No mundo todo universidades buscam obter maior impacto na ciência, na sociedade e na economia com suas pesquisas. Vamos discutir como, ao lado de buscarem excelentes alunos, professores e funcionários, universidades e suas unidades de ensino e pesquisa adotam estratégias para garantirem apoio institucional que defenda o tempo de seus pesquisadores contra atividades não acadêmicas e necessárias para a obtenção de excelência e impacto dos resultados da pesquisa. Além de oficinas, apoio técnico e administrativo e infraestruturas compartilhadas, tal apoio institucional inclui a atração e seleção de excelentes alunos, apoio para que os professores em início de carreira obtenham financiamentos externos e sejam apoiados nas opções de carreiras, facilitação de conexões com governos, empresas, e outras entidades para identificação de desafios em pesquisa e obtenção de financiamentos, e a visibilidade dos resultados de pesquisa para a sociedade. Também discutiremos como universidades acompanham e demonstram a excelência e o impacto de suas pesquisas.

In this seminar, I present the contributions made by Giorgio Parisi in disordered complex systems which granted to him the Nobel prize 2021. Indeed, the theory created by Parisi to deal with disordered complex systems has generated such an overpowering conceptual framework that it spilled over into other areas of knowledge as distinct as mathematics, biology, neuroscience, and computation. The starting point, according to Parisi himself [1], is that disorder generates complexity. In that sense, the spin-glass systems discovered in the mid of 70's last century served as the suitable platform to unveil this relationship between disorder and complexity. Therefore, I explain some experimental aspects of the spin-glass problem in order to situate the problem and the level of difficulty for its description. Next, I discuss theoretical attempts which, although failed to provide a satisfactory description of the original problem, introduced some fundamental technical aspects such as the replica method. In particular, it is reviewed the mean-field Sherrington-Kirkpatrick theory and replica symmetry solution. Then, I move to the famous Parisi solution for breaking the replica symmetry. Firstly showing the one-step replica symmetry breaking (RSB) and the final formulation, the so-called full step RSB. Next, it is examined the nature of the spin-glass phase that emerges from the Parisi solution and the concept of ultrametricity. With that, I give a brief summary of the use of Parisi's theory beyond physics. Finally, the actuality of the Parisi solution is debated in view of the discovery of the presence of spin glass-like phases in several condensed matter systems that are currently being studied.

[1] G. Parisi, Physica A, 194 28-40 (1993).

Em 2021, o Prêmio Nobel de Física foi concedido para estudiosos de "sistemas complexos", com destaque para o clima global. Mas a Ciência do Clima tem uma história que vai muito além desse prêmio, primeiro desvendando os aspectos fundamentais da Física do Sistema Climático e, depois, emitindo alertas há décadas e enfrentando ataques desleais de um negacionismo organizado (em especial sob patrocínio das gigantes do setor de combustíveis fósseis). A grande tragédia é que a história foi muito além do meme "todo filme de catástrofe começa com um cientista sendo ignorado". Uma comunidade científica inteira e a mensagem séria que ela portava foi mais que ignorada: foi sabotada e aviltada. Hoje, o quadro é de emergência climática e se a Ciência do Clima não recebeu atenção, o Sistema Climático já se comunica à humanidade do seu próprio jeito.

Nessa palestra iremos discutir a utilidade de futuros computadores quânticos e os desafios para construir hardware quântico em grande escala.

Both classical and quantum theory hold that every change is reversible at the fundamental level, although we perceive almost everything as irreversible processes. Yet, physicists are not urging to label that perception as illusory: for most, the Second Law of thermodynamics is untouchable, so aberrant the idea of inexhaustible energy is. Building on a narrative pioneered by Watanabe in the 1960s, I shall present an approach to the Second Law as a form of statistical inference given partial information. Irreversibility is unavoidable because it translates the asymmetry between prediction and retrodiction on a given process. This viewpoint applies to both classical and quantum systems. Applied to the “fluctuation relations” that have been the preferential tools to address irreversibility in the last two decades, it both simplifies their derivation and vastly expands their scope.

