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Figure 4.11: Comparison between the rescaled experimental (4.48) and holographic (4.45) temperature evolution of the rescaled pair-breaking term ˜Γ. For generating the holographic data we used η5 = 0.025 and m2b =−3/4, withTc= 0.013.
similar to the experimental one, while at the same time is about the best we can do with our numerical method: m2b =−3/4 andη5 = 0.025. Albeit a small remaining difference in the scaling exponent αe ≈ 5.12 and αh ≈4.85, the two temperature evolutions are remarkably similar.
4.6. Summary and outlook 75
lived enough to form a coherent state. This is directly related to the second charac-teristic feature. The pair-breaking term was measured to be strongly temperature dependent and to be of the same order of magnitude as the gap, in particular at the critical temperature they found that ∆ ≈ 3Γ. Recall that the pair-breaking term of BCS like superconductors is almost temperature independent and negligi-bly small compared to the gap (3.12). This phenomenon can be utilised to furnish the above intuitive picture. At temperatures above Tc, the pairs break up rapidly, preventing the formation of a superconducting ground state. Below the critical temperature it is still strong enough to break up enough pairs to be responsible for a filling of the gap instead of let it close. The third feature, albeit not of the same relevance, is that the pair-breaking term seems to be non-zero at zero temperature, equivalent of a persisting finite width of the peak.
Backed up with the encouraging results of probe fermions in a holographic strange metal and holographic setup capable of generating a gap in the spectrum of a probe fermion in a superconducting background at T = 0, we studied the tem-perature evolution of ∆ and Γ in the background of what supposedly could be a holographic high-temperature superconductor. We derived the finite temperature ansatz of the model constructed in [33] in the background of the simplest holo-graphic superconductor (3.42). By demanding that the normal state, realised by the AdS-RN theory, behaves as effectively as possible like a marginal fermi liquid, we reduced the number of external parameters to only two. The mass mb of the condensing charged scalar field and the coupling strength η5 between the probe fermion and that scalar.
We then studied how the gap and the pair-breaking term are affected by changes of those two parameters and which of the characteristic features can be mimicked with our holographic model. We find that ∆(Tc) = 0 for all of the parameter configurations. This is result is to be expected because above the critical tem-perature the ground state reduces to the AdS-RN strange metal which does not show a gap. Moreover, one expects a continuous transition between the normal and superconducting state due to the continuously vanishing scalar condensate, which effectively multiplies the term in the action responsible for the gap. How-ever, on top of this, we observed that the peaks spread out thermally such that at temperatures T < Tc the gap has filled before it closed. This effect is stronger for smaller values of η5. This was presented in [33] as well. It is not clear whether the superposition of the thermal broadening of the peaks and the closing of the gap can be related to the new interpretation of ∆ in high-temperature superconductors
put into play by in [32].
We did find the strong temperature dependence of the pair-breaking term. More-over, we also observed that Γ ∼ ∆, both in accordance with the experiment.
We were able to quantitatively approach to temperature evolution of Γ, which we mathematically formulated as a powerlaw behaviour (4.47). Knowing the ef-fect of the two external parameters, we were able to identify a set of parameters {m2b, η5}={−3/4,0.025}which leads to a remarkably similar pair-breaking term.
Note that the value for the scalar mass is comparably high and scratches the bound above which the system no longer exhibits a phase transition to supercon-ductivity and that the value of the pair-breaking term at the critical temperature is controlled by the critical temperature. In order to go beyond the scalar mass m2b = −3/4, one should resort to a different numerical method to both, solve the background and also solve the equations of motion for the probe fermion. A possible approach in this direction is to implement a pseudo-spectral method.
Figure 4.11 shows one of the major achievements of this thesis, as a quantitative matching of properties of real physical systems and holographic models is very rare in AdS/CMT. Importantly, this result is directly related to the supercon-ducting state on both sides and shows for the first time a close relation between real and holographic high temperature superconductors. Our results ‘close’ the square consisting of the normal and superconducting state of real and holographic strange metals: the relation between the metallic phases already passed two tests on a quantitative level, namely the linear increase of the electrical resistivity with temperature [34] and a power-law dependence of the electrical conductivity in a mid-infrared regime [71]. The holographic metal and superconductor are directly connected by construction and the relation between the real strange metal and high-temperature superconductor is a widely accepted idea. In this picture this thesis explicitly furnishes the assumed relation between the two superconducting states.
