Quantum Materials Seminar

semester, 2017


Tuesday, January 31st 2017
4:30 pm:
There will be no seminar this week.

Tuesday, February 7th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, February 14th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, February 21st 2017
4:30 pm:
There will be no seminar this week.

Wednesday, March 1st 2017
11:00 am:
Speaker: Maria Navarro Gastiasoro, UMN
Subject: Emergent disorder phenomena in correlated Fe-based superconductors
Note change of time and day for the seminar.

The fundamental pairing mechanism causing high-T superconductivity in Fe-based superconductors remains controversial. Superconductivity is only one of several phases exhibited by these materials, and it is widely believed that the mechanism responsible for pairing may be closely linked to the existence of other proximate ordered phases. Most of these materials are obtained by chemical doping, which besides extra carriers, introduces disorder in the system. Therefore, understanding the role of these impurities is crucial and it has been a main focus of our work.

Faculty Host: Rafael Fernandes

Monday, March 6th 2017
Speaker: Lior Kornblum, Technion - Israel Institute of Technology
Subject: When Functional Oxides Meet Semiconductors
Note: Change of date and time of seminar.

Perovskite oxides of transition metals offer scientists and engineers a theme park of exciting and often unexpected physics, with complex structure-property relations and a wide scope of useful phenomena. Advances in epitaxial growth techniques bring unprecedented possibilities of atomic engineering of these oxides and their interfaces, with exciting prospects of unravelling their underlying physics and harnessing it towards useful applications. However, fundamental material challenges inhibit the introduction of these materials onto semiconductors and thus into the microelectronics technology.

The potential of the oxides’ functionalities to evolve out of the labs and into technology has received a considerable boost with the pioneering of epitaxial growth approaches for perovskites directly on semiconductors. This combination opens a route to bridge between oxide functionalities and microelectronics. Moreover, interface engineering of oxides on semiconductors can lead to new functionalities, made possible by the coupling between the dissimilar materials.

The challenges and the opportunities of oxide-semiconductor integration will be demonstrated with two examples. First, results on integrating oxide interfaces that exhibit 2D electron gas (2DEG) with semiconductors will be presented, and their challenges and opportunities discussed. The second example will revolve around the integration of epitaxial oxides with semiconductors for renewable energy. The ability to synergistically combine the advantages of these two classes of materials, while mitigating their deficiencies, is put to the test in solar water splitting into hydrogen fuel. Detailed analysis of the device energetic structure highlights the advantages of this combination, and charts a clear route for improving the efficiencies using well-established semiconductor technology.

With these examples I hope to provide a glimpse into the prospects of combining functional oxides with semiconductors, and illustrate the promise of combining engineering and physics towards creating new devices and future technologies.

Faculty Host: Bharat Jalan

Tuesday, March 7th 2017
3:30 pm:
Speaker: Damjan Pelc, University of Zagreb
Subject: Percolative aspects of cuprate superconductivity

One of the central problems in the physics of the cuprate high-temperature superconductors is understanding the superconducting emergence regime above the macroscopic critical temperature Tc. The emergence regime provides crucial information about both the normal and superconducting states and has long been the subject of controversy. Experimentally, it is difficult to separate the nascent superconducting response from the complex normal-state behavior, with different experimental probes leading to disparate conclusions. We present a systematic investigation of the emergence of superconductivity in the cuprates using an unconventional probe: nonlinear conductivity. This probe eliminates background subtraction problems because the signal vanishes in the normal state. Through experiments on several cuprate families and as a function of doping, we show that the emergence regime is universally confined to a narrow temperature range, but incompatible with standard Ginzburg-Landau theory. Instead, a single characteristic temperature scale T0 controls superconductivity emergence. To explain the experiments, we introduce a simple superconducting percolation model based on local gap disorder, which provides a quantitative description of our measurements and explains several puzzling prior results. The success of the percolation model shows that intrinsic disorder plays an important role in cuprate superconductivity, enabling us to create an overarching picture of the charge-carrier behavior in the cuprates.

Faculty Host: Martin Greven

Tuesday, March 21st 2017
4:30 pm:
There will be no seminar this week.

Tuesday, March 28th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, April 11th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, April 18th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, April 25th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, May 2nd 2017
4:30 pm:
There will be no seminar this week.

Tuesday, September 12th 2017
4:30 pm:
To be announced.

Tuesday, September 19th 2017
4:30 pm:
There will be no seminar this week.

Tuesday, September 26th 2017
4:30 pm:
See Joint Quantum Materials & Condensed Matter Seminar on Wednesday this week only.

Tuesday, October 3rd 2017
4:30 pm:
Speaker: Eric McCalla, Chemical Engineering and Material Sciences
Subject: A gap in the literature: heat capacity in Nb-doped SrTiO3

Tuesday, October 17th 2017
4:30 pm:
See Joint Quantum Materials & Condensed Matter Seminar on Thursday this week only.

Wednesday, November 1st 2017
10:00 am:
Speaker: Juergen Haase, Felix Bloch Institute for Solid State Physics, University of Leipzig
Subject: Emergence of a new interpretation of NMR of cuprates superconductors

As a fundamental, local, bulk probe nuclear magnetic resonance (NMR) is expected to be at the center of the discussion of the properties of cuprate superconductors. Indeed, in addition to profound insight into chemical structure and bonding, NMR provided vital clues about the electronic spin susceptibility, based on the magnetic hyperfine interaction: the pseudogap (spin gap), single-fluid physics, and spin-singlet pairing were discovered by NMR. However, the data did not appear to contain necessary details for theory. The less often discussed electric hyperfine interaction served a similar chemical purpose, but its understanding in terms of planar charge remained difficult. Here, early on, NMR found that the stoichiometric compounds (Y-1237, Y-1248) appeared to be very homogeneous, as very narrow NMR lines for Cu and O in the CuO_2 plane seemed to prove very small spatial variations of charge. However, all the other materials exhibit mostly broad featureless NMR resonances, indicative rather strong electric field variations in the CuO_2 plane. This conundrum was often considered as proof that charge ordering - apparently not ubiquitous - must be due to chemical inhomogeneity from doping and other crystal imperfections.

Recently, with a number of experiments on different materials we established that a single spin component is not able to explain the NMR shifts, pointing to a different magnetic hyperfine scenario. Very recently, we compiled all literature NMR shift data for planar Cu and with simple plots it becomes already obvious that the hitherto adopted NMR interpretation is wrong, e.g., the magnetic hyperfine scenario is inappropriate. Also recently, we showed that the charges in the plane can be quantified with NMR, which led to the discovery that the sharing of holes between Cu and O (not the doping) is responsible for various cuprate properties, e.g., the maximum T_c . In another set of challenging experiments we just completed a fundamental proof that shows that the above mentioned ‘homogeneous’ materials with sharp NMR lines are in fact highly charge ordered systems, with the order responding to pressure, temperature, and magnetic field. This charge ordering which is ubiquitous to the Y-based systems is likely to be ubiquitous to the CuO_2 plane of all cuprates as it would solve the above mentioned conundrum.

Thus, we view our findings - that will be discussed in more detail - as the emergence of a new interpretation of cuprate NMR, which must have fundamental impact on the understanding of these materials.

Faculty Host: Martin Greven

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