Research
Unconventional superconductivity in complex quantum materials
The stability and robustness of simple superconductors is well understood and can be captured by what is known as Anderson’s Theorem [1]. The current challenge is to account for the phenomenology of complex superconductors, which constitute most of the fascinating quantum materials we have available today. My contribution to this fertile area of research came on three fronts:
First, with the proposal of the Concept of superconducting fitness [2-3], introducing two measures that allow for the engineering of the normal electronic state for the optimization of the superconducting critical temperature. This concept was applied to several families of materials: ruthenates, iron-pnictides, doped topological insulators, and noncentrosymmetric systems.
Second, with the proposal of the Generalized Anderson’s Theorem based on the superconducting fitness measures [4]. This generalization provides a clear explanation for the remarkable robustness of superconductors derived from topological insulators in the presence of impurities. This generalization opens the door for a confident exploration of superconductivity in these materials towards the holy grail of topological superconductivity.
Third, with theoretical proposals that reconcile apparently disparate experimental observations. We have proposed a new order parameter candidate for Sr2RuO4: a chiral orbital anti-symmetric spin triplet [5,6]. We have also explored the newly discovered CeRh2As2, and proposed a general guideline to make superconductors stable at high magnetic fields a twisting of the order parameter (figure on the left) [7,8].
The superconducting fitness measures were later associated with the presence of odd-frequency cor- relations [9], anomalous Hall effect in chiral superconductors [10], and more recently anapole superconductivity [11]. The ubiquity of the fitness measures in theories associated with unusual responses in complex superconducting states reflect the importance and versatility of these measures.
[1] P. W. Anderson, Theory of dirty superconductors, J. Phys. Chem. Solid 11, 26–30 (1959).
[2] Aline Ramires and Manfred Sigrist, Identifying detrimental effects for multi-orbital superconductivity - Application to Sr2RuO4, Phys. Rev. B 94, 104501 (2016).
[3] Aline Ramires, Daniel F. Agterberg and Manfred Sigrist, Tailoring Tc by symmetry principles: The concept of Superconducting Fitness, Phys. Rev. B 98, 024501 (2018).
[4] Lionel Andersen*, Aline Ramires*, Zhiwei Wang, Thomas Lorenz, Yoichi Ando, Generalized Anderson’s theorem for superconductors derived from topological insulators, Science Advances 6, eaaY6502 (2020).
[5] Han Gyeol Suh, Henri Menke, P. M. R. Brydon, Carsten Timm, Aline Ramires and Daniel F. Agterberg [Rapid Communication], Stabilizing Even-Parity Chiral Superconductivity in Sr2RuO4, Phys. Rev. Research 2, 032023(R) (2020)
[6] Sophie Beck, Alexander Hampel, Manuel Zingl, Carsten Timm, and Aline Ramires, The effects of strain in multi-orbital superconductors: the case of Sr2RuO4, https://arxiv.org/abs/2111.13506 [Under review in Phys. Rev. B]
[7] David Möckli and Aline Ramires, Superconductivity in disordered locally noncentrosymmetric materials: an application to CeRh2As2, Phys. Rev. B 104, 134517 (2021)
[8] David Möckli and Aline Ramires, Two scenarios for superconductivity in CeRh2As2, Phys. Rev. Research 3, 023204 (2021)
[9] Christopher Triola, Jorge Cayao, and Annica M. Black-Schaffer, The Role of Odd-Frequency Pairing in Multiband Superconductors, Annalen der Physik 532, 1900298 (2020).
[10] M. D. E. Denys, P. M. R. Brydon, Origin of the anomalous Hall effect in two-band chiral superconductors, Phys. Rev. B 103, 094503 (2021).
[11] Shota Kanasugi, Youichi Yanase, Anapole superconductivity from symmetric mixed-parity interband pairing, arXiv:2107.07096 (2021).