References: F. Buscemi, V. Scarani, Phys. Rev. E 103, 052111 (2021); C. Aw, F. Buscemi, V. Scarani, arXiv:2106.08589 (2021)

A moderna estrutura eletrônica de sólidos baseada no funcional de densidade (DFT) evoluiu muito desde a década de 80 e hoje tem capacidade preditiva. Novos materiais são previstos via simulações computacionais e posteriormente produzidos no laboratório. Usando técnicas de busca como o método CALYPSO, associado a cálculos de DFT, propomos a existência de um novo material, o Li₅C, um eletreto que, em uma faixa de pressões tem estrutura estável. Aliando a moderna estrutura eletrônica com resultados da teoria da supercondutividade (tipo BCS) para materiais com interação elétron-fônon forte, também estudamos a existência de supercondutividade neste material, que segundo nossos estudos, apresenta a maior temperatura crítica entre os eletretos já reportados na literatura.

The Third Law of Thermodynamics states that the entropy of any system in equilibrium has to vanish at absolute zero temperature. At nonzero temperatures, on the other hand, matter is expected to accumulate entropy near a quantum critical point (QCP), where it undergoes a continuous transition from one ground state to another as a function of a non-thermal tuning parameter [1,2]. Based on general thermodynamic principles, we determine the spatial-dimensional profile of the entropy S near a QCP and its steepest descent in the corresponding multidimensional stress space. We demonstrate this approach for the canonical quantum critical compound CeCu6-xAux near its onset of antiferromagnetic order [3]. We are able to link [4] the directional stress dependence of S to the previously determined geometry of quantum critical fluctuations [5]. Our demonstration of the multidimensional entropy landscape can be generalized to other tuning parameters and may provide the foundation to understand how quantum criticality nucleates novel phases.
In CePdAl, a Kondo-lattice state coexists with geometric frustration leading to an unusual antiferromagnetic phase in which one third of the Ce3+ magnetic moments are frustrated and do not participate in the long-range order observed below 2.7 K. To study the interplay between Kondo effect and frustration, we suppress the partial order by applying magnetic fields B and by replacing Pd with isoelectronic, but smaller Ni atoms [6,7]. The (x,B,T) phase diagram is established by using thermal expansion, magnetostriction, and magnetization measurements of CePd1-xNixAl single crystals with 0 ≤ x ≤ 0.14. We construct the entropy landscape and investigate its stress dependence along the quantum phase boundary. Surprisingly, the quantum critical behavior observed in the thermal expansion and the Grüneisen parameter changes with increasing Ni concentration and magnetic field. The effect of geometric frustration on these results is discussed and compared to the the suggested [8] global phase diagram of heavy-fermion compounds.

[1] P. Coleman, and A. J. Schofield, Nature 433, 226 (2005).
[2] L. Zhu, M. Garst, A. Rosch, and Q. Si, Phys. Rev. Lett. 91, 066404 (2003)
[3] H. v. Löhneysen, A. Rosch, M. Vojta, P. Wölfle, Rev. Mod. Phys. 79, 10185 (2007).
[4] K. Grube, S. Zaum, O. Stockert, Q. Si, H. v. Löhneysen, Nature Physics 13, 742 (2017).
[5] O. Stockert, H. v. Löhneysen, A. Rosch, N. Pyka, M. Loewenhaupt, Phys. Rev. Lett. 80, 5627 (1998).
[6] S. Lucas, K. Grube, C.-L. Huang, A. Sakai, S. Wunderlich, E. L. Green, J. Wosnitza, V. Fritsch, P. Gegenwart, O. Stockert, and H. v. Löhneysen, Phys. Rev. Lett. 118, 107204 (2017).
[7] A. Sakai et al., Phys. Rev. B 94, 22045(R) (2016).
[8] Q. Si, Physica B 378–380, 23 (2006).

