Research Article | Open Access

Xiao-Xiong Zeng, Li-Fang Li, "Holographic Phase Transition Probed by Nonlocal Observables", *Advances in High Energy Physics*, vol. 2016, Article ID 6153435, 13 pages, 2016. https://doi.org/10.1155/2016/6153435

# Holographic Phase Transition Probed by Nonlocal Observables

**Academic Editor:**Davood Momeni

#### Abstract

From the viewpoint of holography, the phase structure of a 5-dimensional Reissner-Nordström-AdS black hole is probed by the two-point correlation function, Wilson loop, and entanglement entropy. As the case of thermal entropy, we find for all the probes that the black hole undergoes a Hawking-Page phase transition, a first-order phase transition, and a second-order phase transition successively before it reaches a stable phase. In addition, for these probes, we find that the equal area law for the first-order phase transition is valid always and the critical exponent of the heat capacity for the second-order phase transition coincides with that of the mean field theory regardless of the size of the boundary region.

#### 1. Introduction

Phase transition is a ubiquitous phenomenon for garden-variety thermodynamic systems. Due to the pioneering work by Hawking [1, 2], a black hole is also a thermodynamic system. Such a fact is further supported by AdS/CFT correspondence [3–5], where a black hole in the AdS bulk is dual to a thermal system without gravity. So one can naturally expect that a black hole can also undertake some interesting phase transitions as the general thermodynamic system. Actually it has been shown that a charged AdS black hole undergoes a Hawking-Page phase transition [6, 7], which is interpreted as the confinement/deconfinement phase transition in the dual gauge field theory [8] and a van der Waals-like phase transition before it reaches the stable state [9]. The Hawking-Page phase transition implies that the thermal AdS is unstable and it will transit to the stable Schwarzschild AdS black hole lastly. The van der Waals-like phase transition has been observed till now in many circumstances. The first observation was contributed by [9] in the plane. Specifically speaking, in a fixed charge ensemble, for a black hole endowed with small charge, there is an unstable black hole interpolating between the stable small hole and stable large hole, and the small stable hole will undertake a first-order phase transition to the large stable hole as the temperature of the black hole reaches a critical temperature. As the charge increases to the critical charge, the small hole and the large hole merge into one and squeeze out the unstable phase so that an inflection point emerges and the phase transition is second order. When the charge exceeds the critical charge, the black hole is always stable. Recently in the extended phase space, where the negative cosmological constant is treated as the pressure while its conjugate acts as the thermodynamical volume, the van der Waals-like phase transition has also been observed in the plane [10–16]. In addition, it was shown in [17] that the van der Waals-like phase transition also shows up in the plane. Particularly, in the Gauss-Bonnet gravity, it is found that the Gauss-Bonnet coupling parameter also affects the phase structure of the space time, and in the plane, a 5-dimensional neutral Gauss-Bonnet black hole also demonstrates the van der Waals-like phase transition [18].

In this paper, we intend to probe the Hawking-Page phase transition and van der Waals-like phase transition appeared in a 5-dimensional Reissner-Nordström-AdS black hole by the geodesic length, minimal area surface, and minimal surface area in the bulk, which are dual to the nonlocal observables on the boundary theory by holography, namely, the two-point correlation function, Wilson loop, and entanglement entropy, individually (recently these nonlocal observables have been used to probe the nonequilibrium thermalization process, and it has been found that all of them have the same effect [19–25]). In fact, there have been some similar works to probe the phase structure by holographic entanglement entropy. In [26], the phase structure of entanglement entropy is studied in the plane for both a fixed charge ensemble and a fixed chemical potential ensemble, and it is found that the phase structure of entanglement entropy is similar to that of the thermal entropy. In particular, the entanglement entropy is found to demonstrate the same second-order phase transition at the critical point as the thermal entropy. Soon after, it is found that the entanglement entropy can also probe the van der Waals-like phase transition in the plane [27]. In [28], Nguyen has investigated exclusively the equal area law of holographic entanglement entropy and found that the equal area law holds regardless of the size of the entangling region. Very recently [29] investigated entanglement entropy for a quantum system with infinite volume; their result showed that the entanglement entropy also exhibits the same van der Waals-like phase transition as the thermal entropy. They also checked the equal area law and obtained the critical exponent of the heat capacity near the critical point.

