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Department of Physics and Astronomy

CMP Seminars and Journal Club

Our meetings are usually on Fridays at 11:00 am in Sumwalt 102. You can find abstracts of the previous talks of the semester at the bottom of this page.
Looking forward to seeing you all!

Most Recent Seminar (Spring 2024):

Observation of phase transitions in IrTe2 using second harmonic generation

Speaker: Govinda Kharal
Date and time: February 23th, 11 am
Abstract: IrTe2 exhibits a trigonal structure at room temperature and experiences a first-order structural transition, with the transition temperature determined by whether the material is cooling or warming. We employed the second harmonic generation (SHG) technique to investigate single crystals of IrTe2 in a temperature range spanning from 7 K to 295 K. Using this method, we observed alterations in surface symmetries consistent with the structural phase shifts at approximately 270 K and 165 K, as previously reported in low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and SHG measurements conducted at near room temperature. Our SHG polarimetry measurements revealed that the (001) plane of IrTe2 maintains a trigonal (3m) symmetry above 270 K and then undergoes a first-order phase transition to a triclinic (1) symmetry below 270 K. Nevertheless, the 3m symmetry persists until about 165 K. Notably, we observed the emergence of a 3m symmetry at 7.2 K which was not documented in prior studies at lower temperatures. This finding supports an earlier STM measurement suggesting that the hexagonal phase with the trigonal symmetry may be related to the superconducting phase.

On thermoelectric properties of SnSe and SnSe2 single crystals

Speaker: Abhinna Rajbanshi
Date and time:
February 23th, 11 am
Abstract:
Thermolelectric materials can serve for conversion between thermal and electrical energy. In the search for new thermoelectric materials, layered SnSe and SnSe2 are promising candidates. We have successfully synthesized SnSe and SnSe2 single crystals by the modified Bridgman method and studied their thermoelectric properties: thermopower (S), thermal conductivity (κ), and electrical conductivity (σ) in the temperature range between 2 K and 400 K. Compared to SnSe2 at 300 K, we find that SnSe exhibits higher electrical conductivity, higher thermopower, and lower thermal conductivity, thus resulting in the higher figure of merit. Hall effect measurements reveals that the Hall mobility in SnSe is an order higher than that in SnSe2, advancing its thermoelectric performance.

Spin transfer torque in F/F devices with non-identical half-metallic layers

Speaker: Volodymyr Shablenko
Date and time:
February 23th, 11 am
Abstract:
Using half-metals (e.g., Heusler alloys) as ferromagnetic elements in spintronic devices sometimes leads to unexpected results. While using identical magnetic materials for both fixed and free layers already generate some interesting properties [1], it may be challenging to implement magnetization switching in such system experimentally because the boundary between materials is not well defined in their parallel state. Moreover, materials’ symmetry cancels the linear term in voltage dependence of the out-of-plane torque. In this study, we calculate electric and spin currents using a Stoner model description of half metals and find the out-of-plane torque and exchange energy of an F/F device. Voltage dependence of the out-of-plane torque is calculated as well. The lack of material’s symmetry generates additional anisotropy in this torque’s angular dependence.


Past Seminars:

Speaker: Daniel Duong
Date and time: February 16th, 11 am
Abstract: Uranium-based compounds are an active source of interest due to the wide variety of exotic states that they can host. UCu0.6Bi2 orders antiferromagnetically below TN ≈ 54 K. For T < TN, there is a first-order metamagnetic transition under the application of magnetic field. The temperature and field-orientation dependence of the transition have been mapped using magnetization and MHz susceptibility measurements in fields of up to 60 T and for temperatures down to 0.65 K. Unusually large hysteresis is observed at the transition under certain conditions; the possible origin of this and other features will be discussed.

Speaker: Govinda Kharal
Date and time: February 16th, 11 am
Abstract: IrTe2 exhibits a trigonal structure at room temperature and experiences a first-order structural transition, with the transition temperature determined by whether the material is cooling or warming. We employed the second harmonic generation (SHG) technique to investigate single crystals of IrTe2 in a temperature range spanning from 7 K to 295 K. Using this method, we observed alterations in surface symmetries consistent with the structural phase shifts at approximately 270 K and 165 K, as previously reported in low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and SHG measurements conducted at near room temperature. Our SHG polarimetry measurements revealed that the (001) plane of IrTe2 maintains a trigonal (3m) symmetry above 270 K and then undergoes a first-order phase transition to a triclinic (1) symmetry below 270 K. Nevertheless, the 3m symmetry persists until about 165 K. Notably, we observed the emergence of a 3m symmetry at 7.2 K which was not documented in prior studies at lower temperatures. This finding supports an earlier STM measurement suggesting that the hexagonal phase with the trigonal symmetry may be related to the superconducting phase.