Uma fração considerável do carbono no meio interestelar (ISM), aproximadamente 20% ou mais, está na forma de hidrocarbonetos aromáticos policíclicos (PAHs) e acredita-se que os PAHs são moléculas orgânicas que podem ter contribuído para a construção do material pré-biótico que formou a vida na Terra. Os espectros no infravermelho médio (MIR) de objetos galácticos e extragalácticos são dominados por bandas de emissão fortes a 3.3, 6.2, 7.7, 8.6, 11.3 e 12.7 μm, geralmente atribuídas aos PAHs e espécies relacionadas. Durante muito tempo acreditou-se que galáxias que possuem buracos-negros supermassivos (SMBH), nomeadas como núcleo ativo de galáxias (AGNs), possuem um intenso e energético campo de radiação advindo do disco de acresção do SMBH capaz de destruir as moléculas de PAHs próximas dos SMBHs dos AGNs. A falta de dados com alta resolução angular e espectral ajudou a dificultar a compreensão das propriedades físico-químico das moléculas de PAHs e seu uso como ferramenta de diagnóstico da fonte de ionização em galáxias. Entretanto, após mais de uma década de estudos usando dados observacionais dos telescópios Spitzer e Gemini, assim como bibliotecas teóricas de PAHs e técnicas de análise de sinal, foi possível demonstrar que as moléculas de PAHs em AGNs, além de serem maiores (> 180 átomos de carbono) que as moléculas encontradas em galáxias com formação estelar (< 180 átomos de carbono) também possuem, em sua maioria, moléculas de PAHs com diferentes graus de ionização.

Fenômenos de auto-organização possibilitam a síntese de materiais com padrões estruturais peculiares e potencialidades tecnológicas. Neste colóquio discutiremos um panorama geral mostrando exemplos que ilustram a variedade de processos e mecanismos físicos que atuam durante a agregação de precursores para a formação de mesocristais. Paralelamente, apresentaremos exemplos de como métodos de simulação computacional podem ser empregados para contribuir para o avanço desta área.

On April 7th 2021, Fermilab’s Muon g-2 experiment announced it’s first results of the precision measurement of the anomalous muon magnetic moment based on it’s 2018 Run-1 dataset. These results align with the Brookhaven National Laboratory experimental value and the combined values increases the tension between experiment and theory from 3.7 to 4.2 sigma. This talk will give an overview of the Fermilab Muon g-2 experiment, discuss the steps necessary to precisely measure wobbling muons, why this result has the physics community abuzz, and what’s next.

Research in material science is becoming more and more challenging due to the discovery of coexisting mutually exclusive scenarios. For example, heavy Fermion systems, Fe-based superconductors, etc. often show coexistence of magnetic order and superconductivity. Since multiple bands form the Fermi sea in these materials, it is difficult to probe the underlying interactions leading to such complexity. In order to investigate the possibility of inducing orbital selective electron dynamics, we studied the polarization-dependent excitation of electrons using time-resolved ARPES technique. We discover that electrons of a selected symmetry can be excited using polarized pump pulse keeping other electrons relatively less perturbed. Using this technique, we showed [1] that the relaxation of mobile electrons occurs faster than local electrons in contrast to the expected behavior. This can be attributed to the electron-phonon coupling induced relaxation which occurs more efficiently for the extended states. In addition, we discover melting of magnetic order at a time scale of 50 fs. Interestingly, other excitations occurring at a time scale of about 200 fs can be induced without perturbing the magnetic order.

