The VOS-viewer is a software instrument for constructing and visualizing (mapping) a broad range of bibliometric networks. These networks may for instance include journals, researchers, or individual publications, and they can be constructed with co-citation, bibliographic coupling, or co-authorship relations. In particular, the VOSviewer also offers a text mining functionality that can be used to construct and visualize conceptual (co-word based) networks of terms extracted from a body of scientific literature, particularly titles and abstracts of publications. The VOS viewer can be uploaded with any type of relational information and particularly with publications records of the WoS as well as of Scopus [ 31 ].

For both approaches we used the recently developed CWTS bibliometric instruments CitNetExplorer and the VOS-viewer [ 31 – 33 ]. The CitNetExplorer is a software instrument specifically designed for analyzing and visualizing citation networks of scientific literature and it can be uploaded with sets of publication records directly from the Web of Science (WoS) or Scopus. Citation networks can then be explored interactively, for instance by drilling down into a network and by identifying clusters of closely related publications [ 32 ].

In the second approach we use natural language processing (text mining) to extract the important, publication-specific concepts (terms such as keywords or noun phrases) from the titles and abstracts of a set of SBs. By measuring all co-occurrences of any possible pair of concepts, co-word maps can be created in which the conceptual structure of the research represented by the set of SBs is visualized. For a recent discussion of the concept mapping methodology we refer to [ 30 ].

First the citation-based approach. As any other publication, SBs have links with other (earlier) publications by their references and it is interesting to find out whether there are SBs that have references in common. This might reveal ‘families’ of SBs, in bibliometric terms these are bibliographically coupled SBs. And the other way around, these common references are co-cited by SBs.

In the foregoing section we showed that authors may have several SBs so that at least at the level of individual scientists there are connections between different SBs. There are two major bibliometric approaches to investigate the cognitive environment of publications, and here SBs in particular. The first one is citation-based, the second is concept-based.

Citation links of Sleeping Beauties

We analyze the citation network for each set of SBs of the three main fields, i.e., the 389 physics, 265 chemistry, and 367 engineering & computer science SBs as defined earlier in this paper (s = 10, cs max = 1.0, a min = a max = 10, ca min = 5, short notation [10, 1.0, 10, 5]). This was done by creating a full record (title, abstract, authors, institutions, references) set of these SBs and uploading this set into the CitNetExplorer. Our search algorithm identifies SBs with the requested variables and creates an Excel-database of these SBs including their WoS UT (unique tag) codes. These UT codes can then be uploaded into the WoS website menu in order to produce the full records of the SBs. Thus, the SBs are the source publications and the references of the SBs define the citation links. This procedure renders a citation network based on the references of the SBs if sufficient citations links are available. In the visualization of the citation network each circle represents a publication. Publications are labeled by the last name of the first author. To avoid overlapping labels, some labels may not be displayed. The horizontal location of a publication is determined by its citation relations with other publications. The vertical location of a publication is determined by its publication year. The lines represent citation relations, citations point in upward direction: the cited publication is always located above the citing publication. Publications are clustered based on their citation relations. The identified clusters have different colors.

We determined the citation network of the 389 physics, 265 chemistry and 367 engineering & computer science SBs. The CitNetExplorer algorithm applies threshold values of important parameters for the construction of the citation network, particularly for the minimum number of citation links and also for the minimum cluster size. In this sense, the CitNetExplorer operates as a community detection tool. We refer to the methodology section in the CitNetExplorer website for details. A high value for the minimum number of citation links (e.g., 10) results in a sparse network and a low value (e.g., 2) gives an overcrowded picture.

The interactive facilities of the CitNetExplorer allow to find an optimal network configuration. By trying out several parameter values, we find a sensible representation of the overall citation network analysis with 5 for the minimum number of citation links (mcl = 5) and 2 for the minimum cluster size (mcs = 2). A minimum number of 5 citation links means that in the set of 389 physics SBs only references (publications cited by the SBs) that occur at least in 5 different SBs are included in the construction of the network. This provides us with a general overview. The next step is a ‘drill down’ to specific SBs which reveals more details of the citation network.

We present the results for the 389 physics SBs in Fig 6. A general overview is shown in the upper panel of Fig 6 (mcl = 5, mcs = 2). Several clusters are detected, indicated by colors. Most of these clusters are small, mainly because of the relatively high threshold for the minimum number of citations links. Major clusters are the blue one (left side of the figure) about supergravity and related theoretical work, the gray one about photometric work in biophysics, and the green cluster about string theory and dark matter. In this network we chose a number of SBs as examples for further analysis. These SBs, with a square, are the papers of Ward et al (1982) [34], van Neerven and Vermaseren (1984) [35], Romans (1986) [36], Kostelecky and Samuel (1990) [37], and Kosowsky et al (1992) [38]. In Table 7 we see that Kostelecky with co-author Samuels have 6 SBs, Romans has 5 SBs, Ward has 3 SBs, and Kosowosky with co-author Turner have 3 SBs. Not all of these SBs are visible in the figure because of the threshold for the number of citation links as discussed above.

