# Homepage

Welcome to the Kumah Synthesis and Characterization Laboratory at NC-State. The Kumah Research Group at NC State uses state-of-the-art atomic layer-by-layer deposition techniques including molecular beam epitaxy to fabricate thin crystalline oxide films. The group is focused on

• Understanding how atomic-scale structural distortions at interfaces can be manipulated to induce novel electronic and magnetic phenomena
• Developing pathways for harnessing these unique functionalities for electronic and energy applications.

Tools used by the group include atomic force microscopy, electron diffraction and synchrotron-based x-ray spectroscopy and diffraction.

## Lab News

September 2021: Congratulations to Aubrey Penn on successfully defending her Doctoral thesis!

September 2021: Two new papers posted to the ArXiV:

• T. Chen, K. Ahmadi-Majlan, Z. H. Lim, Z Zhang, J. H. Ngai, and D.P. Kumah. Effect of Buffer Termination on Intermixing and Conductivity in LaTiO3/SrTiO3 Heterostructures Integrated on Si(100) arXiv:2109.08586 (September 2021)
•   A. H. Al-Tawhid, J. Kanter, M. Hatefipour, D. L. Irving, D. P. Kumah, J. Shabani, K. Ahadi, Oxygen vacancy-induced anomalous Hall effect in a non-magnetic oxide . arXiv:2109.08073 ( September 2021)

July 2021: Congratulations to Tongjie Chen on successfully defending his Doctoral thesis!

May 2021: Collaborative paper with Ahadi group out in Applied Physics Letters.

• A.H. Al-Tawhid, D.P. Kumah,  K. Ahadi, Two-dimensional electron systems and interfacial coupling in LaCrO3/KTaO3 heterostructures, Appl. Phys. Lett. 118, 192905 (2021)

November 2020: Kumah presents at the 2020 National Society of Black Physicists Annual Meeting

November 2020: Congratulations! Athby Al-Tawhid successfully defends his doctoral thesis!

October 2020: Our new paper led by Sanaz Koohfar and NC-State undergraduates Yasemin Ozbek and Hayden Bland is available on the arXiv.

S. Koohfar, Y. Ozbek, H. S. Bland, Z. Zhang, D. P. Kumah, Interface-driven magnetic anisotropy in relaxed La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures on MgO. arXiv:2010.07180 (October 2020) J. Applied Physics 129, 055301 (2021);

August 2020: Collaborative work with the Sun Group (NCSU), Wei group (UNC) and the Zhi-Gang group published in Physical Review Materials.

Eric Vetter, Ian VonWald, Shijia Yang, Liang Yan, Sanaz KoohfarDivine Kumah, Zhi-Gang Yu, Wei You, and Dali Sun, Tuning of spin-orbit coupling in metal-free conjugated polymers by structural conformation, Phys. Rev. Materials 4, 085603, August 2020

July 2020: Kumah gives a colloquium at the 2020 Les Houches Summer School of Physics. The presentation is available online at https://youtu.be/5eBxNwr9E6s

February 2020: Sanaz’s paper demonstrating that ferromagnetism is robust in ultrathin LaSrMnO3 films under both tensile and compressive strain has been accepted for publication in Physical Review B (Phys. Rev. B 101, 064420 (2020)). A copy of the paper is available on the arxiv.

December 2019: Two papers accepted for publication. Former undergraduate group member Jacob Mauthe’s work on suspended crystalline oxides in collaboration with the Ngai group at University of Texas Arlington has been accepted for publication in the Journal of Vacuum Science and Technology.

• Z. H. Lim, M. Chrysler, A. Kumar, J. P. Mauthe, D.P. Kumah, C. Richardson, J. Lebeau, J. Ngai. Suspended single-crystalline oxide structures on silicon through wet-etch techniques: effects of oxygen vacancies and dislocations on etch rates. Journal of Vacuum Science & Technology A 38, 013406 (2020) (published online 27 December 2019)

November 11 2019: Sanaz’s recent paper is available on the Arxiv

• Sanaz Koohfar, Alexandru B. Georgescu, Ingrid Hallsteinsen, Ritesh Sachan, Manuel A. Roldan, Elke Arenholz, Divine P. Kumah, Effect of strain on magnetic and orbital ordering of LaSrCrO3/LaSrMnO3 heterostructures, arXiv:1911.03007. (November, 2019)

November 2019: Congratulations! Sanaz Koohfar successfully defends her doctoral thesis!

July 2019: Dr Kumah visits the University of Sao Paulo and the LNLS – Laboratório Nacional de Luz Síncrotron as part of the University Global Partnership Research Collaboration Fund program. A video of Dr Kumah’s colloquium at USP can be found here.

July 2019:

June 2019: Congratulations to Athby on passing his Doctoral Preliminary examination!

May 2019: Congrats! Sanaz’s paper on “Confinement of magnetism in atomically-thin La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures.” has been published in NPJ Quantum Materials 4, 25 (May 29 2019) [arXiv:1710.07592]

April 2019: Congrats! Tongjie successfully completes his PhD preliminary exam.

April 2019: Congrats! Sanaz gives a contributed talk at the 2019 Spring Materials Research Society meeting in Phoenix, AZ.