[1] G. Adhikary, B. Ressel, M. Stupar, P. R. Ribič, J. Urbančič, G. De Ninno, D. Krizmancic, A. Thamizhavel, and K. Maiti, Phys. Rev. B 98, 205142 (2018).
[2] G. Adhikary, T. Saha, P. R. Ribič, M. Stupar, B. Ressel, J. Urbančič, G. De Ninno, A. Thamizhavel, and K. Maiti, arXiv: 2005.01013

Anisotropic superconductivity and the spin-vortex antiferromagnetism in Ni-doped CaKFe₄As₄
Isabel Guillamón
(Laboratorio de Bajas Temperaturas, Departamento de Física de la Materia Condensada, Instituto de Ciencia de Materiales Nicolás Cabrera, Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid)

Quasiparticle interference and vortex imaging have both been shown to be powerful tools to investigate pnictide superconductors. Here I will review new results in the recently discovered family of 1144 materials, particularly in pure and Ni-doped CaKFe4As4. The 1144 CaKFe4As4 compound is a pnictide superconducting material showing optimal superconducting critical temperature with Tc as large as 38 K [1,2]. There are no signatures of nematic, structural or magnetic transitions. Doping with Ni induces a decrease in Tc and the appearance of magnetism. Instead of the stripe like spin density wave (SSDW) antiferromagnetic order present in the 122 systems, Ni-doped CaKFe4As4 shows a spin vortex (or hedgehog) magnetic order [3]. Here I will discuss scanning tunneling microscopy experiments in CaK(Fe0.95Ni0.05)4As4 [4]. We have determined the superconducting density of states and observed the vortex lattice. Quasiparticle interference measurements show the reconstruction of the Fermi surface due the presence of hedgehog magnetic order and the opening of an anisotropic superconducting gap, giving experimental evidence for the coexistence of both states. Finally, I will show the recent efforts made in STM for very high magnetic fields, presenting first spectroscopic data taken at 20 T.

References

• [1] K. Cho, A. Fente et al., Phys. Rev. B 95, 100502(R) (2017).
• [2] A. Fente et al., Phys. Rev. B 97, 134501 (2018).
• [3] W.R. Meier et al., npj Quantum Materials 3, 5 (2018).
• [4] J. Benito-Llorens et al., Phys. Rev. B 103, L060506 (2021).

Non‑Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects
Yanina Fasano
(Low Temperature Lab & Instituto Balseiro, Centro Atómico Bariloche, Argentina)

Coworkers: Jazmín Aragón Sánchez, Gonzalo Rumi, Raúl Cortés Maldonado, Néstor René Cejas Bolece, Joaquín Puig, Pablo Pedrazzini, Gladys Nieva, Moira I. Dolz, Marcin Konczykowski, Cornelis J. van der Beek, & Alejandro B. Kolton

Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers.

The advent of nanomaterials has brought several challenges on the materials' characterization framework. These challenges open for opportunities on the development of instruments capable to overcome today's technological limitations. In optical spectroscopy, diffraction mimics the capacity of conventional optical setups to extract spectral information at the nanoscale. In this seminar, I will present recent advances on the development of a near-field Raman spectroscopy system, taking place in LabNS, the Nanospectroscopy Lab of the Department of Physics, UFMG. The instrument allows for the investigation of local properties in individual graphene nanoflakes and twisted bilayer graphene, and the information extracted from this local analysis is useful to understand statistical results extracted from measurements performed in the micro and macro scales. Supporting the instrument's technology, we have developed a new scattering-type near-field probe formed by a micro-pyramidal body whose length L is scalable to fine-tune localized surface plasmon resonance (LSPR) modes. These so-called plasmon-tunable tip pyramids (PTTPs) act as monopole antennas, as revealed by electron energy loss spectroscopy (EELS). The monopole character of the PTTP is a consequence of its geometry: the nanopiramidal part is electrically grounded on a flat metallic plateau that acts like a mirror providing the monopole's image that closes the dipole system. The talk ends with a discussion on the coherence properties of scattered fields in the proximity of the source (a material system illuminated by strongly focused by optical fields). I will demonstrate that the spatial extent of near-field correlations relies on local properties of the source which are inaccessible in the far field zone.