In this paper, we will further investigate whether one can probe the phase structure by two-point correlation function and Wilson loop besides the entanglement entropy. We intend to explore whether they exhibit the similar van der Waals-like phase transition as the entanglement entropy and thermal entropy. In addition, we also want to check whether these nonlocal observables can probe the Hawking-Page phase transition between the AdS black hole and thermal gas so that we can get a complete picture about the phase transition of the black holes in the framework of holography.

This paper is organized as follows. In Section 2, we will discuss the thermal entropy phase transition of a 5-dimensional Reissner-Nordström-AdS black hole in the plane in a fixed charge ensemble. Then in Section 3, we will probe these phase transitions by geodesic length, Wilson loop, and holographic entanglement entropy individually. In each subsection, the equal area law is checked and the critical exponent of the heat capacity is obtained for different sizes of the boundary region. Section 4 is devoted to discussions and conclusions.

#### 2. Thermodynamic Phase Transition of the 5-Dimensional Reissner-Nordström-AdS Black Hole

Starting from the actionwhere and is the AdS radius, we shall focus on the case of , in which the charged Reissner-Nordström-AdS black hole can be written as [9]where , , and are hyperspherical coordinates for the 3 spheres, andwith and being the mass and charge of the black hole. Whence we can get the Hawking temperature of this space time asIn addition, it follows from the Bekenstein-Hawking formula that the entropy of the black hole is given bywhere is the outer event horizon of the black hole, namely, the largest root of the equation . With this, the mass of the back hole can thus be expressed as the function of the event horizon:Substituting (5) and (6) into (4), we can get the relation between the temperature and entropy of the 5-dimensional Reissner-Nordström-AdS black hole:In addition, with the relation , the Helmholtz free energy can be expressed asNote that this formula for our free energy has implicitly chosen the pure AdS as the reference space time because the free energy vanishes for pure AdS by this formula. Now let us review the relevant phase transitions in the fixed charge ensemble by (7) and (8) in the plane.

To achieve this, we should first find the critical charge by the following equations:Inserting (7) into (9), we can get the values for the critical charge and critical entropy:Substituting these critical values into (7), we can get the critical temperature:

We plot the discharge curves for different charges in Figure 1. For the case , there is a minimum temperature [30], which is indicated by the red solid line in . When the temperature is lower than , we have only a thermal AdS. When the temperature is higher than , there are two additional black hole branches. The small branch is unstable while the large branch is stable. This can be justified by checking the corresponding heat capacities, which is related to their slopes. The Hawking-Page phase transition occurs at the temperature given by [30], which is higher than and indicated by the red dashed line. This can be observed by the relation in Figure 2(a), where is the horizontal coordinate of the cusp and is the horizontal coordinate for the intersection of the stable branch and the horizontal axis. Obviously, when the temperature is lower than , the thermal AdS is the most stable state. While when the temperature is higher than , the most stable state is taken over by the large black hole branch.

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For the case , the phase structure is similar to that of the van der Waals phase transition. That is, for a small charge, there is an unstable black hole interpolating between the stable small hole and stable large hole. The small stable hole will jump to the large stable hole at the critical temperature , which is labeled by the red dashed line in Figure 1(b). As the charge increases to the critical charge, the small hole and the large hole merge into one and squeeze out the unstable phase. So there is an inflection point in Figure 1(c). The heat capacity is divergent in this case; the phase transition is therefore second order. As the charge exceeds the critical charge, we simply have one stable black hole at each temperature, which can be justified by the slope of the curve in Figure 1(d). The van der Waals-like phase transition can also be observed from the relation. From Figure 2(b), we see a swallowtail structure, which corresponds to the unstable phase in Figure 1(b). The critical temperature for the phase transition is apparently read off by the horizontal coordinate of the junction between the small black hole and the large black hole. As the temperature is lower than the critical temperature , the free energy of the small black hole is lowest, so the small hole is stable. As the temperature is higher than , the free energy of the large black hole is lowest, so the large hole dominates thereafter. The nonsmoothness of the junction indicates that the phase transition is first order. When the charge is arriving at the critical charge , the swallowtail structure in Figure 2(b) shrinks into a point as is shown in Figure 2(c). The horizontal coordinate of the inflection point corresponds to the critical temperature of the second-order phase transition, which is consistent with the analytical result in (11).

For the first-order phase transition in Figure 1(b), we would like to check whether Maxwell’s equal area law holds with the following formula:in which is defined in (7); and are the smallest and largest roots of the equation . After a simple calculation, we find and . With these values, we find that and in (12) equal 1.09481 and 1.09482, respectively. So the equal area law in the plane holds within our numerical accuracy.

