (2015) [4] R.G. Burkovsky, et al.., Sci. Reports, 7, 41512 (2017)

Condensed Matter
Thursday, February 8, 2018
11:00 AM
Physics Building, Room 313

"Available"
Condensed Matter
Thursday, February 15, 2018
11:00 AM
Physics Building, Room 313

"Available"
Condensed Matter
Thursday, February 22, 2018
11:00 AM
Physics Building, Room 313
RESERVED
Condensed Matter
Thursday, March 1, 2018
11:00 AM
Physics Building, Room 313
Aaron Wegner [Host: Despina Louca]
UVA- Department of Physics
"Local structure and the Jahn-Teller effect in TiSe2-xTex charge density waves"
ABSTRACT:

Transition metal dichalcogenide (TMDC) materials exhibit a wide variety of interesting physical phenomena. This diverse family of materials forms a quasi-two dimensional layered hexagonal structure of X-M-X sandwiches (M= Ti, Mo, Hf, W, etc.., X= S, Se, Te) that, depending on composition, may be semiconducting, metallic, or superconducting and many undergo charge density wave transitions. As the materials are layered and can be exfoliated, interest in the TMDCs has increased due to the search for graphene-like materials and the importance of thin film applications. One particularly interesting material is TiSe2, which forms a prototypical commensurate CDW that occurs in the vicinity of superconductivity. The origin of this CDW phase is controversial and has alternatively been attributed to exciton condensation or several possible Jahn-Teller type mechanisms. I will discuss how neutron scattering and local structure refinements give insight into the effect of the lattice on CDW formation in TiSe2 and the doping series in which Te is substituted for Se.

Condensed Matter
Thursday, March 15, 2018
11:00 AM
Physics Building, Room 313
Available
Condensed Matter
Thursday, March 22, 2018
11:00 AM
Physics Building, Room 313
RESERVED
Condensed Matter
Thursday, March 29, 2018
11:00 AM
Physics Building, Room 313
University of Tennessee
"New Directions in Theoretical Studies of High Tc Superconductors"
ABSTRACT:

The discovery of high critical temperature superconductivity in iron-based pnictides and chalcogenides brought to the  forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understandingof iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include:
i) The development of spin-fermion models [1] that allow studies in the difficult nematic regime with a finite
short-range antiferromagnetic correlation length above the ordering critical temperatures. This type of studies
also allow the inclussion of doping, quenched disorder, and the study of transport and real-frequency responses;
ii) The application of the Density Matrix Renormalization Group (DMRG) approach to multi-orbital Hubbard
models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder
iron-based compounds such as BaFe2S3 and BaFe2Se3. In this context,
the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed;
iii) Results for a newly developed multi-orbital spin-fermion model for the CuO2 planes in high Tc cuprates.[3]

[1] S.Liang {\it et al.}, Phys.Rev.Lett.{\bf 109}, 047001 (2012) and Phys. Rev. Lett. {\bf 111} 047004 (2013); Phys. Rev. B{\bf 92} 104512 (2015); C. Bishop {\it et al.}, Phys. Rev. Lett. {\bf 117} 117201 (2016); Phys. Rev. B{\bf 96} 035144 (2017). [2] N.D. Patel {\it et al.}, Phys. Rev. B{\bf 96}, 024520(2017). See also  N.D. Patel {\it et al.}, Phys. Rev. B{\bf 94}, 075119(2016). [3] Mostafa Hussein et al., in preparation.

Condensed Matter
Thursday, April 5, 2018
11:00 AM
Physics Building, Room 313
Available
Condensed Matter
Thursday, April 12, 2018
11:00 AM
Physics Building, Room 313
Clarina Dela Cruz [Host: Despina Louca]
Oak Ridge National Laboratory
"Opportunities in Quantum Materials Research using Neutrons"
ABSTRACT:

Quantum materials will arguably be the key materials to push forward the forefront energy relevant technologies of the future. Having two powerful neutron sources at the Oak Ridge National Laboratory, enables us to be positioned strongly to use neutron scattering in unveiling the structure of, and dynamics in quantum systems that lead to fundamental understanding and control of quantum phenomena such as coherence, entanglement and novel emergent states. quantum critical phenomena among others, with a range of correlation strength in them. With the increasing progress in instrumentation, the instruments at ORNL are able to study systems in extreme environments of ultra-low temperatures, high magnetic field as well as high pressure. In this talk, I will give several examples of quantum materials problems where neutron scattering played a crucial role with some prospects for possible studies in molecular magnets in particular.