Substantial in vitro and physiological experimental results suggest that similar coherent spin physics might underlie phenomena as varied as the biosensing of magnetic fields in animal navigation and the magnetosensitivity of metabolic reactions related to oxidative stress in cells. If this is correct, organisms might behave, for a short time, as “living quantum sensors” and might be studied and controlled using quantum sensing techniques developed for technological sensors. I will outline our approach towards performing coherent quantum measurements and control on proteins, cells and organisms in order to understand how they interact with their environment, and how physiology is regulated by such interactions. Can coherent spin physics be established – or refuted! – to account for physiologically relevant biosensing phenomena, and be manipulated to technological and therapeutic advantage?

We have carried out hard x-ray photoelectron spectroscopy (HAXPES) measurements at the U 4f core level and non-resonant inelastic x-ray scattering (NIXS) at the U O4,5 edges of UT2Si2 compounds that all form in the tetragonal ThCr2Si2 structure but exhibit different ground state properties: UFe2Si2 is a Pauli paramagnet; URu2Si2 is the famous hidden order compound of which the order parameter is still fiercely debated despite 30 years of intense experimental and theoretical studies; UPd2Si2 and UNi2Si2 are antiferromagnets with TN well above 100 K and sizeable ordered magnetic moments. We have determined the degree of the 5f-electron localization (HAXPES) as well as the symmetry of the 5f ground state wave function (NIXS) across these very different compounds.

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O objetivo desta comunicação é apresentar à nossa comunidade acadêmica a Política Institucional de Boas Práticas e Integridade em Pesquisa da Unicamp. Em um primeiro momento discutiremos o contexto, a abordagem conceitual e as prerrogativas da Política. Em um segundo momento, apresentaremos o arcabouço regimental e a forma de funcionamento da Comissão de Integridade em Pesquisa da Unicamp (CIP), assim como os desafios de sua implantação. Por fim, discutiremos, em diálogo com os presentes, situações e exemplos de tratamento de más práticas científicas, como forma de se ilustrar o papel e a relevância da integridade ética para a produção de científica no mundo contemporâneo.

Recently, there have emerged various candidate materials of strongly correlated Dirac semimetals. Theoretically, it is known that such Dirac semimetals can show characteristic quantum criticalities which are distinct from the conventional criticalities of bosonic order parameters. Furthermore, the Landau quantization can enhance the symmetry breaking in presence of a magnetic field, which is called magnetic catalysis.

In this study, we investigate quantum criticality of the magnetic-field-induced charge density wave (CDW) order in a simple model of correlated spinless Dirac fermions at zero temperature as a prototypical example of the magnetic catalysis, by using the infinite density matrix renormalization group. It is found that the CDW order parameter M(B) exhibits an anomalous magnetic field (B) scaling behavior characteristic of the (2+1)-dimensional chiral Ising universality class near the quantum critical point, which leads to a strong enhancement of M(B) compared with a mean-field result. We also establish a global phase diagram in the interaction-magnetic field plane for the fermionic quantum criticality.

Strontium ruthenate (Sr2RuO4) has long been thought to host a spin-triplet chiral p-wave superconducting state. However, the singlet-like response observed in recent spin-susceptibility measurements casts serious doubts on this pairing state. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus c66 at the superconducting transition temperature, the available experiments point towards an even-parity chiral superconductor with kz(kx ± iky)-like Eg symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of Sr2RuO4. Here, we show how the orbital degree of freedom can encode the two-component nature of the Eg order parameter, allowing for a local orbital-antisymmetric spin-triplet state that can be stabilized by on-site Hund’s coupling. We find that this exotic Eg state can be energetically stable once a complete, realistic three-dimensional model is considered, within which momentum-dependent spin-orbit coupling terms are key.

O prêmio Nobel de Física de 2020 vem coroar o que já se denomina como a nova "era áurea" dos buracos negros, certamente uma das previsões mais fascinantes da Relatividade Geral de A. Einstein. Neste colóquio, será feita uma rapidíssima revisão da história da Física de Buracos Negros, desde seus primórdios há mais de 100 anos atrás, passando, obviamente, pelos avanços teóricos dos anos 60 que garantiram a R. Penrose 50% do Prêmio, até chegarmos finalmente às notáveis observações diretas mais recentes, destacando-se os trabalhos de Reinhard Genzel e Andrea Ghez sobre o buraco negro no centro da nossa Via Láctea, os quais lhes garantiram a outra metade do Prêmio Nobel, e a já famosa "fotografia" do buraco negro na galáxia Messier 87, a incríveis 53 milhões de anos luz do nosso planeta Terra, obtida pelo consórcio Event Horizon Telescope (EHT). Todos esses pontos serão discutidos de maneira clara e mais elementar possível, na esperança de que todos possam apreciar esses fantásticos resultado nesta nossa época realmente notável para a Física.

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Twenty years ago, moving from CeIn3, to CeRhin5, CeCoIn5, CeIrIn5 to PuCoGa5, the quantum materials community succeeded in driving up the superconducting transition temperature by a factor of a hundred, from 0.1K to 18.5K. This is a dream that our field longs to replicate in other families of quantum materials, but we are still lacking key pieces of the puzzle to enable a next leap. In the meantime, these miniature high temperature superconductors have proven themselves to be a vital work-horse for studying the same combinations of d-wave pairing, strange metal and quantum criticality that feature at much higher temperatures in their transition metal cousins.

I will discuss the theoretical puzzles raised by these superconductors, reviewing both the theoretical progress, and the key questions - the nature of the strange metal, the quantum criticality and the curious relationship between Kondo, antiferromagnetic and superconducting entanglement, emphasizing the wide-open opportunities for new conceptual understanding in this vibrant area of research.

Work supported by NSF grant DMR-1830707.

Nesse colóquio farei uma breve introdução aos neutrinos no modelo padrão. Discutiremos como os neutrino oscilam, como são produzidos, e como são detectados. Abordarei o que sabemos sobre neutrinos e seus problemas em aberto. Entre os problemas em aberto, enfatizarei a origem das massas, a natureza Dirac/Majorana, algumas anomalias experimentais ainda não compreendidas e, se houver tempo, possíveis conexões com matéria escura.

Os neutrinos foram propostos por Pauli em 1930 através de uma carta na qual ele afirma "Essa é uma ideia que eu não ouso publicar", pois era uma partícula impensavelmente difícil de se detectar. Hoje, 90 anos depois, os neutrinos já renderam mais de 5 prêmios Nóbel e tomam um papel central na física de partículas de fronteira. Mas o que uma partícula já tão estudada pode trazer de novo? Uma das poucas evidências de física além do modelo padrão: A sua massa! É através dela que podemos explorar fenômenos que não podem ser alcançados por grandes aceleradores, mas podem ajudar a resolver problemas em aberto tanto na física de baixas e altas energias e ajudar até a entender o universo primordial.

We present our recent results on the novel spin-triplet superconductor UTe2, which might be at the verge of the ferromagnetic order. The huge upper critical field exceeding the Pauli limit is suggestive of the spin-triplet state. For the field along the b-axis, the field reentrant superconductivity is observed up to Hm~35T, at which the first order metamagnetic transition occurs. The field reentrant superconductivity in UTe2 is similar to those observed in ferromagnetic superconductors, namely URhGe and UCoGe. Applying the pressure in UTe2, the superconducting transition temperature splits and the multiple superconducting phase is detected as a thermodynamic response. We overview the results on UTe2 compared to ferromagnetic superconductors, and present our perspective.