Engineering electronic states in van der Waals materials
Twisted bilayer graphene is a pure carbon-based material platform that has attracted much attention since early 2018, given the observation of correlated insulating behavior and superconductivity in this system [1]. My contribution to this very active area of research came on three fronts:
First, with the proposal of a new set up for the generation and control of artificial gauge fields based on the application of a transverse electric field [2]. Previous proposals are based on strain engineering [3], which requires mechanical manipulation of samples and are therefore not suitable for realistic applications.
Second, with the engineering of triple point fermions, a generalization of the standard Dirac and Weyl fermions [4]. This work goes beyond the standard classification for 3D topological semimetals based on crystalline symmetries [5]. This proposal is a non-trivial extension to 2D van der Waals materials and the emerging research on twistronics.
Third, by proposing that heavy fermion physics can be emulated in twisted trilayer gr aphene [6]. Our work opens the door for more clear parallels between "classical" bulk strongly correlated systems and van der Waals heterostructures to be drawn.
[1] Cao, Y., Fatemi, V., Fang, S. et al. Unconventional superconductivity in magic-angle graphene superlattices, Nature 556, 43–50 (2018); Cao, Y., Fatemi, V., Demir, A. et al. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices, Nature 556, 80–84 (2018).
[2] Aline Ramires and Jose L. Lado [On the cover], Electrically tunable gauge fields in tiny-angle twisted bilayer graphene, Phys. Rev. Lett. 121, 146801 (2018).
[3] Guinea, F., Katsnelson, M. & Geim, A. Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering, Nature Phys 6, 30–33 (2010).
[4] Aline Ramires and Jose L. Lado, Impurity-induced triple point fermions in twisted bilayer graphene, Phys. Rev. B 99, 245118 (2019).
[5] B. Bradly, J. Cano et al. Beyond Dirac and Weyl fermions: Unconventional quasiparticles in conventional crystals, Science 353, aaf5037-1 (2016).
[6] Aline Ramires and Jose L. Lado, Emulating heavy fermions in twisted trilayer graphene, Phys. Rev. Lett. 127, 026401 (2021).
Quantum criticality and strongly correlated systems
Strongly correlated systems display very complex phase diagrams and provide long-standing challenges for condensed matter theorists [1]. The description of quantum critical points and phases, and the associated strange metallic behavior are associated with the biggest open questions in the field [2]. My contribution to this fundamental area of research came on two fronts:
First, with the proposal of a phenomenological theory for the intrinsically quantum critical material, β-YbAlB4 [3]. The theory is based on the idea of a critical nodal metal, for which the faithful microscopic description of the low-lying orbital degrees of freedom was key. We were able to account for the unusual divergencies and scaling behavior observed in the thermodynamic properties of this material within a simple proposal. We have also studied the ERS response of this material [4].
Figure reproduced from S. Takano et al. (2016)
Second, with the generalization of supersymmetric spin representations for controlled calculations in the large-N limit with symplectic symmetry [5]. This generalization allows for a natural theoretical treatment of superconductivity and frustrated magnetism [6], going beyond the usual SU(N) treatments. This approach allowed us to identify new fluctuating fermionic modes at the phase transition, which could account for non-Fermi liquid behavior.
[1] Silke Paschen, Qimiao Si, Quantum phases driven by strong correlations, Nat. Rev. Phys. 3, 9-26 (2021).
[2] Sung-Sik Lee, Recent Developments in Non-Fermi Liquid Theory, Annual Review of Condensed Matter Physics 9, 227-244 (2018).
[3] Aline Ramires, Piers Coleman, Andriy H. Nevidomskyy, and A. M. Tsvelik, β-YbAlB4: a critical nodal metal, Phys. Rev. Lett. 109, 176404 (2012).
[4] Aline Ramires and Piers Coleman, Theory for the Electron Spin Ressonance in β-YbAlB4, Phys. Rev. Lett. 112, 116405 (2014).
[5] Aline Ramires and Piers Coleman, Supersymmetric approach to heavy fermion systems, Phys. Rev. B 93, 035120 (2016).
[6] Aline Ramires, Frustration can be critical, Nature Physics 15, 1212 (2019).