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larger image TIFF original image Download: Fig 6. Upper panel: Physics Sleeping Beauties and their citation links, overall picture. Second panel from above: Five selected physics Sleeping Beauties clusters and their citation links (minimum number of citation links = 5). Third panel from above: Further selection of physics Sleeping Beauties clusters and their citation links, now with lower threshold (minimum number of citation links = 3) and drill down to the McDonald SB. Lower panel: Application-oriented physics Sleeping Beauties and their citation links (minimum number of citation links = 2). https://doi.org/10.1371/journal.pone.0139786.g006

With the ‘drill down’ option of the CitNetExplorer the citation network of specifically these five selected SBs is extended. We show the results in the second panel from above in Fig 6 (mcl = 5, mcs = 2). Remarkably, three clusters are clearly connected. From left to right we see the Kosowsky, Kostelecky and Romans cluster. The Kosowsky cluster consists of SBs (of which three by Kosowski and colleagues) on astrophysical, particularly cosmological topics such as dark matter, gravitational radiation and the origin of matter in the inflationary universe. In the Kostelecky cluster we find SBs (of which six by Kostelecky and colleagues) as well as the highly cited paper of Witten (1986) [39] (not a SB, appears in the cluster as a cited publication with a number of citation links above the threshold of 5). This cluster deals with theoretical physics topics on the nature of matter and gravitation, in particular string theory. The Kosowsky and Kostelecky clusters are connected by the SB of Dine et al (1984) on supersymmetry (a model on the classification of elementary particles) [40] and the very highly cited paper (not a SB) of Guth et al (1981) on the inflationary universe [41]. Via the also very highly cited paper (not a SB) on vacuum configurations for superstrings by Candelas et al (1985) [42], co-authored by the string theory pioneer Witten, and via the SB of De Wit et al (1987) on supergravity [43] we observe a further connection with the Romans cluster, particularly the most famous SB of Romans (1986) on supergravitation in a ten-dimensional space-time[28]. The SB of McDonald (1994) on the extension of the standard model to include dark matter particles [44] is somewhat isolated between the Kosowsky and the Kostelecky cluster. This a most interesting paper as we shall see further on because this paper becomes highly cited and turns out to have a remarkable form of ‘self-awakening’.

In between the above clusters we find the small cluster of Van Neerven and Vermaseren (1984) [35] and Kotikov (1991) [45], both SBs which deal with mathematical methods for describing the interaction between elementary particles. Although these SBs are not connected with the above discussed clusters (at least not above the threshold value for citation links), the position of these SBs is certainly sensible given the importance of mathematical methods in theoretical physics. In the upper left corner of the figure we find the Ward cluster (Ward et al 1980 [46], Bokman and Ward 1981 [47], Ward et al 1982 [34]), all are SBs dealing with biophysical and particularly photochemical topics.

By lowering the threshold for the minimum number of citation links from 5 to 3, more references of the SBs are covered and thus the citation network shows further details. We select the McDonald (1994) [44] SB and drill down again for this SB. The result is shown in the third panel from above in Fig 6 (mcl = 3, mcs = 2). Here we see a detailed citation network with links to the seminal books of Weinberg (1972) on gravitation and cosmology [48] and of Hawking and Ellis (1973) on the large scale structure of space-time [49]. The Kosowsky, Kostelecky and Romans clusters remain connected. Interesting is the separate cluster at the right hand side of the Death SB. It is part of three successive papers (in a row in the same journal volume) on gravitation radiation of colliding black hole of which the first one is not an SB (Death and Payne 1992 [50–52]. Most probably, researchers working on this topic decide to cite only the first paper and leaving the two next papers uncited, thus creating ‘artificially’ SBs by a kind of citation-superfluity.

From the above discussed observations we conclude that the physics SBs with theoretical topics are relatively strongly connected along different paths. What about the connectivity of the application-oriented physics SBs? We find that only the Ward cluster has a some connectivity as shown in the two upper panels of Fig 6. The lowest positioned publication is the SB of Ward et al (1982) [34]. The network shows the reference links to earlier publications of Ward and colleagues, and all are SBs. All other application-oriented physics SBs appear to have a very limited amount of citation links. Next to the Ward cluster we can only identify (upper panel Fig 6) the link between the SBs of Onoda (1982) [53] and Nomura (1983) [54] as a small cluster. This cluster concerns research on the application of ceramics in microwave devices. The Nomura SB is positioned between the SBs marked as Fujisaka and Pandey (Fujisaka and Yamada 1983 [55]; Pandey and Mehta 1983 [56]).

To illustrate the rather isolated positions of application-oriented SBs further, we selected from the set of 389 physics SBs those published in the most recent years of this set, 1992, 1993, 1994, and form this subset the 10 most cited (after awakening), of which five are application-oriented. This selection is further discussed in the next section on characteristics of the papers citing the awakened SBs. We show in the lower panel of Fig 6 (mcl = 2, mcs = 2) the citation links map of these five highly cited application-oriented SBs: Birks (1992) on optical fibers [57]; Li and Ahmadi (1992) on particles in turbulent flows [58]; Anderson et al (1993) on propagation of light pulses in nonlinear optical fibers [59]; the earlier discussed SB of Takeda and Shiraishi (1994) on flat silicene structures [24]; and Lewis (1994) on electrical insulation at nanoscale [60]. We see that only the Takeda and Lewis SBs have some citation links above the threshold but none of the application-oriented SBs has direct or indirect a connection with another application-oriented SB.