Quando a temperatura é abaixada rápido o suficiente para evitar a cristalização, o sistema sofre uma transição de vidro e o material resultante é amorfo. Esta transição também ocorre em sistemas cujo parâmetro de ordem é a densidade. Neste caso, a cristalização é evitada quando a densidade é rapidamente aumentada. Se a densidade aumentar ainda mais, o sistema experimenta uma transição de engarrafamento, a qual apresenta diversas propriedades não lineares com aplicações em áreas que variam de meios granulares a algoritmos de aprendizado de máquina.

Um dos principais problemas para entender os mecanismos envolvidos nestas transições e as propriedades das fases resultantes destes processos é o fato de que os tempos envolvidos nas simulações numéricas deste tipo de sistema são, na prática, proibitivos. No entanto, uma recente proposta revisitada de algoritmo onde as partículas podem trocar seus diâmetros (swap) além de desenvolver suas dinâmicas normais de translação, permitiu grandes avanços nesta área. Nesta apresentação eu vou explicar a ideia de um algoritmo de swap, porque funciona [1] e vou discutir algumas de suas implicações para a física dos materiais amorfos [2].

[1] Theory for Swap Acceleration near the Glass and Jamming Transitions for Continuously Polydisperse Particles, Carolina Brito, Edan Lerner and Matthieu Wyart, Physical Review X, v 8, 031050 (2018)
[2] Fast generation of ultrastable computer glasses by minimization of an augmented potential energy
Geert Kapteijns, Wencheng Ji, Carolina Brito, Matthieu Wyart, Edan Lerner, Physical Review E, v 99, 012106 - (2019)

The occurrence of superconductivity in doped SrTiO3 and related materials at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We present experimental results showing the pressure dependence of the superconducting transition temperature, Tc, that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached, in a way reminiscent of behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of itinerant electrons to the critical modes in ferroelectrics is predicted to be small. We present a superconductivity model to make quantitative comparisons with experiments and show that an enhancement of Tc near to a ferroelectric quantum critical point arises due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a direct coupling to the transverse critical modes.

Spin-triplet superconductors are a promising route in the search for topological superconductivity, and UTe2 is a recently discovered contender. In this talk, I will first give a brief overview of UTe2 before presenting new insights from ac calorimetry, electrical resistivity, and x-ray absorption measurements of UTe2 under applied pressure. I will end with a discussion of some of the most pressing outstanding open questions.

"Poeira de estrelas". Ou, mais precisamente, poeira interestelar: esse é um dos principais ingredientes para a formação de planetas no cosmos. Elementos químicos mais pesados que hidrogênio e hélio, dos quais se forma a poeira cósmica, são produzidos por nucleossíntese estelar. Como a formação de estrelas está ligada à produção de diferentes elementos químicos? O que acontece quando novas gerações de estrelas se formam a partir do gás enriquecido pela geração anterior? Contarei um pouco sobre a história de evolução de galáxias no Universo e sobre a conexão entre a formação de estrelas e o enriquecimento químico. Explicarei como recenseamos galáxias a partir de bancos gigantescos de imagens e espectros obtidos por telescópios terrestres e espaciais, e como essas análises estão ancoradas na mecânica quântica. Finalizarei com breves comentários sobre o papel (obscurecedor) da poeira interestelar em observações astrofísicas.

In spite of important recent improvements in the theoretical handling of correlated materials, it is still difficult to obtain precise and detailed theoretical electronic structure results to compare with experiments, like angular resolved photoemission (ARPES), inverse photo-emission experiments (IPE) or optical conductivity measurements. We calculate and resolve with unprecedented detail the spectral densities at zero temperature of key models for strongly correlated electron materials by means of a highly optimized Dynamical Mean Field Theory which uses the Density Matrix Renormalization Group as the (effective) impurity solver. I will show results for the two-orbital Hubbard model at half-filled and hole-doped situations in the presence of inter-orbital Coulomb and Hund interactions for equal and different band-widths. For the half-filled case we observe the emergence of an in-gap narrow band when at least one of the orbitals is metallic which we identify as formed by inter-orbital holon-doublon bound states. When the system is hole doped, we observe clear subbands within the Mott gap formed by pure holon-doublon pairs which are pulled down from the upper Hubbard band to lower energies by the inter-orbital Coulomb interaction and are split by the magnetic Hund’s interactions. The lower Hubbard band also splits into a coherent narrowly dispersing peak around the Fermi energy, and another subband which evolves with the chemical potential.

