Lifshitz Transition at Critical Substitution in the Nematic Pnictide Ba1 – xSrxNi2As2 and Unveiling the Operating Principles of Volume Vacancy Random Access Memory

The physics of correlated electrons in condensed matter is a rich topic of study, where a hostof unique phases of matter are ripe for investigation. This field of study has important implications for fundamental science, as well as significant connections to cutting-edge applications. In this thesis, I will detail three projects in this field. The first is a fundamental study of the electronic structure of Ba1 &ndash; xSrxNi2As2 , an analogue of the famous iron-based superconductor BaFe2As2 . Through a combination of high resolution Angle-Resolved Photoemission Spectroscopy (ARPES) and detailed density functional theory (DFT) calculations, I will show evidence for a Lifshitz transition as a function of Sr content, which has implications for the enhancement of nematic quantum critical fluctuations, and the enhancement of superconductivity. Next I will discuss two projects concerning the development of novel memory technologies. Correlated Electron Random Access Memory (CERAM) is a new memory concept that, in theory, undergoes a controlled metal- insulator transition driven by the Mott physics in carbon doped NiO. Building off of CERAM, I will also detail my work on another novel memory technology called Volume Vacancy Random Access Memory (VVRAM).</p

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling.

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena

Observation of Topological Surface State in High Temperature Superconductor MgB2

The hunt for the benchmark topological superconductor (TSc) has been an extremely active research subject in condensed matter research, with quite a few candidates identified or proposed. However, low transition temperatures (Tc) and/or strong sensitivity to disorder and dopant levels in known TSc candidates have greatly hampered progress in this field. Here, we use Angle-resolved Photoemission Spectroscopy (ARPES) to show the presence of Dirac Nodal Lines (DNLs) and the corresponding topological surface states (TSS's) on the [010] faces of the Tc=39K s-wave BCS superconductor MgB2. Not only is this nearly triple the current record of superconducting Tc among all candidate TSc's, but the nature of these DNL states should make them highly tolerant against disorder and inadvertent doping variations. This makes MgB2 a promising high temperature platform for the study of topological superconductivity

Universal Non-Polar Switching in Carbon-doped Transition Metal Oxides (TMOs) and Post TMOs

Transition metal oxides (TMOs) and post-TMOs (PTMOs), when doped with Carbon, show non-volatile current-voltage (I-V) characteristics, which are both universal and repeatable. We have shown spectroscopic evidence of the introduction of carbon-based impurity states inside the existing larger bandgap effectively creating a smaller bandgap which we suggest could enable Mott-like correlation effect. Our findings indicate new insights for yet to be understood unipolar and nonpolar resistive switching in the TMOs and PTMOs. We have shown that device switching is not thermal-energy dependent and have developed an electronic-dominated switching model that allows for the extreme temperature operation (from 1.5 K to 423 K) and state retention up to 673 K for a 1-hour bake. Importantly, we have optimized the technology in an industrial process and demonstrated integrated 1-transistor/1-resistor (1T1R) arrays up to 1 kbit with 47 nm devices on 300 mm wafers for advanced node CMOS-compatible correlated electron RAM (CeRAM). These devices are shown to operate with 2 ns write pulses and retain the memory states up to 200 C for 24 hours. The collection of attributes shown, including scalability to state-of-the-art dimensions, non-volatile operation to extreme low and high temperatures, fast write, and reduced stochasticity as compared to filamentary memories such as ReRAMs show the potential for a highly capable two-terminal back-end-of-line non-volatile memory.Comment: 28 pages, 17 figures, accepted in APL Material

Experimental electronic structure of the electrically switchable antiferromagnet CuMnAs

Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable N\'eel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of $\approx-390$ meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. Additionally, 2x1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.Comment: Submitted to Physical Review X. 20 pages. 9 figure

Latarjet operation carries three times the risk of failure in seizure versus non-seizure recurrent anterior dislocation of the shoulder joint: outcome of a systematic review with meta-analysis

Background Recurrent anterior shoulder dislocation (RASD) in cases of seizure disorders (SDs) total 50%–80% of all SD-associated shoulder instabilities. Based on the extent of bone loss, treatment options include bony and soft-tissue reconstructions, arthroplasty, and arthrodesis. The primary objective of this paper was to review the treatment options for RASD in SDs. Methods Several bibliographic databases were searched for RASD treatment options in SD patients. The demographic outcome measures, the failure rate (defined as the relative risk of recurrence of dislocation postoperation), and the postoperative seizure recurrence rate were recorded. Results We pooled 171 cases (187 shoulders) from 11 studies. Of these, one, five, two, two, and one reports studied Bankart's operation with remplissage (27 cases/29 shoulders), the Latarjet procedure (106/118), bone block operation (21/23), arthroplasty (11/11), and arthrodesis (6/6), respectively, in treating SD-associated RASD. The relative risk of failure between SD and non-SD patients was 3.76 (1.36–10.38) after the Latarjet operation. The failure rates were 17% and 13% for Bankart's operation with remplissage and the Latarjet procedure in SD patients, respectively, but 0% each for bone block operation, arthroplasty, and arthrodesis. The total rate of seizure recurrence after operation was 33% of the pooled cases. Conclusions SD recurrence in the postoperative period, the size of the bone block, and the muscular attachments to a small coracoid autograft are the determinants of failure among various reconstructive operations in SD-associated RASD. Level of evidence III

Free fibula flap for lower limb salvage after tumour resection

Context: Post-tumour resection lower limb salvage. Aim/Introduction: Resection of tumours of the femur and tibia around the knee and ankle joints results in large bony defects. Often arthrodesis is an alternative; in case, adequate functional motors cannot be preserved or due to economic constraints. Thus, in an immunocompromised patient, the vascularised fibula is the best form of reconstruction. The vascularised fibular flap (pedicled/free) can be used in combination with an allograft. We refer to such a combination reconstruction as ‘allocombo’. The vascularised fibular graft hypertrophies in due course of time, and till that period, the allograft provides the required mechanical strength to allow early ambulation. Subjects and Methods: A retrospective study of 24 cases of vascularised fibular graft for lower limb reconstruction was conducted from February 2003 to March 2014. The average defect size was 15.5 cm and the average length of fibula harvested was 24.35 cm. A total of 19 free fibular flaps and 5 pedicled fibula were done. Mean age was 26 years. Fibula was nestled in the allograft obtained from the tissue bank. Results: The mean follow-up time was 52 months. Free flap success rate was 96%. Successful healing was achieved at 45 ends (97.8%). Radiological evidence of union at osteotomy sites occurred at an average of 6.8 months. Eight patients eventually succumbed to disease. At the final follow-up, the mean Musculoskeletal Tumour Society functional score of the evaluable patients was 26 (range 20–30). Conclusions: Pedicled fibula is a good option if the defect is within 14 cm of the knee joint at the femoral end. The vessels have to curve around the fibular head, thus its removal improves the reach of the pedicle. The flap is easy to harvest with predictable vascular anatomy and it can provide a large amount of vascularised bone and skin paddle. It results in early ambulation, rehabilitation and reduced morbidity. We realised that fixation is easier and chances of vascular injury are less in free as compared to pedicled fibula

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling.

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena