Quantum Materials Seminar

semester, 2018

Tuesday, January 30th 2018
3:30 pm:
Speaker: Damjan Pelc, University of Minnesota
Subject: New ways to learn about quantum materials
Refreshments will be served at the meeting so don't forget to bring your favorite mug.

I will discuss two unusual experimental techniques currently being developed for the study of quantum materials: nonlinear magnetic response and uniaxial pressure. They can be used to detect and modify the fundamental symmetries of materials, with great potential for new insights. In particular, I will examine the symmetry properties of nonlinear magnetic susceptibility, with applications in the study of chiral superconductors. Furthermore, I will present a novel approach to uniaxial pressure experiments, which can easily be combined with various other techniques to investigate the effects of broken structural symmetries.

Tuesday, February 13th 2018
3:30 pm:
Speaker: Maria Navarro Gastiasoro, University of Minnesota
Subject: "Superconductivity in Strontium Titanate"

Tuesday, February 27th 2018
3:30 pm:
Speaker: Samuel Lederer, MIT
Subject: High temperature superconductivity and strange metal behavior near a metallic quantum critical point

It has long been conjectured that quantum critical points (QCPs) are at the root of some of the most fascinating phenomena in the solid state, including the high temperature superconductivity and “strange metal” behavior of cuprate superconductors. Though much progress has been made in the theory of QCPs, those which occur in metals (and are likely relevant to the high temperature superconductors) are still poorly understood despite more than four decades of effort. Using Quantum Monte Carlo techniques, my collaborators and I have performed the first numerically exact simulations of a model which realizes a metallic QCP towards an Ising nematic ordered phase. I will discuss our results, which include numerous phenomena already observed in experiment, and comment on future avenues towards a solution of this difficult and rich problem in quantum statistical mechanics.

Faculty Host: Rafael Fernandes

Tuesday, March 20th 2018
3:30 pm:
Speaker: Laxman Raju Thoutam and Sajna Hameed.
Subject: Novel Electronic and Magnetic Properties in Rare-Earth Titanates: YTiO3 & NdTiO3.

Tuesday, April 3rd 2018
3:30 pm:
Speaker: John Dewey and Joseph Batley, Chemical Engineering and Material Science
Subject: Integration of perovskites into non-local spin valves for the study of spin transport in oxides“

Friday, April 6th 2018
3:35 pm:
Speaker: Kenji Ishii, Synchrotron Radiation Research Center (SPring-8), Japan
Subject: Momentum-resolved charge and spin excitations in cuprate superconductors

In the last two decades, energy resolution of resonant inelastic x-ray scattering (RIXS) has been significantly improved, and RIXS is now established as a momentum-resolved spectroscopy using synchrotron radiation x-rays. Since the beginning of RIXS, cuprate superconductors have been intensively studied. In addition to the interest of superconductivity at the high transition temperature and related phenomena, such as pseudogap and a competing phase with charge order, doped cuprates are important and suitable for the study of the electronic structure of the doped Mott insulator. This is because relatively simple theoretical models with a few orbitals are applicable to describe the electronic structure near the Fermi energy. I will present charge and spin excitations in cuprates observed with RIXS at Cu K-, Cu L3- and O K-edges focusing on the energy range from eV to sub-eV. In this range, charge and spin excitations show characteristic momentum dependence with respective energy scale of hopping energy of electron (t) and exchange interaction of spin (J).

Faculty Host: Martin Greven

Tuesday, April 17th 2018
3:30 pm:
Speaker: Zach Anderson and Yang Tang, University of Minnesota
Subject: Doping dependence of the antiferromagnetic response of HgBa2CuO4+δ

Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and of the pseudogap phenomena exhibited by the cuprates. Neutron scattering measurements of the antiferromagnetic response have been reported for a number of cuprates, but our study of structurally simple HgBa2CuO4+δ (Hg1201) shows several features distinct from what has been found in other materials. In particular, at most dopings and temperatures we see a gapped Y-shaped magnetic dispersion in contrast to the X-shaped response seen in other cuprates. We will discuss our ongoing efforts to understand the doping dependence of the magnetic spectrum in Hg1201.

Friday, April 27th 2018
2:00 pm:
Speaker: Masaaki Matsuda, ORNL
Subject: Magnetic correlations in the vicinity of the superconducting state in CrAs and MnP
Please note time and date change for this seminar. This week only.

CrAs and MnP exhibit superconductivity under pressure with a maximum transition temperature of ~2 K at 2 GPa and ~1 K at 8 GPa, respectively. Since Cr and Mn have the spin degree of freedom, elucidating the magnetic contribution to the superconductivity is crucial to understand the pairing mechanism. A helical structure is the magnetic ground state at ambient pressure in both materials. We performed neutron scattering studies in both materials under pressure. With applying pressure, the helicity and magnetic moment of the helical structure gradually change in CrAs [1], whereas MnP shows a more complicated phase diagram [2]. Most importantly, we found that both materials show helical structure in the vicinity of the superconducting phase, although the directions of the propagation vectors are different. We also studied the chemical pressure effect on static and dynamic magnetic properties in CrAs [1]. The results suggest a coupling between the magnetism and the superconductivity.

[1] M. Matsuda et al., submitted to PRX.
[2] M. Matsuda et al., Phys. Rev. B 93, 100405(R) (2016).

Tuesday, September 4th 2018
1:25 pm:
Speaker: Nikolaos Biniskos, Jülich Centre for Neutron Science JCNS
Subject: Structural, magnetic and electrical properties of La1-xBixMnO3+δ (δ>0) perovskite compounds
Faculty Host: Martin Greven

Wednesday, September 5th 2018
1:25 pm:
Speaker: Nikolaos Biniskos, Jülich Centre for Neutron Science JCNS
Subject: : Inelastic neutron scattering investigations in the series Mn5-xFexSi3

The magnetocaloric cooling process is based on the magnetocaloric effect (MCE) where entropy changes of a magnetic material in an applied magnetic field are tied to adiabatic changes in temperature. An entropy transfer between crystal lattice and the magnetic spin system has to take place. A large MCE at room temperature and low magnetic field for a material with abundant and environmentally friendly elements opens the way for magnetic cooling devices. The MCE potentially occurs in any magnetic ordering process and inelastic neutron scattering (INS) that microscopically probes the magnetization dynamics is a key tool to tackle the question of the ingredients that favor large MCE.

Mn5-xFexSi3 compounds are showing a moderate MCE (2 to 4 J/kg K depending on x) at low magnetic field change from 0T to 2T, which is promising for magnetic refrigeration applications [2]. The ferromagnetic compound MnFe4Si3 has a magnetic phase transition at about 300K. The magnetic excitation spectrum of the magnetocaloric compound MnFe4Si3 has been investigated by means of polarized and unpolarized INS on single crystals. Spectra were collected in the FM phase (TC ≈ 305 K), as well as in the paramagnetic state, in order to understand the nature of the magnetism in MnFe4Si3. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the q-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of kBTC pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin fluctuations are found to be isotropic around TC and can be suppressed by a magnetic field of 2 T [3].

The parent compound Mn5Si3 undergoes two antiferromagnetic transitions at TN1=66K (AF1) and TN2=99K (AF2). Experiments with unpolarized INS in the paramagnetic (PM) state and in the AF2 and AF1 phases revealed that AF1 is characterized by sharp spin-waves, but AF2 is characterized by a diffuse signal that resembles the one of the PM state, indicating strong spin fluctuations [4]. These fluctuations may play an essential role in the MCE.

[1] O. Tegus et al., Nature 415 (2002), 150.
[2] Songlin et al., J. Alloys Compounds, 334 (2002), 249–252.
[3] N. Biniskos et al.,. Phys. Rev. B 96, 104407 (2017).
[4] N. Biniskos et al.,. Phys. Rev. Lett. 120, 257205 (2018).