The good qualitative and and also quantitative agreement between the holographic and experimental data for Γ(T) is the main result of this chapter. Note however that, in the bottom-up approach, there is no clear procedure to determine the action for bulk fermion. In fact, various effects other than the one studied here can be obtain by adding couplings between fermion and other fields (allowed by gauge invariance), see e.g. [96, 98–101]. Likewise, it is not entirely clear what exactly the coupling between probe fermion and charged scalar utilised in this
4.6. Summary and outlook 77
chapter represents on the dual boundary field theory. A proper understanding of theη5-coupling we implemented in our model in terms of a top-down interpretation should certainly be a goal of future research in this direction.
One of the most abvious questions is addressed in the next chapter 5, namely is it possible to obtain similar results with a different holographic superconductor?
Of particular interest naturally are superconductors with a normal phase closer to the real world strange metals as for example the model studied in [34].
Chapter 5
A holographic superconductor with momentum relaxation
5.1 Introduction and summary
Ever since the first holographic realisation of a superconductor [16], holographic superconductivity has been an active and fruitful field of research. The original model is a bottom-up construction starting with the ‘minimal’ holographic model (2.34) supplemented with a U(1) gauge field and a charged scalar. By now count-less models have been developed and investigated among which are also top-down constructions, see for example [102].
The goal of describing real high-temperature superconductors in mind, one has to include a mechanism of momentum dissipation to the boundary field theory. This is equivalent of breaking translational invariance in the bulk because it leads to a non-conservation of the dual stress-energy tensor. Recall that without momentum dissipation the DC conductivity is infinite also in the normal conducting or metallic phase, see 3.3.2.
The first mechanism to implement momentum dissipation into holographic models were holographic lattices [71, 103–107]. Another way is to explicitly break bulk translation invariance by including a mass term for the graviton [108]. It is how-ever, not clear how those massive gravity theories should be interpreted from the quantum field theory perspective. Reference [109] suggested a relation between lattice models and a specific massive gravity theory.
A comparably simple, yet effective way to realise a breaking of translational sym-79
metry is to include spatially dependent sources within the boundary field theory in the form of massless scalar fieldsξi ∼αxi, withi= 1, ..., d−1, suggested in [110].
Such a scalar shift symmetry has the advantage that the geometry of the bulk is still isotropic and homogeneous. Note that the on-shell gravity action in the presence of those scalar fields looks exactly like the on-shell action of the massive gravity theories in a holographic context. From the boundary field theory per-spective, those scalar fields may be regarded as linearised exponential potentials analogous to the electrostatic periodic potentials in a lattice [111]. Holographic su-perconductors with this method of breaking translational invariance are presented in [35, 36].
In this thesis we study a holographic superconductor model based on the model investigated in [34]. Its distinguishing feature is the origin of the linear increase of the DC electrical resistivity ρDC with temperature: This model has the property that ρDC ∼η ∼s, where η is the shear viscosity and s the entropy density. From a (non-holographic) hydrodynamic perspective this is true for a system which has minimal viscosity, a property directly related to intrinsic strong coupling. The above relation requires in particular that the entropy density scales linearly in temperature as well and vanishes at T = 0. Such a hydrodynamic behaviour in metals can be motivated from [112]. Recall from the discussion in 3.2, that this fea-ture cannot be realised by a model based on the AdS-RN background. Instead the model presented in [34] includes a dilaton field acting as a dynamical gauge cou-pling and leading to the vanishing of the black hole atT = 0, i.e.s(T = 0) = 0 due to the relation given in (2.33). We add a charged scalar which as we show causes a phase transition to superconductivity and change the mechanism for momentum dissipation from a massive graviton in [34] to the neutral scalar fieldsξi mentioned above. Close to when we were finishing our analysis of the superconductor work on similar holographic superconductors appeared [35, 36]. We emphasise that there are many more requirements for the action to faithfully represent strange metals.
For example the holographic model cannot reproduce the temperature dependence of Hall angle found in such materials. It is pointed out in [113] that the correct temperature dependence of DC resistivity and Hall angle can be obtained in more complicated Einstein-Maxwell-dilaton theories with momentum relaxation. Such a setup has been found recently in [114] and is arguably consistent once one relaxes the constraint from the null energy condition [115].
The encouraging results of the previous chapter about the accordance of the tem-perature dependence of the pair-breaking term Γ(T) between the minimal