For the second-order phase transition in Figure 1(c), we are interested in the critical exponent associated with the heat capacity:Near the critical point, writing the entropy as and expanding the temperature in terms of small , we findin which we have used (9). In this case, (13) further implies ; namely, the critical exponent is , which is the same as the one from the mean field theory. In addition, taking logarithm to (14), we have a linear relation:with the slope. In what follows, we will use this logarithm to check the critical exponent for the analogous heat capacities in the framework of holography.

It is noteworthy that by holography the whole phase structure described above is not only for the bulk black hole but also for the dual boundary system, where the thermal entropy is simply given by the black hole entropy, and so on.

#### 3. Phase Transition in the Framework of Holography

In this section we shall investigate the phase structures of some nonlocal observables such as two-point correlation function, Wilson loop, and entanglement entropy in the dual fled theory by holography to see whether they have the same phase structure as the thermal entropy.

##### 3.1. Phase Transition of Two-Point Correlation Function

According to the AdS/CFT correspondence, if the conformal dimension of scalar operator of dual field theory is large enough, the equal time two-point correlation function can be holographically approximated as [31]where is the length of the bulk geodesic between the points and on the AdS boundary. Taking into account the spherical symmetry of the 5-dimensional Reissner-Nordström-AdS black hole, we can simply choose and as the two boundary points. Then with to parameterize the trajectory, the proper length is given byin which . Imagining as time, and treating as the Lagrangian, one can get the equation of motion for by making use of the Euler-Lagrange equation; that iswhich can be solved by imposing the following boundary conditions:To explore whether the size of the boundary region affects the later phase structure, we here choose as two examples. Note that, for a fixed , the geodesic length is divergent, so it should be regularized by subtracting off the geodesic length in pure AdS with the same boundary region, denoted by . To achieve this, we are required to set a UV cutoff for each case, which is chosen to be and , respectively, for our two examples. In this paper, we obtain also by numerics though there is an analytical result for for pure AdS in Einstein gravity. We label the regularized geodesic length as .

We plot the relation between and for different in Figures 3 and 4. As shown in Figures 3 and 4, demonstrates a similar phase structure as the thermal entropy. Moreover, we find that the minimum temperature as well as Hawking-Page phase transition temperature in (a), the first-order phase transition temperature in (b), and second-order phase transition temperature in (c) are exactly the same as those in plane, which justifies our notation. To be more specific, it is easy to check by locating the position of local minimum. But in order to confirm and , we are required to examine the equal area law for the first-order phase transition and obtain as the critical exponent for the second-order phase transition, which are documented as follows.

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In the plane, we define the equal area law asin which is an Interpolating Function obtained from the numeric result and and are the smallest and largest roots of the equation . For the case , we find , . Substituting these values into (20), we find , . For the case , after simple calculation, we find , . It is obvious that for different , , and are equal within our numeric accuracy. Thus, the equal area law also holds in the plane.

In addition, in order to investigate the critical exponent for the analogous heat capacity of the geodesic length. We are interested in the logarithm of the quantities , , in which is the critical temperature defined in (11), and is obtained numerically by the equation . We plot the relation between and for different in Figure 5, where these straight lines can be fitted asIt is obvious that the slope is about 3, which indicates that the critical exponent is for the analogous heat capacity and the phase transition is also second order at for the geodesic length.

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##### 3.2. Phase Transition of Wilson Loop

In this subsection, we are going to study the phase structure of the Wilson loop, which in the bulk corresponds to the minimal area surface by holography. Wilson loop operator is defined as a path ordered integral of gauge field over a closed contour, and its expectation value is approximated geometrically by the AdS/CFT correspondence as [32]where is the closed contour, is the minimal bulk surface ending on with its minimal area, and is the Regge slope parameter. Next we choose the line with and as our loop. Then we can employ to parameterize the minimal area surface, which is invariant under the -direction by our rotational symmetry. Thus, the corresponding minimal area surface can be expressed asin which . Making use of the Euler-Lagrange equation, one can get the equation of motion for . Then with the boundary conditions , , we can further get the numeric result of . Similar to the case of geodesic length, we choose as two examples and the corresponding UV cutoffs are set to be , . We label the regularized minimal area surface as , where is the minimal area in pure AdS with the same boundary region. We plot the relation between and for different in Figures 6 and 7. Comparing Figure 6 with Figure 7, we find they are the same nearly besides the scale of the horizontal coordinate. In other words, affects only the value but not the phase structure of minimal area surface in the plane. The result tells us that the similar phase structure also shows up for the minimal surface area. Here we concentrate only on scrutinizing the equal area law for the first-order phase transition and the critical exponent of the analogous heat capacity for the second-order phase transition.