Faculty Host: Martin Greven

Wednesday, September 26th 2018
Speaker: Inna Vishik, UC Davis
Subject: ARPES studies of the model cuprate HgBa2CuO4+d (Hg1201)

The mechanism of high temperature superconductivity in cuprates is one of the biggest unsettled questions in physics, and big stumbling block in this mature field is the question of universal vs materials-dependent or technique-dependent behavior. HgBa2CuO4+ (Hg1201) is considered to be a model cuprate because it has a structurally-simple crystal structure and can have a relatively long electron mean free path, and as such, it is well-characterized by transport and scattering experiments. However, few angle-resolved photoemission spectroscopy (ARPES) experiments have been performed on this material thus far, even though this technique is instrumental in highlighting critical momentum-space anisotropies in crystalline solids. I will present recent ARPES results on Hg1201, which shed light on single-particle scattering processes and electron-boson coupling, and together with a variety of complementary probes can form a coherent experimental description of a model cuprate.

Faculty Host: Martin Greven

Wednesday, October 17th 2018
Speaker: Raymond Osborn,Materials Science Division, Argonne National Laboratory
Subject: Imaging Nanoscale Disorder in Reciprocal Space

Correlated defects are responsible for the functional properties of many materials that underpin energy-related technologies. Single-crystal diffuse scattering using x-rays or neutrons is a powerful probe of short-range order in crystalline lattices, but its use has been limited by the experimental challenge of collecting data over a sufficiently large volume of reciprocal space and the theoretical challenge of modeling the results. However, instrumental and computational advances at both x-ray and neutron sources now allow the efficient measurement and rapid transformation of reciprocal space data into three-dimensional pair distribution functions, providing model-independent images of nanoscale disorder in real space. By eliminating Bragg peaks before the transformation, 3D-∆PDF measurements image defect-defect correlations directly, displaying only the probabilities of interatomic vectors that deviate from the average structure. I will give examples of the use of this method to probe the structure and correlation length of order-disorder transitions in intercalation compounds, the length scale and dimensionality of nematic correlations in iron arsenides, and the defect correlations in a superionic thermoelectric.
This work was supported by the U.S. Department of Energy, Materials Science and Engineering Division.

Faculty Host: Martin Greven

Wednesday, November 14th 2018
Speaker: Mathias Scheurer, Harvard University
Subject: Gauge theories of fluctuating antiferromagnetism for the cuprate superconductors

One of the most mysterious phases of the cuprate high-temperature superconductors is the “pseudogap” state: despite its Fermi-liquid-like electrical transport properties, the size of the Fermi surface is smaller than that predicted by the Luttinger theorem of Fermi liquid theory. In this talk, I will discuss gauge theories of doped fluctuating antiferromagnets that we propose as effective field theories for the pseudogap phase. By virtue of exhibiting “topological order”, these theories allow to circumvent Luttinger’s theorem while exhibiting the charge transport of a Fermi liquid. The presentation will focus on a direct comparison of predictions of these gauge theories with both numerical studies of the strongly coupled Hubbard model and high-resolution photoemission data. The good agreement can be seen as at least indirect evidence for topological order in the phase diagram of the cuprate superconductors. We will also discuss the additional symmetry breaking, such as the formation of charge-density-wave, nematic, or loop-current order, that can occur at low temperature and comment on the quantum critical point in our theory.

Faculty Host: Rafael Fernandes

Wednesday, December 5th 2018
Speaker: Damjan Pelc
Subject: Inhomogeneity in complex oxide superconductors

Some of the most studied unconventional superconductors, including the cuprate high-Tc materials, are oxides. In this talk, I will present some fresh insights into the physics of oxide superconductors, obtained with the help of two somewhat unusual experimental techniques - nonlinear magnetic response and uniaxial pressure. In particular, I will show that several very different complex oxides have the same superconducting precursor regime above the macroscopic transition temperature, Tc. This universal feature turns out to be related to subtle structural inhomogeneity shared by the different materials. Furthermore, we find that it is possible to manipulate this inhomogeneity by plastic deformation of the samples, in experiments that open up a new avenue in the study of unconventional superconductors. Finally, I will present some preliminary results on the intriguing effects of deformation and annealing on Tc in cuprates and strontium titanate.

Faculty Host: Martin Greven

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