Speaker: Abhinna Rajbanshi
Date and time: February 16th, 11 am
Abstract: Thermolelectric materials can serve for conversion between thermal and electrical energy. In the search for new thermoelectric materials, layered SnSe and SnSe2 are promising candidates. We have successfully synthesized SnSe and SnSe2 single crystals by the modified Bridgman method and studied their thermoelectric properties: thermopower (S), thermal conductivity (κ), and electrical conductivity (σ) in the temperature range between 2 K and 400 K. Compared to SnSe2 at 300 K, we find that SnSe exhibits higher electrical conductivity, higher thermopower, and lower thermal conductivity, thus resulting in the higher figure of merit. Hall effect measurements reveals that the Hall mobility in SnSe is an order higher than that in SnSe2, advancing its thermoelectric performance.

Speaker: Dr. Bryan Chavez
Date and time: February 9th, 11 am
Abstract: Second harmonic generation (SHG) has proven to be an invaluable tool for investigating the structural, electrical, and magnetic properties of materials. The application of SHG has become a key area of research, with recent work in the field of quantum materials, such as with our group’s work on PtBi2, IrTe2, and BaMnSb2. SHG comprises three components: electric dipole (ED), magnetic dipole (MD), and electric quadrupole (EQ). In particular, magnetization-induced SHG (MSHG) has emerged as a powerful tool for studying the symmetry and magnetization of antiferromagnetic (AFM) and ferro/ferrimagnetic (FM) systems. While the three types of SHG have been studied independently, materials typically do not exhibit only one type. Therefore, the analysis of materials with combined SHG remains an active area of study, especially for newer materials like magnetic Weyl semimetals, which can be challenging to analyze due to interference terms arising from the combination of ED, MD, and EQ SHG. Recent studies have attempted to bridge this knowledge gap and better classify the magnetic component. In this talk, the impact of a sample's magnetic ordering (AFM or FM) on the SHG response will be discussed, with a focus on the fingerprints in SHG.

Speaker: Caleb Duff
Date and time: February 2nd, 11 am
Abstract: Structured light is a key component of many modern applications, ranging from super-resolution microscopy to imaging, sensing and quantum information processing. As the utilization of these powerful tools continues to spread, the demand for technologies that enable the spatial manipulation of fundamental properties of light, such as amplitude, phase and polarization grows further. The rapidly advancing field of reconfigurable integrated photonics allows entirely new routes towards beam shaping that not only outperform existing devices in terms of speed but also have substantial potential with respect to their footprints, robustness and conversion efficiencies. In this study, we demonstrate how a multipurpose programmable integrated photonic processor can generate and control a wide range of higher-order free-space structured light beams, all starting from only a single injection waveguide. Our method controls the distribution of light’s amplitude and phase within sub-milliseconds, and it is fully reconfigurable and has no moving parts. These advancements broaden the spectrum of potential methods, applications and devices that utilize spatially tailored light by providing a pathway to combine the strengths and versatility of integrated photonics and free-space structured light.

Speaker: Dan Hancock
Date and time: January 26, 11 am
Abstract: In May 2023, superconductivity was found in La3Ni2O7 at  around 80K, providing the first instance of high Tc superconductivity in nickelates. With this evidence, a deeper search for superconductivity in the La-Ni-O system began. This paper looks at the effects of Pr-doped polycrystalline samples of the bilayer La3Ni2O7. The goal is to broaden the scope of nickelate superconductors and provide insight into the mechanisms of superconductivity. Researchers found superconductivity in Pr-doped samples that occurs in the I4/mmm space group, which differs from the initially proposed superconducting space group of La3Ni2O7 (Fmmm). Recent research (http://arxiv.org/abs/2311.09186) proposes that the superconducting state of La3Ni2O7 might in fact be in the I4/mmm space group, which matches the superconducting phase of Pr-doped samples. In the Pr-doped samples, researchers found superconductivity to begin at 10 GPa, distinctively lower than that of La3Ni2O7 with a maximum onset of superconductivity at 78K under pressure of 15GPa. This new information provides insight into the scope of superconducting nickelates.

Article link

Speaker: Siavash Karbasizadeh
Date and time: January 19, 11am
Abstract: In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ScV6Sn6 was discovered to undergo a first-order structural phase transition with the formation of a √3 × √3 × 3 CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of the first-order type, which distinguishes it from conventional density-wave type condensations. Our results provide an insight into the origin of the structural phase transition in this new and unique kagome lattice intermetallic material.

Article link

Speaker: Paras Regmi
Date and time: January 12, 11 am
Abstract: In the realm of topological semimetals, the exploration of the intriguing phenomenon of topological Hall effect (THE) emerges from the nontrivial magnetic texture in real space, characterized by a non-zero scalar spin chirality. This paper reports a large THE in the antiferromagnetic (AFM) state of a Dirac semimetal EuAgAs. The intrinsic contribution of anomalous Hall effect (AHE) is considered while modelling AHE for data analysis. EuAgAs has AFM ground state below TN. The first principal calculation reveals that the material exhibits Dirac semimetallic ground state.  Subsequently, upon the application of external magnetic field, the transition from Dirac semimetallic state to Weyl semimetallic state is observed. The presence of band structure anomalies like Weyl nodes could also contribute to the Hall response observed in the material.

Article link

 


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