Condensed Matter
Thursday, April 19, 2018
11:00 AM
Physics Building, Room 313
Depei Zhang [Host: Seunghun Lee]
UVA - Department of Physics
"Crystal Structures and Photoluminescence of a Two-Dimensional Perovskite"
ABSTRACT:

Arguably the biggest challenge of the high-efficiency perovskite solar cells, such as CH3NH3PbI3 and CH(NH2)2PbI3, is their device instability. A recent study of 2D perovskite compounds, butylammonium methylammonium lead iodide perovskite, [CH3(CH2)3NH3]2(CH3NH3)n-1PbnI3n+1, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52%, without any obvious degradation over thousands of hours under standard light illumination and humidity test. This talk focuses on the study of temperature-dependent crystal structures, along with the photovoltaic properties of the 2D 1-layer (n = 1) perovskite material. We have performed elastic and inelastic neutron scattering, Raman scattering, and photoluminescence measurements on a powder sample of the 1-layer system ([CH3(CH2)3NH3]2PbI4). Our analysis of the data illuminates the evolution of the lattice structure, rotational and vibrational dynamics with temperature, and their connection to the charge carrier lifetime of the solar cell will be discussed.

Condensed Matter
Tuesday, April 24, 2018
11:00 AM
Physics Building, Room 313
Note special date.
Jing Luo [Host: Gia-Wei Chern]
UVA-Department of Physics
"Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice"
ABSTRACT:

We present an extensive numerical study of a new type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a cross-linking network of Kondo or double-exchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n = 1/2$ and 2/3. In particular, we show that an intriguing glassy magnetic state characterized by ordering wavevectors $\mathbf q = \langle \frac{1}{3},\frac{1}{3}, 1\rangle$ gives a rather satisfactory description of the low temperature phase recently observed in spinel~GeFe$_2$O$_4$.

Condensed Matter
Thursday, April 26, 2018
11:00 AM
Physics Building, Room 313
Timothy Halpin-Healy [Host: Genya Kolomeisky]
"Within & Beyond the Realm of KPZ"
ABSTRACT:

We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang,^1 a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as non-equilibrium stochastic growth, optimal paths in ill-condensed matter, as well as the extremal statistics of random matrices & increasing subsequences in random permutations.

1. J. Stat. Phys. 160, 794 (2015).

Condensed Matter
Thursday, April 26, 2018
3:30 PM
Physics Building, Room 313
Note special time.
Amr Ahmadain [Host: Israel Klich ]
UVA-Department of Physics
"Gravitational Weyl Anomalies in 1+1-Dimensional Lifshitz Field Theories"
ABSTRACT:

In this talk, I will explain the notion of anisotropic gravitational Weyl anomalies in Lifshitz field theories and their potential applications. Anomalies are quantum violations of classical symmetries, in this case under a non-relativistic (non-Lorentz-invariant) symmetry group. More specifically, I will focus on the only anomaly found in 1+1-dimensional Lifshitz field theories that arise after coupling to Newton-Cartan geometry with torsion, and it's relation to the Lorentz (diffeomorphism) anomaly in the quantum effective action of a 1+1 CFT. At the heart of this discussion, I will emphasize the role of temporal torsion in generating this anomaly and show how such an anomaly can be derived from a 2+1-dimensional Weyl-invariant Chern-Simons Horava-Lifshitz theory of gravity. I will finally comment briefly on how the 1+1 Lifshitz Weyl anomaly can potentially be connected to thermal Hall transport and some other potential applications.

To add a speaker, send an email to uc5j@Virginia.EDU Include the seminar type (e.g. Condensed Matter Seminars), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-speakers@Virginia.EDU.]