February 2019: Athby’s paper on tuning growth conditions to create a high mobility two-dimensional electron gas at the interface between insulating LaCrO3 and SrTiO3 is selected as an “Editor’s Pick’ in the Journal of Vacuum Science and Technology A. This work was performed in collaboration with the Dougherty Group.

Athby H. Al-Tawhid, J.R. Frick, D.B. Dougherty, D.P. KumahGrowth-Temperature Dependence of Conductivity at the LaCrO3/SrTiO3 (001) Interface.Journal of Vacuum Science & Technology A 37, 021102 (2019) (Selected as Editor’s Pick);

November 16th, 2018: Tongjie’s paper on understanding how the atomic structure at complex oxide-semiconductor interfaces are related to their electronic properties has been published in Applied Physics Letters. This work was in collaboration with the Kemper theory group at NC-State and the Ngai group at University of Texas, Arlington. Tongjie Chen, Kamyar Ahmadi-Majlan, Zheng Hui Lim, Zhan Zhang, Joseph H. Ngai, Alexander F. Kemper, Divine P. Kumah. Interfacial Structure of SrZrxTi(1-x)O3 films on Ge  arXiv:1806.11140 [cond-mat.mtrl-sci]Appl. Phys. Lett. 113, 201601 (November, 2018);

October, 2018: Dr Kumah gives invited talk in “Future Stars of AVS” session at the 2018 International American Vacuum Society Symposium in Long Beach, CA

July 30, 2018: Athby’s paper which demonstrates that under certain growth conditions, the LaCrO3/SrTiO3 system exhibits metallicity with electron mobilities as high as 10^4 /cm2.  Growth-Temperature Dependence of Conductivity at the LaCrO$_3$/SrTiO$_3$ (001) Interface.  A.H. Al-Tawhid, J.R. Frick, D.B. Dougherty, D.P. Kumah. http://arxiv.org/abs/1807.10845

June 2, 2018: New paper on arxiV: Tongjie Chen, Kamyar Ahmadi-Majlan, Zheng Hui Lim, Zhan Zhang, Joseph H. Ngai, Alexander F. Kemper, Divine P. KumahInterfacial Structure of SrZrxTi(1-x)O3 films on Ge  arXiv:1806.11140 [cond-mat.mtrl-sci]

May 2018: Dr Kumah gives invited talk at the 5th Workshop on Complex Oxides held in Capri, Italy

May 11th, 2018: Tongjie is co-author on paper, “Tuning metal-insulator behavior in LaTiO3/SrTiO3 heterostructures integrated directly on Si(100) through control of atomic layer thickness” ,Appl. Phys. Lett. 112, 193104 (2018);

December 14th, 2017: Tongjie leads lab tour for Carrboro High School visit.

December 13th, 2017: Congratulations! Sanaz passes her prelim.

October, 25,2017: Tongjie Chen is co-author on a paper “Atomic layer control of metal-insulator behavior in oxide quantum wells integrated directly on silicon” which is now available on the arxiv (https://arxiv.org/abs/1710.08597).

October, 22,2017: Sanaz Koohfar’s paper on solving a longstanding problem of confining magnetism in atomically-thin oxide films, is now available on the arxiv (https://arxiv.org/abs/1710.07592). Here we demonstrate achieving robust magnetism in LaSrMnO3 films as thin as 2 unit cells by atomically tuning the film’s interfaces using molecular beam epitaxy (MBE). These results have important implications for the use of atomic layers of materials for storing and processing information in spin-based devices.

July 31,2017: Sanaz’s paper on structural distortions at polar manganite interfaces published in Physical Review B  (Phys. Rev. B 96, 024108 ).

La(1-x)SrxMnO3 films have unique magnetic and transport properties which make them an excellent candidate for spintronic devices. For decades, it has been known that, their unique properties are lost when films of this material are fabricate with thicknesses below 4-10 unit cell (1 unit cell ~ 0.4 nm). Using synchrotron X-ray diffraction experiments, we find that the structures of thin films of LSMO are not uniform and comprise of 3 distinct regions: I) A top region extending 2-3 unit cells with strong polar distortions, 2) a middle region with no distortions and 3) an interfacial region 2-3 unit cells thick where material exchange across the film-substrate interface leads to strong changes in the LSMO composition in this region. We find that the distortions are driven by polar discontinuities at the LSMO interfaces and may be responsible for the observed thickness-dependent magnetic and electronic transitions. Identifying and eliminating these distortions is key to the development of the next generation of nano-scale devices for data storage and processing.

April 22,2017: Sanaz’s leads Middle School lab tours as part of NC-State’s Nanodays outreach event

April 15,2017: Sanaz’s paper on structural distortions at polar manganite interfaces available on arXiv (http://arxiv.org/abs/1704.05114).

March, 2017: Atomic structures of LSMO films grown in the KumahLab measured at Beamline 33ID at the Advanced Photon Source.

October 5, 2016: Tongjie and Sanaz present at the Triangle Student Research Competition, Triangle Research Park, NC

August 2, 2016: Undergraduate Researcher, Jacob Mauthe presents work on SrTiO3 characterization at the 2016 NC-State Summer Undergraduate Research Symposium