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First, in the plane, the equal area law can be similarly defined asin which is an Interpolating Function obtained from our numeric result and and are the smallest and largest roots of the equation , respectively. As the same as that of the geodesic length, for a fixed , we first obtain and and then substitute these values into (24) to produce , . The concrete values are listed in Table 1. Obviously, for both , and are equal within the reasonable numeric accuracy. The equal area law thus holds in the plane, which reinforces the fact that the minimal surface area has the same first-order phase transition behavior as that of the thermal entropy.

Second, in order to check whether the minimal surface area also demonstrates the same second-order phase transition as the thermal entropy, we would like to evaluate the critical exponent of the analogous heat capacity at the critical point in the plane. To this end, we plot the relations between and in Figure 8. The numerical results for these curves can be fitted asWith the slope, we can conclude that the minimal surface area also has the same second-order phase transition as the thermal entropy.

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##### 3.3. Phase Transition of Entanglement Entropy

Holographic entanglement entropy is another nonlocal observable, and it has been used extensively to probe the superconductivity phase transition besides the thermalization process recently [33–40]. In this subsection, we intend to employ it to probe the phase structure of a 5-dimensional Reissner-Nordström-AdS black hole. According to the formula in [41, 42], holographic entanglement entropy can be given by the area of a minimal surface anchored on the boundary entangling surface ; namely,For simplicity, we choose as our entangling surface and employ to parameterize the minimal surface. But with the symmetry of (2), (26) can be rewritten aswith . Similarly, we can solve the equation of motion for numerically and eventually obtain the regularized entanglement entropy . We plot the relation between and for in Figures 9 and 10, respectively. As one can see, it exhibits a similar behavior as the thermal entropy. To be more precise, we would like to check the equal area law with the following equation:in which is an Interpolating Function obtained from the numeric result and and are the smallest and largest roots of the equation . For different , the results of , and , are listed in Table 2. It is obvious that nearly equals regardless of the choice of . That is, the equal area law is also valid for the entanglement entropy.

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To get the critical exponent of second-order phase transition of entanglement entropy, we should find the slope of a linear function represented by and , in which is the critical entropy obtained numerically by the equation . The numeric results for different are plotted in Figure 11. The results for these curves can be further fitted as One can see that the slope is always about 3 for different . So we can conclude that the entanglement entropy also has the same second-order phase transition as the thermal entropy.

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#### 4. Concluding Remarks

Investigation on the phase transition of the black holes is important and necessary. On the one hand, it is helpful for us to understand the structure and nature of the space time. On the other hand, it may uncover some phase transitions of the realistic physics in the conformal field theory according to the AdS/CFT correspondence. It is well known now that the Hawking-Page phase transition in the gravity system is dual to the confinement/deconfinement phase transition, and the phase transition of a scalar field is dual to the superconductivity phase transition in the dual conformal field theory.

In this paper, we investigated the van der Waals-like phase transition in the framework of holography so that we can explore whether there is a realistic similar phase transition in physics. Taking the 5-dimensional Reissner-Nordström-AdS black hole as the gravity background, we investigated the phase structure of the two-point correlation function, Wilson loop, and holographic entanglement entropy. For all the nonlocal observables, we observed that the black hole undergoes a van der Waals-like phase transition. This conclusion is reinforced by the investigation of the equal area law and critical exponent of the analogous heat capacity in which we found that the equal area law is valid always and the critical exponent of the heat capacity coincides with that of the mean field theory regardless of the size of the boundary region. In addition, we found the black hole undergoes a Hawking-Page phase transition before the van der Waals-like phase transition for all the nonlocal observables. We also obtained the minimum temperature and Hawking-Page phase transition temperature. Our investigation thus provides a complete picture depicting the phase transition of charged AdS black hole in the framework of holography.

#### Competing Interests

The authors declare that they have no competing interests.

#### Acknowledgments

The authors would like to thank Rong-Gen Cai for his discussions. This work is supported by the National Natural Science Foundation of China (Grants nos. 11405016 and 11575270), China Postdoctoral Science Foundation (Grant no. 2016M590138), Natural Science Foundation of Education Committee of Chongqing (Grant no. KJ1500530), and Basic Research Project of Science and Technology Committee of Chongqing (Grant no. cstc2016jcyja0364).

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#### Copyright

Copyright © 2016 Xiao-Xiong Zeng and Li-Fang Li. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.