We hope that the results presented here together with the possibility of calculating more precise spectral functions for models of correlated materials will stimulate a closer study of the details of experimental results and, hence, will contribute to unveil the complex and elusive microscopic behavior of strongly correlated materials.

Nesse seminário iremos discutir a importância da dimensionalidade na física e química de materiais em diferentes dimensões: 0D (moléculas e átomos), 1D (polímeros condutores, nanotubos, etc), 2D (grafeno e outros), e 3D (materiais convencionais).

In the 50's physicists were trying to figure out nature by constraining the fundamental rules of nature as dictated by quantum mechanics, relativity and analyticity. Today we know that was naive. We can not constraint theories uniquely by pure thought for many reasons. One reason is that there is no single theory! There is a big space of possible theories. By shifting perspectives and aiming at carving out this space - instead of figuring out individual theories - beautiful progress has been made in the recent year reviving the 50's S-matrix Bootstrap program. I will review some fun news results in this direction.

Strongly correlated metals are known to display a rich variety of quantum phases. Whether and how they also produce electronic topology is of considerable current interest and, yet, a largely open question. A promising setting arises in heavy fermion systems, which may contain not only strong correlations but also a large spin-orbit coupling. Recently, a Weyl-Kondo semimetal state has been concurrently discovered in theoretical [1] and experimental [2] studies. It has several defining characteristics, including: Weyl nodes that are pinned to the Fermi energy; and linearly-dispersing nodal excitations with highly reduced velocity. They have clear signatures in thermodynamic [2] and transport [3] properties that are quite unique to the strongly correlated setting. Nonsymmorphic space-group symmetry plays an important role in realizing the Weyl-Kondo semimetal phase [4]. In this talk, I will summarize these developments, discuss how they open up a general strategy for strong correlations to cooperate with space-group symmetry in creating topological conductors, and draw their implications for the strong correlation physics in general through a global phase diagram.

Work done in collaboration with Sarah Grefe, Hsin-Hua Lai, Sami Dzsaber, and Silke Paschen, and supported by NSF (DMR-1920740) and the Robert A. Welch Foundation (C-1411).

[1] H.-H. Lai et al., PNAS 115, 93 (2018).
[2] S. Dzsaber et al., Phys. Rev. Lett. 118, 246601 (2017).
[3] S. Dzsaber et al., arXiv:1811.02819; arXiv:1906.01182.
[4] S. E. Grefe et al., Phys. Rev. B 101, 075138 (2020).

Magnetic nanoparticles have been building blocks in applications ranging from high density recording to spintronics and nanomedicine. Magnetic anisotropies in nanoparticles arising from surfaces, shapes and interfaces in hybrid structures are important in determining the functional response in various applications. In this talk I will first introduce the basic aspects of anisotropy, how to tune it in nanostructures and ways to measure it. Tuning anisotropy has a direct impact on the performance of functional magnetic nanoparticles in biomedical applications such as contrast enhancement in MRI and magnetic hyperthermia cancer therapy. There is a need to improve the surface functionalization and specific absorption rate (SAR) or heating efficiency of nanoparticles for cancer diagnostics and therapy. Strategies going beyond simple spherical structures such as exchange coupled core-shell nanoparticles, nanowire, nanotube geometries can be exploited to increase saturation magnetization, effective anisotropy and heating efficiency in magnetic hyperthermia. This lecture will combine insights into fundamental physics of magnetic nanostructures along with recent research advances in their application in therapy and diagnostics (theranostics) in nanomedicine.